Ovulation Calculator
Predict ovulation and the fertile window from your last period and typical cycle length.
Runs 100% in your browser — nothing is uploaded.
Result
Formula
Ovulation ≈ next period − luteal phase (≈ cycle − 14 days).
About this calculator
Sperm survive up to five days, so the fertile window opens well before ovulation. Cycles vary — this is an estimate.
How to use
- 1
Enter the first day of your last period.
- 2
Input your typical cycle length and luteal phase length in days.
- 3
Click Calculate to see your estimated ovulation date, fertile window, and next period.
Frequently asked questions
What is ovulation?
Ovulation is the release of an egg from the ovary. It typically occurs once per menstrual cycle and is the most fertile time. The egg survives for about 12–24 hours after release.
How is the fertile window calculated?
The fertile window is calculated by taking the estimated ovulation date and counting back 5 days (since sperm can survive up to 5 days) and adding 1 day after ovulation (since the egg is viable for about 24 hours). This gives a 6-day window when intercourse is most likely to lead to pregnancy.
Why is the luteal phase important?
The luteal phase is the time between ovulation and the start of the next period. It is usually fairly constant for an individual (typically 10–16 days). By subtracting the luteal phase from the cycle length, we can estimate when ovulation occurs.
Please note: Results are estimates for general information only and are not professional or medical/financial advice.