Ovulation calculator

How the ovulation calculator works

Ovulation is when one of your ovaries releases an egg. The NHS says it usually happens about 10 to 16 days before the start of your next period. The time between ovulation and your next period is fairly constant, while the first part of the cycle varies more, so the simplest estimate counts back 14 days from when your next period is due:

next period = first day of last period + cycle length
likely ovulation = next period − 14 days
fertile window = 5 days before ovulation to 1 day after

The fertile window is the time when sex is most likely to lead to pregnancy. Sperm can survive inside the body for up to 7 days, and the egg lives for about 12 to 24 hours after it's released. That's why your most fertile days are the five or so days before ovulation and the day of ovulation itself.

Worked example

If your last period started on Thursday 1 October 2026 and you have a 28-day cycle:

• Your next period is due on 1 October + 28 days = Thursday 29 October 2026
• Likely ovulation is 14 days before that: Thursday 15 October 2026 (day 15 of your cycle)
• Your most fertile days are Saturday 10 October to Friday 16 October
• If you conceive in this cycle, your estimated due date is Thursday 8 July 2027

With a 32-day cycle and the same start date, the next period is due on Monday 2 November, likely ovulation moves to Monday 19 October, and the fertile window runs from Wednesday 14 to Tuesday 20 October.

Ovulation day by cycle length

Day 1 is the first day of your period. In a short cycle you can ovulate very soon after your period ends, which is why it's possible to get pregnant from sex during or just after a period.

Signs you might be ovulating

• Cervical mucus becomes wetter, clearer and more slippery, a bit like raw egg white, around ovulation.
• Body temperature rises slightly after ovulation. Tracking it every morning shows the pattern after a few cycles, but illness or a bad night's sleep can throw it out.
• Ovulation predictor kits from pharmacies test your pee for the surge in luteinising hormone (LH) that happens shortly before ovulation.

Cramps, breast changes and mood changes are not reliable signs on their own.

Trying for a baby

You don't need to time sex exactly. The NHS and fertility charities suggest having sex every 2 to 3 days throughout your cycle, which means there will usually be sperm waiting when you ovulate. The NHS says around 84% of couples conceive naturally within a year if they have regular unprotected sex every 2 or 3 days. Start taking 400 micrograms of folic acid a day while you're trying, and keep taking it until you're 12 weeks pregnant.

If you've been trying for a year without success, speak to your GP. Go sooner, after 6 months, if you're 36 or over, or if you have irregular or very painful periods or a known fertility problem.

Once you've had a positive test, the pregnancy due date calculator shows how many weeks pregnant you are, and the maternity pay calculator helps you plan your leave. To count the days to a date, use the date calculator.

This is not contraception

Calendar calculators like this one can't be used to avoid pregnancy. Cycles change from month to month, and ovulation can come earlier or later than expected because of stress, illness, travel or recently stopping hormonal contraception. Natural family planning, done properly, means tracking several signs every day, ideally after learning the method from a trained teacher. Speak to your GP or a sexual health clinic about contraception that suits you.

Last reviewed October 2026 using 2026/27 figures. This calculator gives an estimate for guidance only.