Period Calculator
Predict your next periods and ovulation dates for the coming six months from your last period and cycle length.
About the Period Calculator
A menstrual cycle runs from the first day of one period to the day before the next. Cycles of 21–35 days are normal in adults. Ovulation usually happens about 14 days before the next period starts, whatever the cycle length.
The formula
Next period = last period start + cycle length
- Ovulation
- ≈ next period − 14 days
Worked example
Example: Next period starts = Wed, 15 Apr 2026
- Inputs: First day of your last period: 15 March 2026; Average cycle length: 31 days; Period length: 6 days.
- Formula: Next period = last period start + cycle length
- Result: Next period starts = Wed, 15 Apr 2026
Things to keep in mind
- This is an estimate for general information, not medical advice. Talk to a doctor or other qualified professional about your health.
- Stress, illness, travel and hormonal contraception shift cycles; predictions are estimates.
Questions people ask
Is a 35-day cycle normal?
For adults, 21–35 days is considered typical. Talk to a doctor if your cycles are often outside this or change suddenly.
How do I find my cycle length?
Count the days from the first day of one period to the first day of the next for a few months and average them.