Ovulation calculator: fertile days
Estimated ovulation and fertile window
This is not a method of contraception. Ovulation can shift even in a regular cycle, and with irregular cycles a calendar estimate isn't reliable. Your dates stay in this browser.
- Estimated fertile window
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- Next period expected
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Worked out in your browser. No sign-up.
The ovulation calculator estimates when you ovulate from the first day of your last period and your usual cycle length. It counts back 14 days from your next expected period and shows a six-day fertile window: the five days before ovulation plus the day itself. It helps when trying for a baby; it cannot be used as contraception.
How it works
Adding your cycle length to the first day of your last period gives the date your next period should start. Ovulation is placed 14 days before that. Sperm can survive for several days, so the fertile window here covers the five days before ovulation and the day itself, six days in total.
Both the cycle and the phase after ovulation vary from person to person and month to month. An ovulation (LH) test or a scan can easily land a day or two away from this estimate. The calendar can't confirm that an egg was released.
Worked example
Your last period started on 1 January 2026 and your cycle is usually 28 days. The next period is due on 29 January, estimated ovulation is 15 January, and the fertile window runs from 10 to 15 January.
With a 30-day cycle, all of those dates move two days later.
Questions people ask
Do I always ovulate on day 14?
No. Day 14 only fits a 28-day cycle. Ovulation tends to come about two weeks before the next period, so with a 32-day cycle this calculator puts it four days later, on day 18.
Which date do I enter if my period lasts five days?
The first day of proper bleeding, not the last. How long the bleeding lasts doesn't change the calculation.
Can I use this to find safe days?
No. A calendar estimate can't rule out pregnancy on any day, and it is even less dependable when cycles are irregular. Talk to a GP or a family planning clinic about contraception.
Why does my ovulation test show a different day?
An LH test picks up the hormone surge in your actual cycle, while the calculator assumes ovulation 14 days before your next period. When they disagree, go with the test.
Sources
- NHS: Periods (ovulation about 12 to 16 days before the next period) — checked 13.11.2026
- ASRM: Optimizing natural fertility (six-day fertile window) — checked 13.11.2026
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Updated: 13 November 2026