# AskANumber

> AskANumber is an AI-native calculator platform: 59 expert-checked calculators fronted by a plain-English question bar. Every answer shows its formula, cites its source, and gets a permalink that encodes all inputs. AI parses words; a deterministic, audited formula computes the number — the model never does the math.

Every calculator page includes: a crisp one-line answer summary, the plain-text formula with variable definitions, assumptions, limitations, authoritative sources with URLs, a worked example with stable default inputs, an FAQ, and a last-reviewed date. Inputs are encoded as URL query parameters, so any calculation can be cited by URL and reproduced exactly.

## Calculators

### Finance

- [Mortgage Calculator](https://askanumber.com/mortgage-calculator): Monthly payment, total interest, and amortization for a fixed-rate mortgage — principal and interest only, with an extra-payment field to test payoff. Formula: M = P · [r(1+r)^n] / [(1+r)^n − 1]
- [Loan Calculator](https://askanumber.com/loan-calculator): Monthly payment and total interest for any fixed-rate loan from three inputs — amount borrowed, APR, and term — using standard amortization. Formula: M = P · [r(1+r)^n] / [(1+r)^n − 1]
- [Student Loan Calculator](https://askanumber.com/student-loan-calculator): Estimate the monthly payment, total interest, and payoff time on a student loan from your balance, rate, and term — plus how extra payments finish it sooner. Formula: M = P · r / (1 − (1+r)^−n);  extra payments shorten n
- [Auto Loan Calculator](https://askanumber.com/auto-loan-calculator): Monthly car payment from the amount financed — vehicle price with sales tax added, minus down payment and trade-in — over 36 to 84 months. Formula: P = (price · (1 + tax)) − down − trade; M = P · r / (1 − (1+r)^−n)
- [Compound Interest Calculator](https://askanumber.com/compound-interest-calculator): Future value of an investment with regular monthly contributions — compounded annually, quarterly, monthly, or daily, with growth split from deposits. Formula: A = P(1 + r/n)^(nt) + PMT · ((1+r/n)^(nt) − 1) / (r/n)
- [Simple Interest Calculator](https://askanumber.com/simple-interest-calculator): Interest accrued without compounding — I = P·r·t, so the math is linear and doubling the time or the rate exactly doubles the interest. Formula: I = P · r · t
- [Retirement Calculator](https://askanumber.com/retirement-calculator): Projected nest egg at retirement age from your current savings, monthly contributions, and expected return — growth compounded monthly. Formula: FV = P(1+r)^n + PMT · ((1+r)^n − 1)/r
- [Savings Goal Calculator](https://askanumber.com/savings-goal-calculator): How much you need to save monthly to hit a target — a future-value problem solved in reverse, counting what your current balance grows into. Formula: PMT = (FV − P(1+r)^n) · r / ((1+r)^n − 1)
- [ROI Calculator](https://askanumber.com/roi-calculator): Return on investment as a total percentage and annualized as CAGR — making gains over different holding periods directly comparable. Formula: ROI = (V − C) / C · 100;  CAGR = (V/C)^(1/t) − 1
- [Paycheck Calculator](https://askanumber.com/paycheck-calculator): Gross-to-net paycheck estimate for a single filer — 2024 federal brackets, FICA, a flat state rate, and 401(k) contributions taken pre-tax. Formula: net = gross − 401k − federal − FICA − state
- [Credit Card Payoff Calculator](https://askanumber.com/credit-card-payoff-calculator): How long a fixed monthly payment takes to clear a balance — credit card, student loan, or any debt — and what paying more saves. Formula: n = −ln(1 − Br/M) / ln(1+r)
- [Refinance Calculator](https://askanumber.com/refinance-calculator): Lifetime savings from refinancing a mortgage — monthly payment difference, break-even point on closing costs, and the new payment amount. Formula: savings = (M_old − M_new) · n − closing_costs
- [Rent vs Buy Calculator](https://askanumber.com/rent-vs-buy-calculator): Compare the cost of renting vs buying a home over your time horizon — mortgage, upkeep, and down payment against rent with 3% annual inflation. Formula: buy = mortgage + maintenance + down;  rent = ∑ rent · (1.03)^t
- [Tip Calculator](https://askanumber.com/tip-calculator): Calculate the tip on any bill and split the total per person — set any percentage, see the math, and share an exact permalink. Formula: tip = bill · %;  per person = (bill + tip) / split
- [Sales Tax Calculator](https://askanumber.com/sales-tax-calculator): Add sales tax to a pre-tax price to get the out-the-door total — the tax amount is itemized so you can see exactly what the rate adds. Formula: total = price · (1 + rate/100)
- [Discount Calculator](https://askanumber.com/discount-calculator): Final price after a percent-off discount, plus exactly what you save — 30% off means you pay 70% of the original price. Formula: final = price · (1 − %/100)
- [Inflation Calculator](https://askanumber.com/inflation-calculator): What money will cost — or used to be worth — at a given inflation rate, compounded annually, with the loss in purchasing power alongside. Formula: future = amount · (1 + r)^y;  past = amount / (1 + r)^y
- [Income Tax Calculator](https://askanumber.com/income-tax-calculator): Estimated US federal income tax from the 2024 brackets and standard deduction — with your effective rate, marginal bracket, and taxable income. Formula: tax = Σ bracket-slice × rate, on income − standard deduction

