Ovulation Calculator
Enter the first day of your last period and your usual cycle length to estimate when you ovulate and which days you are most likely to get pregnant. It is an estimate for people trying to conceive, not a method of contraception.
Free, no sign-up. What you enter stays in your browser: it is not saved or sent to HerFound. Reviewed .
From the first day of one period to the first day of the next. Leave it at 28 if you are not sure.
Enter the first day of your last period to see your estimated ovulation day and fertile window.
This is general information, not medical advice. It cannot take your own health history into account. For anything that worries you, speak to a doctor, midwife or pharmacist.
How it works
The calculator uses the calendar method: estimated ovulation day = first day of your next expected period minus 14 days. Your next expected period is the first day of your last period plus your cycle length. ACOG notes that ovulation happens about 14 days before the next period starts.
The fertile window is the 5 days before ovulation plus the day of ovulation, 6 days in total. Sperm can live for up to about 5 days inside the body and an egg lives for about a day, which is why the days before ovulation count. A 1995 study in the New England Journal of Medicine found that almost all pregnancies came from sex in this 6-day window.
With a 28-day cycle this puts ovulation 14 days after the first day of your period. With a 32-day cycle it is 18 days after, and with a 24-day cycle it is 10 days after.
What it cannot tell you
- Do not use this calculator to avoid pregnancy. The real day of ovulation can be several days earlier or later than the estimate, even in regular cycles. If you do not want to get pregnant, speak to a doctor, nurse or sexual health clinic about contraception.
- It is an estimate from dates only. It does not measure hormones, temperature or cervical mucus, so it cannot confirm that you ovulated or when.
- It is much less reliable if your cycles are irregular, if you recently stopped hormonal contraception, or if you are breastfeeding or close to menopause.
- It cannot tell you anything about your fertility. If you are under 35 and have been trying for a year without success, or 35 or over and have been trying for 6 months, speak to a doctor.
Apps that do this for you
A calculator answers once. These apps keep track over time. Compare all period tracking apps.
Clue
A period tracker: cycle tracking with predictions for your period, PMS and ovulation.
Cycles
A period tracker for iPhone: predict your periods and fertile days and share your cycle phases with a partner.
Euki
A period tracker: log your cycle and sexual health, with your entries kept on your phone.
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Common questions
You ovulate about 14 days before your next period starts, not a fixed number of days after your last one. With a 28-day cycle that is about 14 days after the first day of your period. With a longer or shorter cycle the day moves later or earlier.
The fertile window is about 6 days long: the 5 days before ovulation and the day of ovulation itself. Sperm can survive for several days, so sex before ovulation can lead to pregnancy. The chance is highest in the 2 or 3 days just before ovulation.
An ovulation calculator gives a rough estimate, not an exact day. It assumes ovulation comes 14 days before your next period, but the NHS says the real gap is anywhere from about 10 to 16 days, and cycle length changes from month to month. Ovulation test strips or tracking body signs are more precise.
No, you should not use an ovulation calculator to avoid pregnancy. The estimate can be wrong by several days, and sperm can live for days inside the body. If you do not want to get pregnant, ask a doctor, nurse or sexual health clinic about reliable contraception.
Yes, the day you ovulate can change from one cycle to the next. Stress, illness, travel and normal variation can all move ovulation earlier or later. This is why a date-based estimate is only a guide.