Ovulation Calculator
Quick answer
Enter the first day of your last period and your average cycle length to estimate your ovulation day, your 6-day fertile window and your next expected period — the key dates when trying to conceive (or avoid).
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Your details
Everything runs in your browser — nothing is stored or sent anywhere.
How ovulation is estimated
Ovulation ≈ first day of last period + (cycle length − 14) days
Fertile window = 5 days before ovulation through 1 day after
Next period ≈ last period + cycle length
The luteal phase (ovulation → next period) is about 14 days for most women; the follicular phase varies.
Worked example
- 1
Last period started March 1, cycle length 28 days.
- 2
Ovulation ≈ March 1 + 14 days = March 15.
- 3
Fertile window ≈ March 10–16.
- 4
Next period expected ≈ March 29.
Interpreting your fertile window
| Your result | Category | What it means |
|---|---|---|
| Cycle day ~14 on a 28-day cycle | Ovulation day | Estimated at cycle length minus 14. The egg typically survives only 12–24 hours. |
| 5 days before through 1 day after ovulation | Fertile window | The days pregnancy is possible. Sperm may survive up to 5 days, so the days before ovulation matter most. |
| 35-day cycle → ~day 21 · 21-day cycle → ~day 7 | Personal timing | Ovulation shifts with cycle length, and so does your fertile window. The estimate is a guide, not a guarantee. |
Frequently asked questions
Sources & further reading
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Open the logThis calculator provides general estimates for informational purposes only and is not medical advice. For personal guidance, consult a qualified healthcare provider.