Period Calculator

Predict your next few period start dates based on your last period and average cycle length. Useful for planning ahead — pair with our Ovulation Calculator if you're also tracking your fertile window.

Period Calculator

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First day of last period

Average cycle length (days)

Next period (estimated)

Enter a date above

Following period

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Third period

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Predictions assume a consistent cycle length — actual timing can shift.

How this is calculated

Next period ≈ Last period start + cycle length (repeated for each future prediction)

Example

Last period started June 1, 2026, with a 28-day cycle: next period ≈ June 29, 2026, then July 27, then August 24.

Step-by-step guide

Enter the first day of your last period.
Enter your average cycle length — count from the first day of one period to the first day of the next.
Read your next three predicted period dates.

Common mistakes

Using a one-off unusually short or long cycle instead of your typical average, which skews future predictions.

Expecting exact precision several months out — small natural cycle variations compound the further into the future you predict.

Frequently asked questions

How accurate are these predictions?

They're most reliable for your very next period if your cycle is fairly regular, and become less certain further into the future since cycle length can naturally vary month to month.

What if my period is late?

Occasional variation is common and can have many causes — stress, travel, illness, or changes in routine. If you're regularly experiencing missed or irregular periods, a healthcare provider can help identify the cause.

Can I use this to track ovulation too?

This tool focuses on predicting period start dates. For an estimated ovulation day and fertile window, see our dedicated Ovulation Calculator.

Related calculators

See more in the Health Calculators section, or try the Ovulation Calculator and Pregnancy Due Date Calculator.