How does this differ from the Ovulation calculator?

This calculator projects future period start dates forward from your last period. The Ovulation calculator instead works backward from your next expected period to estimate your fertile window, they use the same inputs but answer different questions.

Why might my actual next period differ from the prediction?

Stress, illness, travel, medication, and normal biological variation can all shift cycle length from month to month, so predictions become less reliable the further out they project, especially if your cycles aren't very regular.

Period Calculator

Predict your next several period dates from your last period and cycle length.

How this is calculated

Each future period = last period + (cycle length × number of cycles ahead)

Assumptions used in this calculation

Regular cycles: Assumes your cycle length stays consistent; actual cycles can vary month to month.

About this calculator

The period calculator predicts upcoming period start dates for several future cycles, based on the first day of your last period and your average cycle length.

Worked example

Last period Jan 1, 2026, 28-day cycle, 3 cycles ahead

Result: Next periods: Jan 29, Feb 26, Mar 26

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Frequently asked questions

Ovulation Calculator

Estimate your ovulation date and fertile window from your last period and cycle length.

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Period Calculator

About this calculator

Frequently asked questions

How does this differ from the Ovulation calculator?

Why might my actual next period differ from the prediction?

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