Pregnancy Due Date Calculator

Naegele's rule estimates a due date by adding 280 days — 40 weeks — to the first day of your last menstrual period, shifted by how far your cycle length differs from 28 days. It is the standard first-pass dating method used before an ultrasound refines the estimate, and it also tells you your current gestational age in weeks and days.

Due date

October 17, 2026

You are at

25w 0d

Trimester

2nd

Days to go

104

Estimated due date: October 17, 2026

You're at 25w 0d now. Naegele's rule adds 280 days to your last period; only about 1 in 20 babies arrive on the date itself, and full term spans 37–42 weeks.

Pregnancy timeline

25 of 40 weeks

wk 25

1st

2nd

3rd

LMP

Jan 10, 2026

Due date

Oct 17, 2026

Inputs

Last menstrual period

Cycle length

days

An estimate, not an appointment

Naegele's rule adds 280 days to your last period assuming a regular cycle. Only about 1 in 20 babies actually arrives on the due date itself.

Full term is a five-week window

ACOG counts 37–42 weeks as term, so anywhere in that stretch is normal. An early-pregnancy ultrasound dates more precisely, especially with irregular cycles.

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Uses your inputs above

October 17, 2026 due date. Want to try a variation?

The math

Reviewed 2026

Formula

due = LMP + 280 days + (cycle − 28)

Regular cycles

Sources: ACOG dating guidelines

Ultrasound dating is more accurate

Related calculators

Age

Date difference

How to calculate due date by hand

Write down the first day of your last menstrual period (lmp).
Add 280 days (40 weeks).
Add or subtract the difference between your cycle length and 28 days — a 31-day cycle adds 3 days.
The result is the estimated due date; days elapsed since the LMP, divided by 7, give gestational age in weeks.

How does the pregnancy due date calculator work?

The calculator applies Naegele's rule: due date = LMP + 280 days, plus a correction of (cycle length - 28) days so longer or shorter cycles shift the estimate. The 280-day figure assumes ovulation around day 14 of a 28-day cycle, which implies about 266 days of gestation from conception. ACOG's dating guidance is the reference, and it is explicit that first-trimester ultrasound measurement is the most accurate way to establish gestational age — LMP arithmetic depends on an accurately remembered date and regular cycles. Weeks elapsed since the LMP give the gestational age and trimester shown alongside the date.

References: ACOG dating guidelines.

Last reviewed July 2, 2026 · Editorial policy

Frequently asked questions

How does cycle length change the due date?

Each day your cycle runs beyond 28 days pushes the due date one day later, and shorter cycles pull it earlier — a 31-day cycle adds 3 days. The correction reflects that ovulation tends to come later in longer cycles.

Why 280 days when pregnancy is about nine months?

Because the 280 days (40 weeks) are counted from the last period, not from conception — they include roughly two weeks before conception happens. Counted from conception, gestation is about 266 days.

What trimester cut-offs does this calculator use?

It marks the second trimester at 13 weeks and the third at 27 weeks, counted from the first day of the last menstrual period. Sources vary by a week in where they draw these lines, so treat the boundary weeks loosely.

What does this calculator assume?

Regular cycles See the math card above for the full list.

What doesn't this account for?

Ultrasound dating is more accurate For a more complete picture, combine with related calculators below.

How accurate is this due date calculator?

The math is deterministic — the same inputs always produce the same output, and the formula is shown above. Accuracy of the answer for your situation depends on how well your inputs match reality and how well the formula models the question.