Free tool
Ovulation calculator
Enter the first day of your last period, how long your cycle usually is and how many days your period lasts. You'll see your fertile window, your likely ovulation day and when your next period is due.
Free, no sign-up. Your dates stay on your device.
How this works
- Ovulation usually happens about 14 days before your next period, so we count back from the date your next period is due.
- Sperm can live up to five days, so your fertile window is the five days before ovulation, ovulation day and the day after. The two days before ovulation are the most fertile.
- This is an estimate from an average. Stress, illness, travel and PCOS can all move ovulation, which is why tracking your real cycle gives truer dates.
Questions women ask
How accurate is an ovulation calculator?
It's a good starting point if your cycles are regular. It assumes ovulation 14 days before your period, but in real cycles this varies by a few days. Logging your periods and symptoms over a few months, or using ovulation test strips, gives a truer picture.
What are the signs of ovulation?
Clear, stretchy discharge like raw egg white, a slight rise in body temperature after ovulation, mild one-sided pain in the lower belly, tender breasts and a higher sex drive. Noting these each month shows you your own pattern.
Can I get pregnant outside my fertile window?
It's unlikely but possible, because ovulation can come earlier or later than predicted. If you're trying, have sex every two to three days through the month. If you're avoiding pregnancy, don't rely on a calculator. Use contraception.
My cycles are irregular. Will this work for me?
If your cycle length changes by more than a week from month to month, a calculator can be off by several days. Irregular cycles can be a sign of PCOS or a thyroid problem, so it's worth tracking them and talking to a gynaecologist.
This calculator gives an estimate, not a diagnosis, and isn't a form of contraception. If your periods are very irregular, very painful or very heavy, see a gynaecologist.
Your cycle, worked out every month
Grace's My Cycle keeps your fertile days and next period up to date, and learns from what you log.
Open My Cycle in Grace