### Health & Fitness

- [BMI Calculator](https://askanumber.com/bmi-calculator): Body Mass Index — a screening measure of body fat from height and weight, classified against the WHO adult ranges in US or metric units. Formula: BMI = mass (kg) / height (m)²
- [BMR Calculator](https://askanumber.com/bmr-calculator): Basal metabolic rate — daily calories burned at complete rest, computed with the Mifflin-St Jeor equation from sex, age, height, and weight. Formula: M: 10·kg + 6.25·cm − 5·age + 5 | F: 10·kg + 6.25·cm − 5·age − 161
- [TDEE Calculator](https://askanumber.com/tdee-calculator): Total daily energy expenditure — calories burned including activity, from your BMR times an activity factor between 1.2 (sedentary) and 1.9. Formula: TDEE = BMR · activity factor
- [Calorie Deficit Calculator](https://askanumber.com/calorie-deficit-calculator): Daily calorie target for a desired weekly weight loss, using the 3,500 kcal per pound approximation — enter your TDEE and a weekly goal. Formula: target = TDEE − (loss_lb · 3500 / 7)
- [Body Fat Calculator](https://askanumber.com/body-fat-calculator): US Navy body fat estimate from tape-measure circumferences — height, neck, and waist, plus hips for women — no calipers or scan needed. Formula: Navy: 86.01·log10(waist−neck) − 70.04·log10(height) + 36.76
- [Ideal Weight Calculator](https://askanumber.com/ideal-weight-calculator): Ideal body weight range from height and sex — the Devine, Robinson, and Miller formulas side by side, with their average as the headline. Formula: Devine M: 50 + 2.3·(in over 5ft);  F: 45.5 + 2.3·(in over 5ft)
- [Water Intake Calculator](https://askanumber.com/water-intake-calculator): Recommended daily water intake based on weight and activity — 0.5 to 0.75 fl oz per pound, shown in fluid ounces, liters, and cups. Formula: oz = weight_lb · activity_factor (0.5–0.75)
- [Pregnancy Due Date Calculator](https://askanumber.com/due-date-calculator): Estimated due date from your last menstrual period using Naegele's rule — LMP plus 280 days, adjusted for cycle length — with weeks and trimester. Formula: due = LMP + 280 days + (cycle − 28)
- [Heart Rate Zones Calculator](https://askanumber.com/heart-rate-zones-calculator): Training heart rate zones from age and resting heart rate — the Karvonen heart-rate-reserve method, with zone boundaries in actual bpm. Formula: HRmax = 220 − age;  zone = rest + (HRmax − rest) · %
- [Macro Split Calculator](https://askanumber.com/macro-split-calculator): Daily protein, carb, and fat gram targets from a calorie budget — balanced, low-carb, or high-protein splits at 4 and 9 kcal per gram. Formula: g = kcal · ratio / (4 for P/C, 9 for F)
- [Ovulation Calculator](https://askanumber.com/ovulation-calculator): Estimated ovulation day and fertile window from your last period and cycle length — assuming a typical 14-day luteal phase, plus your next period date. Formula: ovulation ≈ next period − 14 days (luteal phase)

