Ovulation Calculator

How It Works

Ovulation — the release of an egg — usually happens about 14 days before the next period starts. This calculator works out when your next period is due (last period + cycle length), then counts back 14 days. The fertile window runs from five days before ovulation through ovulation day: sperm survive up to about five days, while the egg survives roughly a day.

The method assumes fairly regular cycles. It is a calendar estimate, not a measurement — stress, illness and travel can shift ovulation by days, and cycles that vary by more than a few days make any calendar prediction unreliable.

Worked Example

With a last period starting on 1 November 2026 and a 28-day cycle, the next period is due on 29 November 2026, ovulation falls on 15 November 2026 and the fertile window runs from 10 to 15 November. With a 32-day cycle, the next period would be due on 3 December, ovulation on 19 November and the fertile window 14–19 November.

How to Use This Calculator

Enter the first day of your last period and your average cycle length, averaged over a few months. You get your estimated ovulation day, fertile window, next period date and the following cycle’s ovulation day.

Tips and Practical Advice

Track your cycles for a few months first: the average of three or more cycles is far more useful than one. Combine the calendar estimate with body signs — cervical mucus and basal body temperature — or ovulation predictor kits. And never use a calendar estimate as contraception: ovulation can shift, and this tool cannot tell you when you are safe.

Source: standard calendar method — ovulation about 14 days before the next period; fertile window 5 days before ovulation through ovulation day. Reviewed Nov 13, 2026.

Frequently Asked Questions