Ovulation Calculator

Estimate your ovulation day, fertile window and next period dates for the next 6 cycles from the first day of your last period and your usual cycle length.

Cycle length is from the first day of one period to the first day of the next (21–45 days). Leave the luteal phase at 14 unless you have tracked it (9–17 days).

Estimated ovulation (this cycle)

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Enter your last period to see your dates.

Fertile window—
Most fertile days—
Next period—
Pregnancy test from—
Cycle day of ovulation—
Due date if you conceive this cycleOvulation + 266 days—

Predictions assume a regular cycle. If your cycles vary, use your shortest and longest lengths to see the full range.

Not a method of contraception. These dates are statistical estimates. Ovulation can shift by several days from cycle to cycle, so this calculator must not be used to avoid pregnancy. It is not medical advice — talk to a doctor if your cycles are irregular, shorter than 21 or longer than 35–45 days, or if you have been trying to conceive for 12 months (6 months if you are 35 or older).

How the ovulation calculator works

The luteal phase — the time from ovulation to your next period — is fairly constant — usually 10 to 16 days, about 14 on average, while the first half of the cycle varies. So the calculator works backwards: next period = last period + cycle length, then ovulation = next period − luteal phase. With a 28-day cycle and a 14-day luteal phase, ovulation falls on cycle day 15 (day 14 after your period started).

The fertile window is the 5 days before ovulation, ovulation day itself and the day after. Sperm can live inside the body for about 3–5 days (UK guidance says up to 7), but the egg can be fertilized for only about 24 hours, so the two days before ovulation and ovulation day give the highest chance of pregnancy. The due date if you conceive is ovulation + 266 days, the same as 280 days from the last period in a standard cycle.

Next periodPeriod start + cycle length
OvulationNext period − luteal phase
Fertile windowOvulation − 5 days to ovulation + 1
Pregnancy testFrom the day your period is due
Due date if conceivedOvulation + 266 days

Ovulation predictor kits (LH tests), cervical mucus changes and basal body temperature can confirm what the calendar estimates. Everything is calculated in your browser; your dates are not sent or stored.

Sources

Already pregnant? Use the pregnancy due date calculator for weeks pregnant and a prenatal timeline.

Frequently asked questions

How is ovulation day calculated?
After ovulation, the luteal phase lasts a fairly steady 10–16 days (about 14 on average) before the next period starts. So the calculator predicts your next period from your cycle length and counts back the luteal phase: with a 28-day cycle and a 14-day luteal phase, ovulation is day 15, counting the first day of your period as day 1.
What is the fertile window?
It is the six days ending on ovulation day plus the following day. Sperm can survive in the reproductive tract for about 3–5 days (sometimes longer), while the egg can be fertilized for only about 24 hours after ovulation, so sex in the days before ovulation gives the best chance of pregnancy.
Can I use this calculator as birth control?
No. Calendar predictions can be off by several days because ovulation varies from cycle to cycle, especially with irregular cycles, after childbirth or while breastfeeding. Use a reliable contraceptive method if you do not want to get pregnant.