What is the Period Calculator?
Predict the dates of your next three periods from your cycle length.
Enter first day of last period, cycle length and period length and the Period Calculator returns next period, following period and period after that. Results update as you type, and nothing you enter leaves your device.
How it is calculated
Next period = first day of last period + cycle length
This is a general estimate for adults and not medical advice. Talk to a doctor or dietitian about your own health.
How to use it
- Enter first day of last period.
- Enter cycle length in days.
- Enter period length in days.
- Read next period in the results panel. It updates as you type.
- Change any value to compare scenarios, or use Share to send the result as a link.
Worked example
Common questions
What formula does the Period Calculator use?
It uses: Next period = first day of last period + cycle length.
What do I need to use the Period Calculator?
You need first day of last period, cycle length and period length. Sensible starting values are filled in so you can see how it works straight away.
Is the Period Calculator free, and is my data stored?
It is free with no sign-up. The calculation runs in your browser, so the values you enter are not sent to or stored on our server.