Ovulation Calculator
Estimate ovulation day, fertile window and next periods from your cycle.
Runs in your browser — your data never leaves your device.
About the Ovulation Calculator
If you are trying to conceive, knowing your most fertile days can help. Enter the first day of your last period and your usual cycle length to estimate when you will ovulate, your fertile window and when your next period is due — for the next three cycles.
This method assumes regular cycles. Ovulation can vary from month to month, so ovulation test kits or a doctor's advice give more certainty. This calculator must not be used as contraception.
How to use the Ovulation Calculator
- Enter the first day of your last period.
- Enter your usual cycle length (28 days if you are not sure).
- Click Calculate.
Formula
Ovulation usually happens about 14 days before the next period, so ovulation day ≈ period start + (cycle length − 14). The fertile window is the 5 days before ovulation plus the day after, because sperm can survive up to about five days.
Examples
- Last period started 1 October, 28-day cycle → ovulation about 15 October, fertile days 10–16 October, next period about 29 October.
- With a 32-day cycle, ovulation moves to about day 18.
Frequently asked questions
Is this medical advice?
No. It gives a general estimate for information only. Please speak to a doctor or qualified health professional about your own situation.
Can I use this to avoid pregnancy?
No. Cycle-based estimates are not reliable enough for contraception. Speak to a doctor about suitable methods.