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Ovulation Calculator

Enter the first day of your last period and your usual cycle length to estimate when you're likely to ovulate, your fertile window, and when your next period is due. This is a calendar estimate — it is not a method of contraception.

Count from the first day of one period to the day before the next. The NHS says around 28 days is typical and 21 to 35 days is a normal range.
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Fertile window (estimated)—
Next period expected—
Following cycle's ovulation (estimated)—

Estimate only. It assumes ovulation falls 14 days before your next period, but the NHS says the real timing is anywhere from about 10 to 16 days before, and it can shift from cycle to cycle — so your actual dates may differ. This calculator is not a contraceptive method and is not medical advice; if you're trying to conceive or avoid pregnancy, speak to a GP, nurse or sexual health clinic.

How the estimate works

Day 1 of your cycle is the first day of your period. The calculator places ovulation on cycle day (cycle length − 14) — day 14 of a 28-day cycle, day 21 of a 35-day cycle — which matches NHS guidance that ovulation is usually about 14 days before your next period starts. The fertile window runs from 5 days before that day to 1 day after it, because the NHS notes sperm can survive inside the body for up to 7 days, so sex in the days before ovulation can still lead to pregnancy. Your next period is expected one full cycle after the last one began.

FAQ

How accurate is an ovulation calculator?
It's a rough guide, not a measurement. The dates come purely from your period date and cycle length, and the NHS says ovulation happens around 10 to 16 days before your next period rather than on a fixed day. If your cycles are irregular, or you're unsure of your cycle length, the estimate is less reliable. Ovulation tests, temperature charting and cervical fluid changes give better real-time signals than the calendar alone.
Can I use this to avoid getting pregnant?
No. This tool is not a contraceptive. The NHS says fertility awareness methods are between 91% and 99% effective at preventing pregnancy only when used correctly all the time, and just 76% effective if the instructions aren't followed exactly — and those methods involve daily temperature and cervical fluid records, not just period dates. Natural family planning also doesn't protect against sexually transmitted infections. If you don't want to get pregnant, use a recognised method of contraception.
Can I get pregnant just after my period?
Possibly. The NHS notes it's possible (though not very likely) to get pregnant soon after your period finishes if you ovulate early or have a short cycle, because sperm can survive for up to 7 days inside the body. With a 21-day cycle, for example, this calculator puts the fertile window at cycle days 2 to 8.

Sources

Method: Day 1 = first day of your period. Estimated ovulation = cycle day (cycle length − 14). Fertile window = 5 days before to 1 day after that day. Next period = last period start + cycle length. The 14-day figure, the 10–16 day range, the 21–35 day normal cycle range and the 7-day sperm survival figure are taken from the NHS pages below, checked on the date this page was built. The 5-day lead and 1-day trailing margin are this calculator's conventions, not NHS figures.