### Math

- [Percentage Calculator](https://askanumber.com/percentage-calculator): What is X% of Y, Y is what percent of X, and percent change between two values — three modes for the percentage questions people mix up. Formula: of: X·Y/100;  is: X/Y·100;  change: (Y−X)/X·100
- [Fraction Calculator](https://askanumber.com/fraction-calculator): Add, subtract, multiply, or divide two fractions — the result is reduced to lowest terms, with its decimal equivalent alongside. Formula: + / − : (n₁d₂ ± n₂d₁) / (d₁d₂) | × : (n₁n₂) / (d₁d₂) | ÷ : (n₁d₂) / (d₁n₂)
- [Scientific Calculator](https://askanumber.com/scientific-calculator): Evaluate a full expression with trig, logs, powers, roots, and factorials — standard order of operations, with pi and e built in. Formula: standard mathematical evaluation
- [Statistics Calculator](https://askanumber.com/statistics-calculator): Mean, median, mode, range, and standard deviation from any list of numbers — paste values separated by commas or spaces and get the count too. Formula: mean = Σx/n;  σ = √(Σ(x−μ)² / n)
- [GCD & LCM Calculator](https://askanumber.com/gcd-lcm-calculator): Greatest common divisor and least common multiple of two integers — the GCD via the Euclidean algorithm, the LCM from |a·b| / GCD. Formula: GCD: Euclidean algorithm;  LCM = |a·b| / GCD
- [Quadratic Equation Solver](https://askanumber.com/quadratic-equation-solver): Solve ax² + bx + c = 0 with the quadratic formula — real or complex roots, plus the discriminant and the vertex of the parabola. Formula: x = (−b ± √(b² − 4ac)) / 2a
- [Triangle Calculator](https://askanumber.com/triangle-calculator): Right triangle solver: enter two legs to get the hypotenuse from the Pythagorean theorem, plus area, both acute angles, and perimeter. Formula: c = √(a² + b²);  area = ab/2
- [Area Calculator](https://askanumber.com/area-calculator): Area of a rectangle, circle, or triangle from the standard formulas — unit-agnostic, so feet in gives square feet out. Formula: rect: a·b;  circle: πr²;  triangle: ½·b·h
- [Volume Calculator](https://askanumber.com/volume-calculator): Volume of a box, sphere, or cylinder from the standard formulas — results come out in the cube of whatever length unit you enter. Formula: box: w·h·d;  sphere: 4/3·πr³;  cylinder: πr²·h
- [Exponent & Logarithm Calculator](https://askanumber.com/exponent-log-calculator): Compute x^y, logarithms in any base, and nth roots in one place — logs use the change-of-base identity, roots are fractional powers. Formula: logₐ(b) = ln(b)/ln(a);  ⁿ√x = x^(1/n)
- [GPA Calculator](https://askanumber.com/gpa-calculator): Grade point average on the unweighted 4.0 scale from letter grades and credit hours — each course's points weighted by its credits. Formula: GPA = Σ(grade points × credits) / Σ credits
- [Final Grade Calculator](https://askanumber.com/final-grade-calculator): What you need to score on the final exam to hit your target grade — plus the scores that would land an A, a B, or a C at your exam weight. Formula: needed = (target − current·(1 − w)) / w
- [P-Value Calculator](https://askanumber.com/p-value-calculator): P-value from a z-score, one- or two-tailed, with a significance verdict at α = 0.05 and the critical z value for comparison. Formula: two-tailed: p = 2·(1 − Φ(|z|));  one-tailed: p = 1 − Φ(|z|)
- [Confidence Interval Calculator](https://askanumber.com/confidence-interval-calculator): Confidence interval for a mean from sample statistics — a z-interval at 90%, 95%, or 99%, with the margin of error and standard error shown. Formula: CI = mean ± z · (sd / √n)

