Calculate ovulation

Mark a calendar estimate for ovulation, the surrounding fertile interval, and the next period from a regular cycle.

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ToolOvulation Calculator
Input
Output
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Preview

How it works

The next period is placed one usual cycle length after day 1, and subtracting 14 days gives the estimated ovulation date. The fertile interval extends from 5 days before that estimate through 1 day after; a 28-day cycle is the conventional default used for the calendar arithmetic.

Formula
estimated ovulation offset = usual cycle length − 14 days
  • The menstrual date must be a real past or current ISO date rather than a JavaScript-normalised value.
  • This is a regular-cycle estimate and does not detect whether ovulation occurred.

Worked example

A 28 day cycle
Last period started on 1 March 2026
Input
											Last period: 2026-03-01
Cycle length days: 28
Output
												Ovulation date: 2026-03-15
Fertile window start: 2026-03-10
Fertile window end: 2026-03-16
Next period date: 2026-03-29

When to use this

Period calendars use a regular cycle length, preconception plans mark the six-day interval around estimated ovulation, and fertility consultations compare that estimate with cervical mucus or luteinising-hormone testing.

Edge cases

  • An impossible calendar value such as 2026-02-30 is rejected instead of being rolled forward into March.
  • A cycle lasting 14 days or less is rejected because the fixed luteal subtraction would place estimated ovulation on or before cycle day 1.
  • Irregular cycles, anovulation, hormonal contraception, postpartum changes, and month-to-month variation leave no signal in the inputs, so the calendar dates are not adjusted.

References