Ovulation calculator

Estimate your fertile window, your most fertile days and when you'll ovulate, for the next three cycles. Free, private and no sign-up.

Your fertile window, worked out

Add the first day of your last period and you'll see your next three cycles: period dates, fertile window and estimated ovulation.

Estimates only, based on the calendar method. Not a method of contraception and not medical advice. Also try the period calculator.

How this ovulation calculator works

Ovulation usually happens about 12 to 14 days before your next period, so the calculator places it 14 days before each predicted period (Cleveland Clinic). In a 28-day cycle that's around day 14; in a 32-day cycle, around day 18.

An egg lives for about 12 to 24 hours after it's released, but sperm can survive for up to five days (Cleveland Clinic). That's why the fertile window starts five days before ovulation and runs to one day after.

The most fertile days are the two days before ovulation and ovulation day itself. In a landmark study, the chance of conceiving from sex on a single day rose from about 10% five days before ovulation to about 33% on ovulation day (Wilcox et al., 1995).

Signs you're ovulating

A calendar estimate is a starting point. Your body gives clearer clues:

Cervical mucus turns clear, slippery and stretchy, like raw egg white, in the days before ovulation.
Basal body temperature (your temperature first thing in the morning) rises slightly, about 0.5 to 1 °F (0.3 to 0.6 °C), after ovulation.
Ovulation tests detect the hormone surge (LH) that triggers ovulation. A positive test usually means ovulation within about 36 hours.
Ovulation pain, a one-sided twinge or cramp low in the tummy, is common for some people.

Source: Cleveland Clinic. More in our guide to the ovulation phase.

How accurate is it?

Less exact than a period prediction. In more than 600,000 cycles tracked in one study, the time from period to ovulation ranged from about 10 to 30 days, even though the time from ovulation to the next period was steadier (Bull et al., 2019). The researchers note that tracking body signs, not just cycle length, is needed to pinpoint fertile days.

If you're trying to conceive, use the window as a guide and combine it with the signs above.

Not a method of contraception

Because ovulation can move from cycle to cycle, calendar estimates alone can't reliably prevent pregnancy. If you want to avoid pregnancy, talk to a doctor or nurse about contraception that suits you.

Questions people ask

How do I work out when I ovulate?

Subtract 14 from your average cycle length to get the likely cycle day. For a 30-day cycle that's around day 16, counting the first day of your period as day 1. Your body's signs, such as egg-white cervical mucus or a positive ovulation test, are more precise.

How many days after my period do I ovulate?

It depends on your cycle length. In a 28-day cycle with a 5-day period, ovulation is around 9 days after your period ends. In longer cycles it comes later.

Can I get pregnant outside my fertile window?

It's less likely, but ovulation doesn't always happen on schedule, so the real fertile window can shift. That's why calendar estimates aren't birth control.

Is ovulation pain normal?

A mild twinge or cramp on one side of the lower tummy around ovulation is common. If the pain is severe, lasts a long time or comes with fever or heavy bleeding, see a doctor.

Do you save the dates I enter?

No. The calculator runs entirely in your browser. Nothing you enter is stored, added to the page address or sent to us, and it's gone when you close the page.

Sources

Ovulation: Calculating, Process, Pain & Other Symptoms, Cleveland Clinic
Luteal Phase of the Menstrual Cycle: Symptoms & Length, Cleveland Clinic
Follicular Phase, Cleveland Clinic
Timing of sexual intercourse in relation to ovulation (Wilcox, Weinberg and Baird, 1995), New England Journal of Medicine
Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (Bull et al., 2019), npj Digital Medicine

Written by the Periodiva team for general information, not medical advice. If something about your cycle worries you, such as very heavy bleeding, severe pain or a missed period, talk to a doctor.