When am I fertile?
Ovulation usually happens about 14 days before your next period. You are most fertile in the 5 days before it and on the day itself. With a 28 day cycle that is around day 9 to 14. Enter your details to see your fertile days for the next 3 cycles.
Enter a date to calculate.
Everything is calculated on your own device. We store nothing and send nothing anywhere.
This is an estimate, not medical advice and not contraception. If in doubt, ask your GP or midwife.
How we calculate
The second half of your cycle, after ovulation, is fairly constant for most women: about 14 days. So we count back from your next expected period. Sperm can live up to about 5 days and an egg about a day, so your fertile window is the 5 days before ovulation plus the day itself.
Want to know for sure?
A calculator estimates. Ovulation tests, your basal body temperature and your cervical mucus show when you really ovulate.
Not contraception
Never use this calculator to avoid pregnancy. A cycle can always shift.
Want to know every month, without the maths?
PERIOD. learns from your own cycles and keeps your predictions up to date. Free, no ads, and we never sell your data.
Frequently asked questions
When do I ovulate?
Usually about 14 days before your next period. With a 28 day cycle that is around day 14, with 32 days around day 18.
How many days per cycle are you fertile?
About 6 days: the 5 days before ovulation and the day of ovulation itself.
Can I use this calculator to avoid pregnancy?
No. It is an estimate, not contraception.
Does it work if my cycle is irregular?
The estimate is less reliable then. Ovulation tests or tracking your temperature give more certainty.
Read more
Other free tools
Sources: NHS, Periods and fertility in the menstrual cycle; NHS, Pregnancy due date; ACOG, Methods for estimating the due date.