Period Calculator
Predict your next few period dates and fertile windows from your cycle history.
days
days
Predictions assume a regular cycle — actual dates can vary due to stress, illness, and other factors.
Worth Knowing
This projects your upcoming period dates and fertile windows by extending your average cycle length forward from your last period. Its accuracy depends entirely on how regular your cycles are — for someone with steady 28-to-30-day cycles it's quite reliable, but stress, travel, illness, and hormonal changes can all shift timing. Track a few real cycles to refine the average, and remember calendar prediction is an estimate, not a contraceptive method or a guarantee of fertility timing.
How to Use
- Enter First day of your last period into the first field of the Period Calculator.
- Fill in the rest: Average cycle length, Average period length.
- Your result appears instantly as you type — there's no submit button or page reload.
- Predictions assume a regular cycle — actual dates can vary due to stress, illness, and other factors.