Pregnancy & Fertility
Period Cycle Calculator
Know your rhythm: predict the next period, the ovulation day, and the fertile window from your last period and average cycle.
The menstrual cycle is counted from the first day of one period to the day before the next. A typical cycle lasts 21–35 days. The calculator predicts the next period and fertile window from your average cycle length.
Typical cycle: 21–35 days.
Next period = last period + cycle length.
Track 3+ cycles for accuracy.
Calculate Your Period Cycle
Your Cycle Length
What Is Period Cycle?
The menstrual cycle runs from the first day of one period to the day before the next. Cycles of 21–35 days are typical for adults; teens' cycles often range wider.
This calculator projects the next period, ovulation, and the fertile window from your average, useful for planning, travel, and understanding your own patterns.
How It Is Calculated
Next period = last period start + average cycle length
Example: Example: LMP Aug 1, 28-day cycle → next period ≈ Aug 29, ovulation ≈ Aug 15
Cycle phases (28-day example)
| Phase | Days | Hormone driver |
|---|---|---|
| Menstruation | 1–5 | Low estrogen & progesterone |
| Follicular | 6–13 | Rising estrogen |
| Ovulation | 14 | LH surge |
| Luteal | 15–28 | Progesterone dominant |
Limitations
- Predictions need 3+ tracked cycles to be meaningful.
- Irregular cycles (variation >7–9 days) deserve clinical evaluation.
- Not a contraception method.
Sources & Review
References used for this calculator’s formulas and thresholds:
Period Cycle FAQ
How long is a normal cycle?
21–35 days for adults, counted from day 1 of bleeding to the day before the next bleed.
Why is my period late?
Stress, travel, illness, weight change, and exercise all shift ovulation, which shifts the period. More than 7 days of variation for several cycles warrants a check.
When should I see a doctor about cycles?
If cycles are shorter than 21 days, longer than 35, wildly irregular, very painful, or you bleed between periods — get evaluated.
Does cycle length affect fertility?
Yes, longer cycles mean later ovulation. The luteal phase stays ~14 days; the follicular phase varies.
A note before you begin: this tool offers educational estimates only. It cannot replace advice from your doctor or another qualified professional.