What is a Period Calculator?
A Period Calculator predicts the start and end dates of your next three periods from the first day of your last period, your cycle length and how long your period lasts.
Cycles often vary by a few days from month to month. Stress, illness, travel and some medicines can change timing. Treat the dates as a guide.
How to use the Period Calculator
- Enter first day of your last period.
- Enter average cycle length.
- Enter period length.
- Tap Calculate to see the result.
- Tap Reset to clear the fields and try again.
Who should use it
- Anyone tracking their menstrual cycle
- People planning travel or events
- Those noticing changes in their cycle
Key terms
- Cycle length
- Days from the start of one period to the start of the next.
- Period length
- How many days bleeding usually lasts.
Frequently asked questions
What is a normal cycle length?
Between 21 and 35 days is common for adults.
Why are my periods irregular?
Many things can affect timing. See a doctor if it changes a lot.
Is this medical advice?
No. Consult a doctor.
Is my data stored?
No. Everything you enter stays in your browser.
Last updated: 1 November 2026