### Conversion

- [Length Converter](https://askanumber.com/length-converter): Convert meters, feet, inches, kilometers, miles, yards, and more using exact international definitions (1 in = 2.54 cm exactly). Formula: Multiply or divide by the conversion factor to a base unit.
- [Weight Converter](https://askanumber.com/weight-converter): Convert kilograms, pounds, ounces, grams, stones, and metric tons — the pound is defined as exactly 0.45359237 kg. Formula: Multiply or divide by the conversion factor to a base unit.
- [Temperature Converter](https://askanumber.com/temperature-converter): Convert between Celsius, Fahrenheit, and Kelvin with the exact formulas — F = C·9/5 + 32 and K = C + 273.15 — shown as one full equation. Formula: F = C·9/5 + 32;  K = C + 273.15
- [Volume Converter](https://askanumber.com/volume-converter): Convert liters, US gallons, cups, quarts, pints, fluid ounces, and tablespoons — including the kitchen measures that trip up recipes. Formula: Multiply or divide by the conversion factor to a base unit.
- [Area Converter](https://askanumber.com/area-converter): Convert square meters, square feet, acres, hectares, and square miles — land, floor-plan, and map units in one place. Formula: Multiply or divide by the conversion factor to a base unit.
- [Speed Converter](https://askanumber.com/speed-converter): Convert mph, km/h, meters per second, knots, and Mach — road, running, sailing, and aviation speeds on one dial. Formula: Multiply or divide by the conversion factor to a base unit.
- [Energy Converter](https://askanumber.com/energy-converter): Convert joules, calories, kilocalories (food Calories), watt-hours, kWh, and BTU — the food-label and utility-bill units, reconciled. Formula: Multiply or divide by the conversion factor to a base unit.
- [Time Converter](https://askanumber.com/time-converter): Convert seconds, minutes, hours, days, weeks, months, and years — using the average Gregorian month and year, not 30-day approximations. Formula: Multiply or divide by the conversion factor to a base unit.
- [Currency Converter](https://askanumber.com/currency-converter): Convert between 31 world currencies at daily ECB reference rates — mid-market, with no retail spread — including the rate and its inverse. Formula: out = (amount / rate_from) · rate_to
- [Data Storage Converter](https://askanumber.com/data-converter): Convert bytes, KB, MB, GB, and TB — and the binary units (KiB, MiB, GiB) that explain why a "1 TB" drive shows up as 931 GiB. Formula: Multiply or divide by the conversion factor to a base unit.

### Date & Time

- [Date Difference Calculator](https://askanumber.com/date-difference-calculator): How many days, weeks, months, and years between two dates — an exact calendar day count that handles leap years automatically. Formula: days = (to − from) / 86400 s
- [Age Calculator](https://askanumber.com/age-calculator): Exact age in years, months, and days from a date of birth — using real calendar month lengths, with totals in days, weeks, and hours. Formula: age = floor((on − birth) / year)
- [Time Zone Converter](https://askanumber.com/time-zone-converter): Convert a clock time between cities worldwide — daylight saving applied automatically for today's date, plus the exact hour difference between them. Formula: UTC = local − from_offset;  destination = UTC + to_offset  (offsets read live from the IANA tz database, DST included)
- [Business Days Calculator](https://askanumber.com/business-days-calculator): Business days (Mon–Fri) between two dates, counted inclusively — weekends excluded, public holidays left to your own calendar. Formula: count weekdays from start to end inclusive
- [Countdown Calculator](https://askanumber.com/countdown-calculator): How long until a future event — live days, hours, and minutes from this moment, switching to time elapsed once the date has passed. Formula: count down to event date
- [Hours Calculator](https://askanumber.com/hours-calculator): Hours worked between two clock times, minus unpaid breaks — with decimal hours for payroll and overnight shifts handled past midnight. Formula: hours = end − start − break  (overnight wraps past midnight)

## Guides

Answer-first interpretation guides — what the numbers mean, what counts as good, and how much for your situation — each cited and linked to its calculator.

