How we find your fertile days
Ovulation is counted back from your next period
Ovulation usually happens about 14 days before your next period. In a 28-day cycle that is around day 14, but in longer or shorter cycles the day moves.
The 6-day fertile window
Your fertile window is the 6 days ending on ovulation day, because sperm can live for several days after sex.
Irregular cycles
If your cycle length changes a lot, the dates are harder to predict. Ovulation tests can help, and a doctor can check why cycles are irregular.
This is not birth control
This is not birth control. Cycles vary, and ovulation can come early or late. Do not use this tool to avoid pregnancy.
When to see a doctor
Doctors usually suggest a check-up after 12 months of trying without pregnancy, or after 6 months if you are 35 or older. Over 40, or if your periods are irregular or you know of a health problem, see a doctor sooner (CDC).
Questions people ask
When do I ovulate?
In most women, around 10 to 16 days before the next period (NHS). In a 28-day cycle that is around day 14, but it moves in longer or shorter cycles.
Which days are best to get pregnant?
The days just before ovulation and ovulation day itself. You are most likely to conceive around ovulation, usually about 14 days before your next period (NHS).
How long is the fertile window?
We show 6 days: the 5 days before ovulation and ovulation day. Sperm can live up to 7 days (NHS), so sex in the days before ovulation can lead to a pregnancy.
How often should we have sex when trying?
The NHS says that if you are 39 or under and have sex without contraception every 2 to 3 days, you are likely to get pregnant within a year (NHS).
Can I use this to avoid pregnancy?
No. With typical use, about 24 in 100 women who track their fertility get pregnant in a year (NHS). Ask a doctor, nurse or pharmacist about contraception.
What if my cycles are irregular?
Then ovulation is harder to predict. Ovulation tests or tracking body temperature can help, and a doctor can check why cycles are irregular.
When should we see a doctor?
The NHS says to see a GP after 1 year of trying (NHS). Go sooner if you have a health condition or are worried. In the US, the CDC advises seeing a doctor after 6 months of trying if you are 35 or older (CDC).
Read the full guide: Ovulation calculator
Sources
- NHS: Periods and fertility in the menstrual cycle · checked October 26, 2026
- NHS: Trying to get pregnant · checked October 26, 2026
- NHS: Natural family planning (fertility awareness) · checked October 26, 2026
- CDC: Infertility FAQs · checked October 26, 2026