Ovulation calculator: find your fertile window

Work out when you are likely to ovulate

This ovulation calculator uses the first day of your last period and your usual cycle length to estimate your ovulation date, your fertile window, your most fertile days, the date your next period is due and the earliest sensible day for a pregnancy test. It then projects the same estimates over your next 6 cycles in a small colour-coded calendar.

Cycle dates are health data, so everything is calculated in your browser and nothing is sent anywhere. Your entries are only kept on this device if you tick the box at the bottom, unticked by default.

How to use it

1. Pick the "First day of your last period": the first day of full bleeding, not spotting.
2. Enter your "Average cycle length (days)", from 21 to 45. Leave 28 if you are not sure.
3. Keep 14 in "Luteal phase (days)" unless you know your own value (10 to 16).
4. Adjust "Period length (days)" if you like: it only affects the calendar colours.
5. Use "Previous month" and "Next month" to move through the calendar.

How the dates are calculated

The luteal phase is the part of the cycle between ovulation and your next period. The calculator subtracts it from your cycle length: estimated ovulation = first day of last period + cycle length − luteal phase.

The fertile window lasts 6 days: the 5 days before ovulation plus ovulation day. Sperm can survive for up to 5 days inside the body, while the egg only lives for 12 to 24 hours after it is released. The "Most fertile days" are the 2 days before ovulation and ovulation day itself.

Worked example: a 30-day cycle

Say your last period started on 5 January 2026 and your cycles last 30 days, with a 14-day luteal phase. Estimated ovulation falls on 21 January. The fertile window runs from 16 to 21 January, the most fertile days are 19 to 21 January, and your next period is due on 4 February 2026. That is also the first day suggested for a pregnancy test: most home tests advise waiting until the day your period is due.

A longer cycle moves ovulation later

With a first day of 1 November 2026 and a 35-day cycle, ovulation is estimated on 22 November, the fertile window runs from 17 to 22 November and the next period is due on 6 December. With a 28-day cycle from the same date, you would get ovulation on 15 November and a window from 10 to 15 November.

Reading the 6-cycle calendar

The "Your next 6 cycles" table repeats the calculation on the assumption that every cycle keeps the same length. For the 30-day example, cycle #1 starts on 4 February with ovulation estimated on 20 February. In the calendar, each day carries a symbol as well as a colour, so it can be read without colour: ● period, △ fertile day, ▲ one of the most fertile days, ★ estimated ovulation. Today is outlined.

Limits you should know about

The result is a statistical estimate. Ovulation does not always happen on the same day, even with regular cycles, and the dates are less reliable if your cycles are irregular, after stopping hormonal contraception, while breastfeeding or around the menopause. This calculator is not a substitute for advice from your GP, midwife or gynaecologist, and it must never be used as a method of contraception.

Frequently asked questions

Can I use it if my cycles are irregular?

Yes, by entering the average length of your recent cycles, but the dates will be less accurate. If your cycle length varies a lot, it is worth talking to your doctor.

Why does the fertile window start 5 days before ovulation?

Because sperm can survive for up to 5 days, sex a few days before ovulation can still lead to a pregnancy.

Can I use these dates to avoid getting pregnant?

No. A calendar estimate does not protect against pregnancy. Ask a healthcare professional about contraception that suits you.

Is my data stored?

Nothing is sent. If you tick "Remember my entries on this device", they stay on this device only, and "Clear all" resets the form.