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Fertile window calculator

Enter the first day of your last period and how long your cycle usually is.

Today's estimated chance is 27 percent. The moon shows tonight's real phase. Around it, a cycle of 28 days: 6 carry an estimated chance, highest on September 29 at 33 percent. Solid marks are days already past; dashed marks are predictions.
Estimated chance today
27%

Full moon· turn the moon to see another day

Estimated most fertile daySeptember 29

My period started on and my cycles usually last .

September 2026
MTWTFSS

Each day shows the moon of that night.

28days
2145

If you are not sure, leave it at 28 — the estimate already allows for that.

Try it
With few cycles, the estimate stays wide.Example
01 · The estimate

A calendar gets the day of ovulation right at most 21% of the time. So we show the chance for each day rather than a single window.

How we calculate this

Worked out from what you entered. The days ahead are still a prediction.

General information. Not a substitute for your doctor.

Day-specific estimates: Wilcox, Weinberg & Baird, NEJM 1995.

Sources: Wilcox, Weinberg & Baird, NEJM 1995 · Li & Urteaga, JAMIA 2021

Due dateHome