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

This is a general estimate for planning only, not medical advice or a method of contraception. Fertility varies — talk to a healthcare professional. See our Disclaimer.

What is an ovulation calculator?

An ovulation calculator estimates the days you are most likely to be fertile, based on the first day of your last period and your cycle length. It points to your estimated ovulation day and the fertile window around it, useful whether you are trying to conceive or simply tracking your cycle.

How it's calculated

Ovulation typically happens about 14 days before your next period, because the luteal phase is fairly constant. So the tool estimates ovulation as your last-period date plus (cycle length minus 14). The fertile window is the five days before ovulation through ovulation day, since sperm can survive several days. It also estimates your next period (LMP plus cycle length) and an approximate due date if conception occurs.

Example

For a last period starting 1 June 2026 with a 28-day cycle, ovulation is estimated on 15 June 2026, with a fertile window of 10 to 15 June and the next period due 29 June. With a 35-day cycle, ovulation shifts to 22 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 do I ovulate?+

About 14 days before your next period; in a 28-day cycle that is around day 14.

What is the fertile window?+

The roughly six days ending on ovulation day, because sperm can survive up to about five days.

How accurate is this?+

It is an estimate based on average timing; real ovulation varies, so use it as a guide, not a guarantee.

Can I use this for birth control?+

No; it is not a reliable contraceptive method.

What if my cycle length varies?+

Use your average cycle length; the more irregular your cycle, the less precise the estimate.

How is ovulation related to my due date?+

If conception happens around ovulation, the due date is about 280 days from the last period; see our Due Date Calculator.

What is the luteal phase?+

The time from ovulation to your next period, usually about 14 days and fairly constant.

Should I see a professional?+

Yes, for fertility or contraception decisions; this tool is for general planning only.

References

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