Ovulation Calculator
Free ovulation calculator. Predict your most fertile days and ovulation date based on cycle length and last period — for conception planning or tracking.
Quick answer
Ovulation day = first day of last period + (cycle length − 14). Fertile window starts 5 days before ovulation and ends the day after — sperm survive ~5 days, egg survives ~24 hours. Tracking BBT or LH strips improves accuracy over calendar prediction.
Ovulation Calculator
Estimated fertile window
How it works
Ovulation typically happens 14 days before the next period starts. The fertile window covers the 5 days before ovulation through the day after — sperm can survive 3-5 days in the female reproductive tract, while an egg lives only 12-24 hours.
When to use it
Trying to conceive (or trying not to), tracking cycles for irregularity, or planning a pregnancy test date (~2 weeks after ovulation).
Common mistakes
Assuming a 28-day cycle when yours isn't. Cycles vary widely — 21 to 35 days is all considered normal. Track 3 cycles before relying on this calculator. For more accuracy, use ovulation predictor kits or basal body temperature tracking.
How the ovulation calculator works
For a typical 28-day cycle, ovulation occurs on day 14 (counting from the first day of the last period). The luteal phase (post-ovulation) is consistently 14 days; the follicular phase varies. So ovulation day = cycle length − 14 days. The fertile window is the 5 days before ovulation plus ovulation day itself, with peak fertility 1-2 days before ovulation. The calculator outputs predicted ovulation date and fertile window dates for the next several cycles.
When to use it
Planning timed intercourse for couples trying to conceive. Avoiding pregnancy with fertility-awareness methods (use multiple methods together — calendar alone has high failure rates). Identifying mid-cycle symptoms (mid-cycle pain or 'mittelschmerz' often coincides with ovulation). Logging cycle data for fertility apps or fertility specialist consultations.
Common mistakes
- Assuming day 14 ovulation for non-28-day cycles. The luteal phase is fixed; the follicular phase varies. A 32-day cycle ovulates on day 18, not 14.
- Relying on the calendar method alone. Calendar-only fertility awareness has a typical-use failure rate of 24%. Add basal body temperature tracking and/or LH urine tests for higher accuracy.
- Confusing fertile window with ovulation day. The fertile window spans about 6 days. Sex on any of those days can result in pregnancy, but odds are highest 1-2 days before ovulation.
Frequently asked questions
How do I know when I'm ovulating?
Calendar prediction: cycle length minus 14 days from the first day of your last period. More accurate methods: BBT (basal body temperature) tracking shows a sustained rise after ovulation; LH urine tests detect the surge that precedes ovulation by 12-36 hours.
How long is the fertile window?
About 6 days total — the 5 days before ovulation plus ovulation day itself. Sperm survive up to 5 days in fertile cervical mucus; the egg survives 12-24 hours. Conception is most likely 1-2 days before ovulation.
Can I get pregnant outside the fertile window?
It's rare but possible, especially with cycle irregularity. Sperm can survive longer than expected in some cervical environments, and ovulation can occur earlier or later than predicted. Calendar-based contraception is unreliable on its own.