Ovulation & Fertility Estimate
Estimate ovulation and the fertile window from the first period day and usual cycle length.
Your results
- Estimated ovulation
- 2026-10-17
- Estimated fertile window
- 2026-10-12 to 2026-10-18
- Estimated next period
- 2026-10-31
Calculations run in your browser. Results are estimates where described below.
How the calculation works
Estimated next period = start + cycle length. Estimated ovulation = next period − 14 days. Approximate fertile window: five days before through one day after.
Worked example
A cycle starting 1 January 2026 with 28 days gives estimated ovulation 15 January and next period 29 January.
Assumptions and limits
Luteal phases and cycles vary. These calendar estimates cannot identify safe days for contraception or diagnose fertility.