Ovulation Calculator
Estimate your fertile window and ovulation day.
Free · No sign-up · Works in your browser
About the Ovulation Calculator
Estimate your fertile window free — enter the first day of your last period and your average cycle length, and this ovulation calculator estimates your ovulation day, your most fertile 5–6 day window, and your next expected period. It uses the standard method (ovulation is roughly your cycle length minus 14 days). Cycles naturally vary month to month, so use this as a planning guide — it isn't a substitute for ovulation tests or medical advice.
How to use the Ovulation Calculator
- 1Enter the first day of your last period.
- 2Enter your average cycle length.
- 3See your fertile window, ovulation day, and next period.
Frequently asked questions
How is ovulation estimated?
The luteal phase (ovulation to next period) is fairly constant at about 14 days, so ovulation ≈ cycle length − 14 days after your period starts. Your fertile window is the ~5 days before plus ovulation day.
How accurate is it?
It's an estimate — cycle length and ovulation timing vary. Ovulation predictor kits and tracking basal body temperature give more personalized results.
Can I use this for birth control?
No — calendar estimates are not reliable contraception on their own because ovulation shifts. Talk to your healthcare provider about contraception.