Ovulation Calculator

Your fertile days and estimated ovulation date from your last cycle. The window lasts about seven days, not one.

Estimated ovulation

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Estimated ovulation—

Fertile days—

Likeliest day—

Day of the cycle you ovulate—

Next period—

How this was worked out

Indicative result. The calendar method is the least reliable way there is of predicting ovulation, and it does not work as contraception. If you are trying to conceive and it is not happening, speak to a professional.

The formula

ovulation = cycle length − 14 days

Where it comes from

What almost nobody knows: the luteal phase, between ovulation and the next period, lasts almost exactly 14 days in every woman. What varies between one and another is the first half of the cycle. That is why ovulation is worked out backwards from the end rather than forwards from the start.

How to work it out by hand

Take your real cycle length
Subtract the 14 days of the luteal phase
That is the day of the cycle you ovulate
The fertile window is the five days before plus the day after

What is worth knowing

The fertile window lasts about seven days because sperm survive up to five days in the female reproductive tract, while the egg lives only twelve to twenty-four hours. That is why the likeliest day is the one before ovulation, not the day itself. Methods more reliable than the calendar: luteinising hormone urine tests, and basal body temperature, which confirms ovulation but only after it has happened.

Frequently asked questions

When are the fertile days?

The five days before ovulation and the day after. On a 28-day cycle, days 10 to 16.

When exactly do you ovulate?

About 14 days before the next period. On a 28-day cycle, around day 14; on a 35-day one, around day 21.

How long does an egg live?

Between twelve and twenty-four hours. Sperm last up to five days, which is why the fertile window is so wide.

Does it work as contraception?

No. The calendar method has a very high failure rate because ovulation shifts easily.