Ovulation calculator: predict your fertile window and next ovulation date from cycle length and last period. Free, private, runs in your browser, no signup.
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Every figure above is calculated locally in your browser from the assumptions shown. No inputs are sent anywhere. See the methodology section below for how this tool works.
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Ovulation calculator: predict your fertile window and next ovulation date from cycle length and last period. Free, private, runs in your browser, no signup.
Luteal phase of 11 to 17 days moves ovulation plus or minus 3 days.
Transparent methodology
How this calculator works
Reviewed August 2026 · BoringToolsKit Editorial Team
Formula
Ovulation ≈ last period + (cycle length − 14) days; fertile window ≈ 5 days before through 1 day after ovulation.
BoringToolsKit. “Ovulation Calculator.” boringtoolskit.com/ovulation-calculator/ (reviewed August 2026). Free to reference in articles, syllabi, and answer posts with a link.
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Worked example: a September cycle
Take a period starting Tuesday, September 1, 2026 with a 28-day cycle. The formula adds cycle minus 14 to the start date: 28 - 14 = 14 days after September 1, which lands on Tuesday, September 15 as the estimated ovulation day. The fertile window subtracts 5 days from ovulation and adds 1: September 10 through September 16, a 7-day span. The next period is 28 days after the start, September 29.
Change only the cycle length to 26 days and the arithmetic shifts: 26 - 14 = 12, so ovulation moves to September 13. At 32 days it is 32 - 14 = 18, giving September 19. That is a 6-day swing from a cycle-length change of 6 days, because the luteal phase after ovulation is held fixed at 14 days and all the variation is pushed into the time before ovulation. Shorter cycles put the window earlier, not closer to the next period.
Where the 14-day assumption breaks
The luteal phase, the days between ovulation and the next period, averages about 14 days but ranges roughly 11 to 17 in healthy cycles. A woman with a true 12-day luteal phase and a 28-day cycle actually ovulates around day 16, September 17 in the example above, 2 days later than the calculator's September 15 estimate. With a 17-day luteal phase the estimate is 3 days early instead. So a 28-day cycle carries about a plus or minus 3-day uncertainty from this assumption alone.
Cycle-length irregularity compounds it. If cycles alternate between 26 and 32 days, the ovulation estimate for any single cycle can be off by up to 3 days before the luteal-phase error is added. Sperm survive up to 5 days, which is why the 7-day window partially absorbs the error, but the ovulation date itself should be treated as a midpoint, not a prediction. Studies using ovulation predictor kits find only a minority of women ovulate exactly 14 days before menstruation.
