Menstrual Cycle Calculator
Predicts your next period, your ovulation day and your fertile window from the first day of your last period and your usual cycle length.
Recent
Formula
How the calculation works
Enter the first day of your last period, which is day one of bleeding and not the day the period ended.
Enter your average cycle length, counted from the first day of one period to the first day of the next, and how many days your period usually lasts.
The tool adds the cycle length to your last period for the next period, takes 14 days back from that date for ovulation, and marks the fertile window as five days either side.
Common mistakes
- Using the last day of your period instead of the first, which shifts every predicted date forward by the length of your bleeding.
- Assuming a 28-day cycle for everyone, when real cycles range from about 21 to 35 days and a longer cycle moves ovulation much later.
When to use it
- Use it to plan the days when conception is most likely if you are trying to get pregnant and want a simple window to focus on.
- Use it to know when to expect your next period, so you can carry supplies and time a pregnancy test if your period is late.
Worked example
If your last period began on 1 February 2026 and your cycle is 28 days, your next period is due on 1 March 2026, ovulation falls around 15 February 2026 and the fertile window runs from 10 to 20 February 2026.
Common questions
How accurate is this prediction?
It is a calendar estimate that assumes a regular cycle, so if your cycles vary or you want confirmation, combine it with ovulation test strips or a basal body temperature chart.
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