- [How much house can I afford?](https://askanumber.com/guides/how-much-house-can-i-afford): A common starting point is the 28/36 rule: keep housing costs under about 28% of your gross (pre-tax) monthly income, and total debt payments under about 36%. On an $80,000 salary — roughly $6,667/month gross — that's about $1,867/month for housing. The home price that payment buys depends on your mortgage rate, down payment, and taxes and insurance.
- [What is a good mortgage rate?](https://askanumber.com/guides/what-is-a-good-mortgage-rate): A good mortgage rate is one at or below the current national average for your loan type. As of July 2026, that benchmark is about 6.43% for a 30-year fixed and 5.79% for a 15-year fixed (Freddie Mac PMMS). Because rates move weekly, "good" is relative to today's average, not a fixed number, and your own rate depends on your credit and loan details.
- [What's a good interest rate on a car loan?](https://askanumber.com/guides/whats-a-good-interest-rate-on-a-car-loan): A good car loan rate is at or below the average for your credit tier. As a benchmark, the average rate on a 60-month new-car loan is about 7.52% APR (Federal Reserve G.19, Q1 2026); used-car loans run higher. Super-prime borrowers (781+ credit score) beat that average handily, while subprime borrowers pay far more. Rates change over time, so this figure is a moving target.
- [Is my student loan payment too high?](https://askanumber.com/guides/is-my-student-loan-payment-too-high): As a rule of thumb, your student loan payment is generally considered manageable when it stays under about 8-10% of your gross monthly income. Some lenders use a stricter test (under 10% of take-home pay) or a broader one (total debt under 36% of income). Above these thresholds, the payment is often called high relative to earnings. Run your own numbers in the Student Loan Calculator.
- [How much should I save each month to retire?](https://askanumber.com/guides/how-much-to-save-each-month-to-retire): A common starting point is to save about 15% of your gross income for retirement, including any employer 401(k) match (Fidelity's guideline). A parallel target is a nest egg near 25x your expected annual expenses, which pairs with the "4% rule." The exact monthly dollar figure depends on your income, age, and existing savings, so run your own numbers in the Retirement Calculator.
- [Should I refinance my mortgage?](https://askanumber.com/guides/should-i-refinance-my-mortgage): Refinancing your mortgage is generally worth considering when you can lower your interest rate by roughly 0.5-1 percentage point and you plan to stay in the home past the break-even point, which equals your closing costs divided by your monthly savings. Closing costs typically run about 2-5% of the loan amount, so the longer you stay, the more likely refinancing pays off.
- [Is a BMI of 27 healthy?](https://askanumber.com/guides/is-a-bmi-of-27-healthy): A BMI of 27 falls in the "overweight" range under the World Health Organization's adult classification, which spans 25.0 to 29.9 — above the healthy range of 18.5–24.9 but below the obese threshold of 30. So a BMI of 27 sits just past the healthy band. BMI is a screening number, not a body-fat measurement, so context matters.
- [What is a healthy body fat percentage?](https://askanumber.com/guides/what-is-a-healthy-body-fat-percentage): A healthy body-fat percentage depends on sex: the American Council on Exercise puts the "fitness" range at roughly 14–17% for men and 21–24% for women, with 18–24% (men) and 25–31% (women) counting as average/acceptable. Women naturally carry more essential fat, so their healthy numbers sit higher than men's across every category.
- [How many calories should I eat to lose weight?](https://askanumber.com/guides/how-many-calories-should-i-eat-to-lose-weight): To lose weight, you eat below your total daily energy expenditure (TDEE, or maintenance). A deficit of about 500 kcal/day yields roughly 1 lb (0.45 kg) of loss per week, since about 3,500 kcal equals 1 lb of body fat; around 750 kcal/day gives about 1.5 lb/week. Start from your TDEE and subtract the deficit.
- [How much water should I drink a day?](https://askanumber.com/guides/how-much-water-should-i-drink-a-day): The U.S. National Academies set adequate total water intake at about 3.7 liters (~15.5 cups) a day for men and 2.7 liters (~11.5 cups) a day for women, counting both beverages and food. Since roughly 20% comes from food, that works out to about 13 cups of fluids a day for men and about 9 cups for women.
- [What heart rate should I exercise at?](https://askanumber.com/guides/what-heart-rate-should-i-exercise-at): The American Heart Association points to two target zones built on your estimated maximum heart rate (roughly 220 minus your age): 50–70% of that max for moderate-intensity exercise and 70–85% for vigorous. At age 40, max ≈ 180 bpm, so moderate falls around 90–126 bpm and vigorous around 126–153 bpm. These are estimates, not personal prescriptions.
