🗓️ Period Calculator
Plan ahead by predicting your next periods. Enter the first day of your last period, your average cycle length and how many days your period usually lasts.
How the Period Calculator works
Each new period is estimated to start one cycle length after the previous one, and to last the number of days you enter.
Step-by-step calculation
- 1Enter last period and cycle length.
- 2Add one cycle for each next period.
- 3Add period length for end dates.
Worked example
With a last period starting March 1, a 28-day cycle and 5-day periods, the next periods are about March 29–April 2, April 26–30 and May 24–28.
Period Calculator examples
| Average cycle length (days) | Next periods |
|---|---|
| 21 | Thursday, January 22, 2026 |
| 28 | Thursday, January 29, 2026 |
| 35 | Thursday, February 5, 2026 |
| 42 | Thursday, February 12, 2026 |
Fixed values: First day of last period: Jan 1, 2026. Other inputs kept at the calculator's default values. Results calculated by this calculator.
Why use a Period Calculator?
Plan ahead for upcoming periods.
Important factors that can affect your results
- Irregular cycles.
- Birth control.
- Stress and travel.
Frequently asked questions
My cycle is irregular. Will this work?
Use your average cycle length. Predictions will be less accurate if your cycles vary a lot.
When should I talk to a doctor?
If cycles are often shorter than 21 days or longer than 35 days, very painful, or periods stop unexpectedly.
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Last reviewed November 2026 · How we build and check our tools · Report a problem