Ovulation Calculator
Enter a valid date and a cycle length between 21-45 days.
This assumes a 14-day luteal phase (typical, but not universal) and a regular cycle. Sperm can survive up to 5 days, so the fertile window includes the days before ovulation, not just the day itself. Cycle tracking apps or ovulation predictor kits are more precise if you're actively trying to conceive.
Free ovulation calculator that estimates your fertile window and ovulation date from your last period and cycle length — the same underlying calculation as a conception calculator, in one tool.
How it works
- Enter the first day of your last period
- Enter your average cycle length
- See your estimated fertile window, ovulation date, and next period
Frequently asked questions
How is the fertile window calculated?
Ovulation is estimated as your cycle length minus 14 days after your last period (the luteal phase is typically about 14 days). The fertile window includes the 5 days before ovulation, since sperm can survive that long, plus the day of ovulation itself.
How accurate is this?
It's an estimate based on average cycle timing, which varies even for people with regular cycles. Ovulation predictor kits or cycle tracking over several months give a more personalized picture if you're actively trying to conceive.
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From the blog
Update history
- 2026-09-03 Bounded numeric fields are now sliders instead of number inputs.
- 2026-09-01 Added: a cover image — used as the card thumbnail, social share image, and blog title image.
- 2026-09-01 Launched: ovulation calculator with fertile window, ovulation date, and next period estimate. Covers the standalone Conception Calculator use case as the same calculation. Entirely client-side.