Ovulation calculator
Ovulation falls roughly 14 days before your next period, not 14 days after your last one. That is why this calculates a fertile window rather than a single date.
Calculate your fertile window
Your fertile window
No calculation possible
Without a regular period there is nothing to calculate: ovulation cannot be derived from a calendar. The question is not when you ovulate, but whether you ovulate.
A calculation says when ovulation is expected, not whether it happened. That can be measured: progesterone, taken about seven days after the expected ovulation, shows whether you actually ovulated that cycle.
See the fertility testThis calculation is an estimate based on your cycle length, not a measurement of your ovulation. Do not use it as contraception. If you are unsure about your cycle, or conceiving is not happening, discuss it with your doctor.
Why a window and not a date
The first half of your cycle is the part that moves. Across more than 600,000 cycles it ranged from roughly 10 to 30 days, while the second half is far more stable. That is why we count back from your next period and give you a window.
Knowing whether you actually ovulate
A calculation says when ovulation is expected, not whether it happened. That can be measured: progesterone, taken about seven days after the expected ovulation, shows whether you actually ovulated that cycle.
Frequently asked questions
How do you calculate ovulation?
By counting back from your next period, not forward from your last one. For most women the second half of the cycle lasts about 14 days and varies little, while the first half varies a great deal. So ovulation falls roughly 14 days before your next period.
Why do I get a window instead of one date?
Because a single date implies more precision than the calculation can deliver. We know your cycle length, but not the length of your own second half, and that differs between women. The window shows that uncertainty instead of hiding it.
How many days am I fertile?
Research into the timing of intercourse and pregnancy found conception only within the six days ending on the day of ovulation. The chance is highest on the day of ovulation itself and the day before.
Can I use this as contraception?
No. The spread is too wide and the calculation measures nothing. Use a method designed and tested for that purpose, and discuss your options with your doctor.
Is my data stored?
No. The calculation runs entirely in your browser. Your period date and cycle length are never sent to our servers and are not stored anywhere.
Further reading
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This calculation is an estimate based on your cycle length, not a measurement of your ovulation. Do not use it as contraception. If you are unsure about your cycle, or conceiving is not happening, discuss it with your doctor.