Pregnancy tools
Ovulation calculator
Find your most fertile days and when you are likely to ovulate. See your cycle on a calendar, your next three cycles, and the first day a pregnancy test is likely to work.
Your fertile days appear hereEnter the first day of your last period.
How the ovulation calculator works
Ovulation usually happens 12 to 14 days before your next period starts, not 14 days after your last one. So the calculator finds your next period from your cycle length, then counts back. On a 28-day cycle that is around days 15 to 17, counting the first day of your period as day 1. On a 32-day cycle it is around days 19 to 21.
Sperm can live for up to 7 days inside the body, so the days before ovulation count too. The calculator marks your most fertile days as the 5 days before the likely ovulation days, plus the ovulation days themselves.
Getting the timing right
Couples who have regular unprotected sex every 2 or 3 days throughout the cycle have the best chance of conceiving, rather than aiming only for the day of ovulation. Stress, illness and changes to your routine can move ovulation, so treat these dates as a guide. This calculator is not a method of contraception.
When to take a pregnancy test
Most pregnancy tests work from the first day of a missed period. If you do not know when your period is due, test at least 21 days after you last had unprotected sex. Some very sensitive tests work a little earlier. If your test is positive, the due date calculator shows your estimated due date and the UK appointments ahead.
Questions
When do I ovulate on a 28-day cycle?
Usually around days 15 to 17, counting the first day of your period as day 1. That is 12 to 14 days before your next period.
Can I use this if my periods are irregular?
It is less reliable, because the dates move whenever your cycle length changes. Ovulation test kits can help you pinpoint it. If your periods are very irregular, speak to a GP.
Is this a form of contraception?
No. Calendar estimates are not reliable enough to use as contraception.
How long should we try before seeing a GP?
The NHS suggests seeing a GP if you have not conceived after a year of trying. If you are 36 or over, or already know of a possible fertility problem, see a GP sooner.
When can I take a pregnancy test?
Most tests work from the first day of a missed period. If you do not know when your period is due, test at least 21 days after unprotected sex.
Last reviewed 13 October 2026. These tools give estimates for information only and are not medical or legal advice. Your midwife, GP or fertility clinic confirms your dates; your employer and gov.uk confirm your leave and pay.