Ovulation calculator
An estimate of when you ovulate and the days around it, from your cycle length.
- Runs in your browser — nothing you enter is uploaded
- No sign-up, no quotas, free to use as often as you like
How it works
Ovulation is timed from the end of the cycle, not the beginning. The luteal phase — between ovulation and the next period — is the stable part, usually around fourteen days, while the first half is what varies. Counting forwards from your period without accounting for that is why the common "day 14" answer is wrong for most people.
How it is worked out
Ovulation is estimated by counting back the luteal phase length from the expected next period, rather than counting forward from the last one. For a 32-day cycle with a 14-day luteal phase, that puts ovulation around day 18, not day 14.
The fertile window
The window shown runs from five days before ovulation to one day after. Sperm can survive up to about five days in the right conditions, while an egg is viable for roughly 24 hours — so the fertile days are mostly before ovulation, not after it. That surprises people, and it is the single most useful thing on this page.
How much to trust it
Calendar methods are estimates. Cycle length varies month to month, illness, stress and travel all shift ovulation, and a cycle can be anovulatory without any outward sign. If timing matters to you, ovulation predictor kits and basal body temperature tracking measure what is actually happening rather than predicting from an average.
Not contraception
This must not be used to avoid pregnancy. Calendar methods have a high failure rate in ordinary use precisely because the estimate moves. It is not contraception and it is not medical advice.
Ovulation calculator — questions
Is day 14 always ovulation?
No. That is only true for a 28-day cycle with a 14-day luteal phase. Ovulation is better estimated by counting back from the next period.
Why does the fertile window start before ovulation?
Because sperm survive up to about five days while the egg lasts around a day, so the days before ovulation matter most.
Can I use this as contraception?
No. Calendar methods fail often enough in ordinary use that they are not a reliable way to avoid pregnancy.
What is a luteal phase?
The time between ovulation and your next period. It is usually 12 to 16 days and is the more consistent half of the cycle.
Is my data stored?
No. Nothing you enter is transmitted or saved — it is calculated in your browser.