- [How many calories do I burn a day?](https://askanumber.com/guides/how-many-calories-do-i-burn-a-day): Most adults burn roughly 1,600 to 3,000 calories a day. Your total daily energy expenditure (TDEE) equals your resting metabolic rate (BMR) — the calories your body uses at rest — multiplied by an activity factor from about 1.2 (sedentary) to 1.9 (very active). Exact numbers depend on your sex, age, height, weight and how much you move.
- [How do I calculate a percentage?](https://askanumber.com/guides/how-to-calculate-a-percentage): To find X% of a number, multiply the number by X/100. For example, 20% of 150 = 150 × 0.20 = 30. A percentage is simply a fraction out of 100, so "20%" means 20 per 100. That single multiply-by-a-decimal move handles most everyday percentage questions, from tips to discounts to test scores.
- [How do I calculate percentage change?](https://askanumber.com/guides/how-to-calculate-percentage-change): Percentage change = (new − old) ÷ old × 100. Subtract the old value from the new, divide by the old value, then multiply by 100. A positive result is an increase; a negative one is a decrease. For example, 80 rising to 100 is (100 − 80) ÷ 80 × 100 = +25%, while 100 falling to 80 is −20%.
- [What is a good GPA?](https://askanumber.com/guides/what-is-a-good-gpa): On the standard U.S. 4.0 scale, a "good" GPA is roughly 3.0 or higher — a solid B average — while 3.5+ is considered strong and often earns honors or cum laude, and 4.0 is straight-A. But "good" is contextual: many colleges look for 3.0+, competitive graduate and professional programs often expect 3.5+, and scholarships commonly set 3.0 or 3.5 minimums.
- [What grade do I need on my final exam?](https://askanumber.com/guides/what-grade-do-i-need-on-my-final): The grade you need on your final = (target − current × (1 − final weight)) ÷ final weight, with weight written as a decimal. Say your current grade is 88%, the final is worth 30% (0.30), and you want a 90% overall: (90 − 88×0.70) ÷ 0.30 = 94.7%. If that result tops 100%, the target isn't reachable on the final alone.
- [How much should I tip?](https://askanumber.com/guides/how-much-should-i-tip): In the U.S., the customary restaurant tip is 15–20% of the pre-tax bill: about 15% for adequate service, 18–20% for good, and 20% or more for excellent. Many people tip on the total shown for simplicity. Common norms elsewhere: $1–2 per drink at a bar, 15–20% for taxis and rideshare, and $1–2 per bag for hotel porters.
- [How do I calculate my age?](https://askanumber.com/guides/how-to-calculate-your-age): Your age in full years equals the current year minus your birth year, then subtract 1 if your birthday hasn't happened yet this year. Born July 10, 1990, with today being July 4, 2026? That's 2026 − 1990 = 36, but July 10 hasn't arrived, so your age is 35. For age to the day, count the calendar days between the two dates.
- [How many business days are between two dates?](https://askanumber.com/guides/how-many-business-days-between-two-dates): To count business days between two dates, take the total calendar days in the range and subtract every weekend day and any public holiday that falls inside it. Business days are weekdays, Monday through Friday. For example, a 7-calendar-day span starting on a Monday contains 5 business days, because it skips one Saturday and Sunday.
- [How many days until a specific date?](https://askanumber.com/guides/how-many-days-until-a-date): The number of days until a specific date is the count of calendar days from today to that date. From July 4 to December 31, for example, that count is 180 days. The exact total depends on whether today, the end date, or both are counted, and it shrinks by one each day. A leap day falling inside the range adds one more.

## Pages

- [All calculators (A–Z)](https://askanumber.com/calculators): full directory
- [Finance calculators](https://askanumber.com/calculators/finance)
- [Health & Fitness calculators](https://askanumber.com/calculators/health)
- [Math calculators](https://askanumber.com/calculators/math)
- [Conversion calculators](https://askanumber.com/calculators/convert)
- [Date & Time calculators](https://askanumber.com/calculators/date)
- [About](https://askanumber.com/about): what AskANumber is and who maintains it
- [Methodology](https://askanumber.com/methodology): how formulas are sourced, tested, and reviewed
- [Contact](https://askanumber.com/contact): report a calculation error

## Citing us

Cite calculators by URL — e.g. https://askanumber.com/mortgage?price=620000&down=124000&rate=6.5&years=30 reproduces that exact calculation. Formulas are plain text in the HTML, methodology is sourced, and pages carry Schema.org SoftwareApplication + FAQPage markup.