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Menstrual Cycle Calculator

Predict your next three period start dates and fertile windows based on your last period and average cycle length.

Enter Cycle Details

First Day of Last Period

Average Cycle Length

days

Average Period Length

days

Next 3 Predicted Cycles

This is an estimate for informational purposes only and is not medical advice. Consult a healthcare provider.

How to Calculate & Formula Details

Mathematical Model Formula

Next Period = Last Period Start + (Cycle Length x N). Ovulation Window = Period Start - 14 days, minus a 5-day fertile window.

This calculator predicts your next three menstrual cycles based on the first day of your last period and your average cycle length, giving you both period start dates and estimated fertile windows for planning.

How Cycle Prediction Works

Menstrual cycles are counted from the first day of one period to the first day of the next. By projecting forward using your average cycle length, this tool estimates your next few period dates.

Frequently Asked Questions

How far in advance can I predict my period?

This calculator projects three cycles ahead, but accuracy decreases the further out you go, especially if your cycle length varies naturally month to month.

What is a normal cycle length?

A normal menstrual cycle typically ranges from 21 to 35 days, with 28 days being the commonly cited average. Consistently tracking your own cycles gives the most accurate personal average.

Why do my predictions seem off some months?

Stress, illness, travel, and hormonal changes can all shift ovulation and period timing. This tool provides a statistical estimate, not a guarantee.

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