Ovulation Calculator
Find your fertile window, ovulation day, next periods and estimated due date from your last period and average cycle length.
Pick the first day of your last period — the fertile window is worked out as you type.
- Estimated ovulation day
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- Fertile window (6 days)
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- Most fertile days
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- Next period expected
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- Due date if you conceive
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- Cycle day today
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| Period starts | Fertile window | Ovulation |
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These dates are estimates based on a regular cycle, not medical advice. Ovulation shifts with stress, illness, medication and irregular cycles, so this tool cannot confirm that you are fertile or prevent pregnancy. Talk to a doctor or midwife about contraception, fertility or any cycle that worries you.
How it works
Ovulation is timed backwards from your next period, not forwards from your last one.
The luteal phase — the stretch between ovulation and the first day of bleeding — is the stable
part of the cycle and lasts about 14 days for most people. So the calculator adds your average
cycle length to the first day of your last period to predict the next period, then subtracts
the luteal phase to land on the estimated ovulation day: ovulation = last period +
cycle length − luteal phase. On a 28-day cycle that is day 14; on a 32-day cycle it is
day 18.
Sperm survive up to five days, while the egg lives roughly 24 hours, so the fertile window runs from five days before ovulation to the day after it — six days in total, with the two days before ovulation and ovulation day itself being the most fertile. The tool also projects the next three cycles, shows which cycle day you are on today, and adds 266 days to ovulation for the due date if you conceive this cycle. Everything is calculated in your browser with plain JavaScript: your dates never leave your device, are never stored, and there are no ads, accounts or trackers.
Frequently asked questions
How do I calculate my ovulation day?
Ovulation is timed backwards from your next period, not forwards from your last one. Add your average cycle length to the first day of your last period to get the next period date, then subtract the luteal phase — the stretch between ovulation and bleeding, which is about 14 days for most people. So ovulation = last period + cycle length − luteal phase. On a 28-day cycle that lands on day 14; on a 32-day cycle it is day 18, not day 14. The calculator does this for you and also projects the next three cycles.
When is the fertile window and which days are most fertile?
Sperm can survive up to five days in the reproductive tract while the egg only lives around 24 hours, so the fertile window runs from five days before ovulation to the day after it — six days in total. The two days immediately before ovulation and ovulation day itself carry the highest chance of conception, which is why the tool highlights them separately. Having sex every day or every other day across that window is the usual advice for people trying to conceive.
Is an ovulation calculator accurate enough to avoid pregnancy?
No. These dates are estimates that assume a regular cycle of the length you entered, and real ovulation shifts with stress, illness, travel, medication, breastfeeding and conditions such as PCOS. Because sperm survive for days, an ovulation that arrives a few days early or late can make a supposedly safe day fertile. Use the results as a planning aid only, confirm timing with ovulation tests or basal body temperature if you are trying to conceive, and talk to a doctor or midwife about contraception — this tool is not medical advice.