Ovulation and fertile window calculator
An estimate of your most fertile days, based on your cycle length.
Runs entirely in your browser. Nothing you type is sent anywhere.
From the first day of one period to the first day of the next.
Next fertile window—
- Likely ovulation
- —
- Next period due
- —
This is not a method of contraception. Timing varies from cycle to cycle, even in regular cycles.
For general wellness only. These results are estimates from published formulas and guidelines, not a diagnosis or medical advice. They may not suit pregnancy, children, older adults, athletes or people with health conditions. Talk to a qualified health professional about your health. In an emergency, call your local emergency number.
How to use the ovulation and fertile window calculator
The tool estimates ovulation as 14 days before the next expected period, a common clinical estimate, and the fertile window as the 6 days ending on the ovulation day: the 5 days before and the day itself (Wilcox et al., 1995). A later study (Wilcox et al., 2000) found that only about 30% of women had their whole fertile window between days 10 and 17 of the cycle, and that timing can be unpredictable even when cycles are regular. Ovulation tests, temperature charting or a clinician can narrow it down.
- Pick the first day of your last period.
- Enter your usual cycle length, or drag the slider.
- Read the estimated fertile window and ovulation day for the next three cycles.
Sources
- Wilcox, Weinberg & Baird (1995): Timing of sexual intercourse in relation to ovulation, N Engl J Med
- Wilcox, Dunson & Baird (2000): The timing of the “fertile window” in the menstrual cycle, BMJ
Sources checked on 9 October 2026. Formulas and thresholds are used as published; the wording on this page is our own.