Period Calculator
Predict your next period date and fertile window from your last period and cycle length.
- Free to use
- No registration required
- Works on all devices
- Last updated
- 13-09-2026
Enter your last period date and cycle length to predict your next period.
days
Your result
12-09-2026
Next expected period
Estimated ovulation date29-08-2026
Fertile window (start)24-08-2026
Fertile window (end)30-08-2026
AI explanation
Formula
Next period = last period date + cycle length (days)Worked example
Last period 2026-01-01, 28-day cycle
| Field | Value |
|---|---|
| First day of last period | 01-01-2026 |
| Average cycle length | 28 |
| Next expected period | 29-01-2026 |
Assumptions
- Assumes your cycle length stays consistent from month to month — actual cycles can vary, especially due to stress, illness, or other factors.
Frequently asked questions
How is my next period predicted?
By adding your average cycle length to the start date of your last period — the more consistent your cycle, the more accurate this prediction.
What's a normal cycle length?
Cycle length varies widely between individuals — anywhere from about 21 to 35 days is generally considered within the normal range, though "normal" varies for each person.
What if my period is late compared to this prediction?
A late period can result from many factors (stress, illness, pregnancy, hormonal changes) — if this is a persistent or concerning pattern, consult a healthcare provider.
Related calculators
Sources
- Optimizing natural fertility: a committee opinion — American Society for Reproductive Medicine. Effective 01-01-2017, checked 13-09-2026.
This calculator provides a general estimate only and does not constitute personalized medical advice.
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