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Ovulation calculator
Ovulation usually happens 12 to 16 days before your next period, so in a 28-day cycle it falls around day 14. Enter the first day of your last period to see your likely ovulation days and fertile window for the next three cycles.
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- Nothing you enter is stored
- Ranges that fit real, changing cycles
Your likely ovulation days
Enter your dates above and your estimates will show here. Nothing you enter leaves this page.
An estimate, not medical advice or birth control. Don’t use these dates to avoid pregnancy.
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How we estimate ovulation
Counted back from your next period.
Ovulation usually happens 12 to 16 days before your next period, whatever your cycle length (NHS). So we first estimate when your next period will start, then count back 14 days. Because your period start is a range, ovulation is a range too.
Your fertile window
Sperm can live for up to 5 days and an egg for about a day after ovulation (ACOG). That makes the fertile window the five days before ovulation plus the day itself. The two or three days just before ovulation are the most likely to lead to pregnancy.
Narrowing it down
A calendar can’t see what your body is doing this month. Ovulation tests (LH strips), changes in cervical mucus and your basal body temperature can confirm when it’s happening. Having sex every 2 to 3 days through the cycle covers the window without needing to time it exactly.
Ovulation day by cycle length
Ovulation is about 14 days before the next period, so it moves with your cycle length.
| Cycle length | Likely ovulation | Fertile window | Next period starts |
|---|---|---|---|
| 21 days | day 7 | days 2–7 | day 22 |
| 24 days | day 10 | days 5–10 | day 25 |
| 26 days | day 12 | days 7–12 | day 27 |
| 28 days | day 14 | days 9–14 | day 29 |
| 30 days | day 16 | days 11–16 | day 31 |
| 32 days | day 18 | days 13–18 | day 33 |
| 35 days | day 21 | days 16–21 | day 36 |
Days are counted from the first day of your last period (day 1). Real cycles move, so treat each as the middle of a range.
Sources: NHS: Trying to get pregnant · ACOG: Fertility awareness-based methods
How do I calculate my ovulation day?
Estimate the first day of your next period, then subtract 14 days. With a 28-day cycle that’s around day 14; with a 32-day cycle it’s around day 18. Because the timing varies, treat it as a range of a few days.
How many days after my period do I ovulate?
It depends on your cycle length, not your period. Counting from the first day of your period, ovulation is roughly your cycle length minus 14 days, so day 14 of a 28-day cycle or day 21 of a 35-day cycle.
What are the most fertile days?
The two or three days before ovulation and the day of ovulation. The fertile window opens about five days before ovulation because sperm can survive that long.
How accurate is an ovulation calculator?
It’s a good starting point if your cycle is fairly regular, but it’s an estimate. Ovulation can shift from month to month, especially with irregular cycles. LH tests or temperature tracking can confirm it.
Can I ovulate twice in one cycle?
You can release more than one egg, but it happens within about 24 hours. You don’t ovulate at two separate times in the same cycle.
Is my data saved anywhere?
No. The calculator runs in your browser. The dates you enter are never sent to us, never put in the page address and never stored. Close the tab and they’re gone.
Can I use this as birth control?
No. These are estimates from a calendar, and ovulation can move from cycle to cycle. Don’t use them to avoid pregnancy. If you need contraception, talk to a doctor, nurse or pharmacist about methods that are tested for it.