Period Calculator
Predict your next period date and fertile window from your last period and cycle length.
- Free to use
- Accurate results
- No registration required
- Works on all devices
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.
- Informational only — not a substitute for tracking symptoms or consulting a healthcare provider about irregular cycles.
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, reviewed 13-09-2026.
This calculator provides a general estimate only and does not constitute personalized medical advice.
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