Period Calculator

Predict the start dates of your next several periods and your fertile days from your last period, cycle length, and how long your period lasts.

Next period starts

—

Next period ends—

Days until next period—

Fertile window this cycle—

Upcoming periods

Cycle	Period start	Period end

Where you are in your cycle

Menstrual, follicular, fertile, and luteal phases, with a pointer at today.

How it works

Each cycle is counted from the first day of one period to the first day of the next. Adding your cycle length repeatedly projects future start dates, and the period length sets how long each one lasts. Ovulation, and therefore the fertile window, falls about 14 days before the next period.

Next period = last period + cycle length
Fertile window = (next period − 14) − 5 days … + 1 day

Frequently asked questions

What if my cycle is irregular?

Predictions assume a steady cycle. If yours varies a lot, use the average of your last few cycles, and treat the dates as a rough guide.

Can I use this to avoid pregnancy?

Calendar predictions are not a reliable form of contraception on their own, because ovulation can shift. Use a dedicated method if you are preventing pregnancy.