Health & Fitness · Pregnancy
Ovulation Calculator — Fertile Window and Next Periods
Estimate when you'll ovulate and your most fertile days from the first day of your last period and your usual cycle length. See the fertile window for the next six cycles, the expected start of each period, and the earliest day a home pregnancy test is reliable. Adjust the luteal phase if you know yours.
How this is calculated
Next period = last period + cycle length · Ovulation = next period − luteal phase (≈ 14 days) · Fertile window = ovulation − 5 days to ovulation day
- Luteal phase
- Days between ovulation and the next period — fairly constant for each person
Ovulation happens about two weeks before your next period, not two weeks after the last one, so in a 32-day cycle it's around day 18. Sperm can survive up to five days and an egg about one day, which makes the five days before ovulation and ovulation day itself the fertile window; the two days before ovulation carry the highest chance.
Assumptions & sources
- Regular cycles of the length entered; calendar dates only.
- 1Timing of the 'fertile window' in the menstrual cycle: day specific estimates from a prospective study (Wilcox, Dunson & Baird) — BMJ 2000;321:1259
- 2Ovulation and the fertile window — University of California, San Francisco
Calculator ID HLTH-CYCLE-001 · version 1.0.0
Common questions
When do I ovulate in a 28-day cycle?
Around day 14 — and the fertile window runs from about day 9 to day 14.
Can I use this to avoid pregnancy?
No. Calendar estimates miss many cycles' real ovulation; use reliable contraception.
When should I take a pregnancy test?
From the day your period is due (about 14 days after ovulation); a test is most reliable a few days later.