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Period Calculator

Predict your next period and your fertile window from the start of your last period and your average cycle length.

How it works Report a problem

Cycle Predictor

Predict your next 3 periods

About the Period Calculator

Predict your next period and your fertile window from the start of your last period and your average cycle length.

How to use the Period Calculator

  1. Enter your Last Period Start date and your Avg Cycle (Days).
  2. Press Calculate.

Formula and method

Next period = last start + cycle length. Ovulation usually happens about 14 days before the next period.

Worked example

A period starting on the 1st with a 28-day cycle suggests ovulation around the 14th and the next period on the 29th.

Tips

  • Track several cycles to find your true average.
  • Do not use this as birth control.

Frequently asked questions

Is a 28-day cycle normal?
It is the average, but anything from about 21 to 35 days can be normal.