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Ovulation Calculator

Ovulation typically happens about 14 days before the next period starts — not 14 days after the last one began — so the estimate is your cycle length minus 14, counted from the first day of your last period. The fertile window spans roughly the five days before ovulation through the day after, reflecting how long sperm survive and how briefly the egg is viable.

Estimated ovulation
June 29, 2026
Fertile window
Jun 24 – Jun 30
sperm survive ~5 days
Next period
Jul 13
Cycle
28 days
Estimated ovulation around June 29, 2026
Fertile window Jun 24 – Jun 30. Based on an assumed regular cycle and a ~14-day luteal phase — ovulation shifts month to month, so it isn't reliable contraception.
Inputs
First day of last period
Cycle length
days
Not a contraception method
This predicts a fertile window from an assumed regular cycle and a ~14-day luteal phase, but ovulation shifts month to month. ACOG doesn't treat calendar timing as reliable birth control.
Aim across the whole window
You're fertile the ~5 days before ovulation plus the day itself, since sperm survive up to five days. For conception, intercourse every 1–2 days across the window is the usual advice.
Ask a follow-up
Uses your inputs above
June 29, 2026 estimated ovulation. Want to try a variation?

The math

Reviewed 2026
Formula
ovulation ≈ next period − 14 days (luteal phase)
Regular cycles
~14-day luteal phase
Estimates only — ovulation tests and tracking are more reliable

Related calculators

Due date
Date difference

Example: how ovulation is calculated

Step-by-step with default inputs

Suppose you put the default values into Ovulation Calculator:

First day of last period
2026-06-15
Cycle length
28 days

Plug those into the formula ovulation ≈ next period − 14 days (luteal phase) and the result is:

Estimated ovulation
June 29, 2026

How to calculate ovulation by hand

  1. Note the first day of your last period (lmp) and your typical cycle length (len) in days.
  2. Subtract 14 from the cycle length — ovulation is estimated that many days after the LMP (day 14 of a 28-day cycle).
  3. Count back 5 days from ovulation for the start of the fertile window, and 1 day past it for the end.
  4. Add the full cycle length to the LMP to predict the next period.

How does the ovulation calculator work?

This is the calendar arithmetic behind fertility-awareness methods as described in ACOG's patient guidance: the luteal phase between ovulation and the next period is comparatively fixed at about 14 days, so ovulation is projected at cycle length minus 14 days after the first day of the last period. The fertile window is drawn from five days before ovulation, since sperm can survive about that long, through one day after. The math assumes regular cycles of the length you enter; the follicular phase before ovulation is what actually varies between people and between cycles, which is exactly what a fixed formula cannot see.

References: ACOG: fertility awareness-based methods.

Last reviewed July 2, 2026 · Editorial policy

Frequently asked questions

When do I ovulate with a 30-day cycle?

Around day 16 — cycle length minus the roughly 14-day luteal phase. Longer cycles push ovulation later because the phase before ovulation stretches while the luteal phase stays comparatively fixed.

Which days am I most fertile?

The estimated window runs from five days before ovulation through the day after — with the default June 15 start and 28-day cycle, June 24 to 30 around a June 29 ovulation. It opens well before ovulation because sperm can survive several days waiting for the egg.

Why is the luteal phase treated as fixed?

Because across regular cycles the time from ovulation to the next period varies much less than the time before ovulation — about 14 days is the standard planning assumption in calendar-based methods. Cycle-length differences are therefore assigned to the pre-ovulation phase.

What does this calculator assume?

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

What doesn't this account for?

Estimates only — ovulation tests and tracking are more reliable For a more complete picture, combine with related calculators below.

How accurate is this ovulation 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.