Period Calculator
Quick answer
Enter the first day of your last period and your average cycle length to predict when your next three periods are likely to start — handy for planning trips, events and everyday life.
Last updated:
Your details
Everything runs in your browser — nothing is stored or sent anywhere.
How it works
Next period = first day of last period + cycle length
Following periods repeat at the same interval
Average cycle length is 28 days, but anywhere from 21 to 35 days is considered normal.
Worked example
- 1
Last period started March 1, cycle 30 days.
- 2
Next period ≈ March 31.
- 3
Then ≈ April 30 and ≈ May 30.
Interpreting your cycle length
| Your result | Category | What it means |
|---|---|---|
| 21 – 35 days | Normal range | Most cycles fall here. Twenty-eight days is the average, not a universal rule. |
| A few days early or late | Usual variation | Stress, travel, illness and training load shift timing in otherwise healthy women. |
| Consistently under 21 or over 35 days | Worth a check-up | Persistent irregularity can relate to hormonal or thyroid conditions — a healthcare provider can help. |
Frequently asked questions
Sources & further reading
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Open the logThis calculator provides general estimates for informational purposes only and is not medical advice. For personal guidance, consult a qualified healthcare provider.