Ovulation and fertile days calculator
Work out when you ovulate and which are your most fertile days over the next cycles.
Ovulation calculator: what it is and how it works
Ovulation is when an ovary releases an egg. In a regular cycle it happens about 14 days before the next period: on day 14 of a 28-day cycle or day 18 of a 32-day cycle. The egg lives for about 24 hours, but sperm can survive up to five days, so the fertile window covers the five days before ovulation, the day itself and the day after.
This calculator estimates your ovulation day, fertile window and next period for the next four cycles. If your cycles are irregular the estimate is less accurate; ovulation tests or tracking your basal body temperature can help.
How to use it
- Enter the first day of your last period.
- Enter your usual cycle length.
- See your ovulation day, fertile window and upcoming cycles.
Ovulation = period start + cycle length − 14 · Fertile window = ovulation − 5 to ovulation + 1Worked example
If your last period began on 1 November 2026 and your cycle is 28 days, you will ovulate around 15 November; your fertile days run from 10 to 16 November and your next period is due on the 29th.
Frequently asked questions
Which days are most fertile?
The two or three days before ovulation and ovulation day itself.
Does it work with irregular cycles?
The estimate is less reliable. Ovulation tests or basal temperature can help detect it.
Can I use it as contraception?
No. Calendar-based methods have a high failure rate.
How can I tell I am ovulating?
Signs include clear, stretchy cervical mucus, mild abdominal pain and a rise in basal temperature after ovulation.