Health & fitness · FREE ONLINE TOOL

Ovulation Calculator

Estimate ovulation timing and a possible fertile window from cycle length and luteal-phase assumptions, without confirming ovulation.

Your inputs

First day of last menstrual period

Calculations run in your browser. No account needed.

EXAMPLE RESULTCalculated locally
Next period estimate
2026-07-29
Estimated ovulation
2026-07-15
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My workspace

Calculate with your inputs to save a result or compare scenarios.

Detailed breakdown

All 6 rows

The complete calculation, row by row.

Swipe or scroll horizontally for more columns.
Detailed breakdown · All 6 rows
CycleNext periodOvulation estimatePossible fertile window
12026-07-292026-07-152026-07-10 to 2026-07-15
22026-08-262026-08-122026-08-07 to 2026-08-12
32026-09-232026-09-092026-09-04 to 2026-09-09
42026-10-212026-10-072026-10-02 to 2026-10-07
52026-11-182026-11-042026-10-30 to 2026-11-04
62026-12-162026-12-022026-11-27 to 2026-12-02

How it works

A useful distinction

Change one timing assumption at once

Holding a 28-day cycle fixed, changing the luteal assumption from 14 to 12 days moves estimated ovulation two days later. This comparison shows model sensitivity, not evidence that ovulation actually occurred on either date.

Understanding the limits of cycle predictions

The ovulation calculator estimates a window from the cycle information you enter. A typical cycle length does not establish when ovulation occurs in any particular cycle, and variation can shift the dates. Review the result as a calendar estimate rather than a confirmation. It should not be used as a reliable contraceptive method or to diagnose a fertility concern.

Cycle estimate: formula and assumptions

Next period = start + cycle length. Estimated ovulation = next period − luteal phase.

Cycle length varies. The fertile window shown is the five days before estimated ovulation plus ovulation day.

Worked example

Using the Cycle estimate example:

Inputs
First day of last menstrual period
2026-07-01
Typical cycle length (days)
28
Assumed luteal phase (days)
14
Number of cycles
6
Method

Next period = start + cycle length. Estimated ovulation = next period − luteal phase.

Result

Example result

Next period estimate: 2026-07-29

Estimated ovulation: 2026-07-15

These are illustrative inputs. Change the values to explore a different scenario.

What to keep in mind

Do not use calendar estimates alone to prevent pregnancy. This tool does not confirm ovulation.

Formula references

References explain the underlying methods; they do not imply endorsement or individual professional review of Finflo.

Common questions

Why does changing the luteal phase move the ovulation estimate?

The model subtracts the entered luteal-phase length from the estimated next period date.

Will this identify ovulation accurately for an irregular cycle?

It cannot reliably establish ovulation from a fixed calendar assumption. Variable cycles can shift the actual timing substantially.

Prepared by Finflo. See our methodology and editorial policy. Read the relevant professional-advice limitations.