Ovulation Calculator

Enter the first day of your last period and your average cycle length to estimate your most fertile days, ovulation date and next period.

Reviewed by the WorldCalcs team · Methodology · Last reviewed: June 2026

First day of last periodAverage cycle length (days)

Estimated ovulation day

15 June 2026

Fertile window

10 June 2026 – 15 June 2026

Next period

29 June 2026

Estimated due date if conceived

8 March 2027

What is ovulation?

Ovulation is the moment an ovary releases an egg, which can then be fertilised for about 24 hours. In a typical cycle it happens about 14 days before the next period starts. The days leading up to and including ovulation are the “fertile window,” when conception is most likely because sperm can survive in the body for up to about five days.

How the dates are calculated

This calculator uses the standard calendar method, which assumes the time from ovulation to your next period (the luteal phase) is about 14 days. So ovulation is estimated as the first day of your last period plus your cycle length minus 14. The fertile window is the five days before ovulation through ovulation day, and your next period is the last period date plus your cycle length.

Example

With a last period starting 1 June and a 28-day cycle, ovulation is estimated at 1 June + (28 − 14) = 15 June. The fertile window runs from 10 June to 15 June, and the next period is expected around 29 June.

All calculations happen in your browser. Nothing is sent, stored, or tracked.

Results are estimates and may contain errors — for general information only, not professional advice. Always verify before relying on them. Disclaimer

How to use

Pick the first day of your last period and your average cycle length (default 28 days). The calculator estimates your ovulation day, fertile window, next period and an estimated due date if you conceive this cycle.

This is an estimate only and not a form of birth control or medical advice.

Frequently asked questions

When am I most fertile?+

The most fertile days are the two or three days leading up to and including ovulation. Across a 28-day cycle, ovulation is usually around day 14.

How long is the fertile window?+

About six days: the five days before ovulation plus ovulation day, because sperm can live up to five days and the egg about a day.

What if my cycle isn't 28 days?+

The calculator adjusts automatically. Ovulation is always estimated about 14 days before your next period, so a longer cycle moves ovulation later.

Is this a reliable form of birth control?+

No. The calendar method is only an estimate and cycles vary, so it should not be relied on to prevent pregnancy.

Why does ovulation timing vary?+

Stress, illness, travel and hormonal changes can shift ovulation. The calculator gives a typical estimate, not a guarantee.

References

American College of Obstetricians and Gynecologists (ACOG) — fertility awareness and the menstrual cycle (14-day luteal phase).