About this tool
Type the first day of your last period, how long your cycle usually lasts and how many days the period lasts. The calculator estimates the next period, the ovulation day - about 14 days before the next period, because the second half of the cycle (the luteal phase) is the most constant one - and the fertile window: the five days before ovulation plus the day after, when sperm and egg can meet. A bar shows where you are in the cycle today, and the list gives the dates of the next six cycles. It works for regular cycles and is calculated in your browser: nothing is sent or saved.
How to use it
- 1 Choose the first day of your last period.
- 2 Set your usual cycle length and how many days your period lasts.
- 3 Read the next period, the fertile window and the ovulation day.
- 4 Copy the link to keep your dates or check the next cycles in the list.
Frequently asked questions
How is the fertile window calculated?
Ovulation usually happens about 14 days before the next period. Sperm can survive up to five days and the egg about one day, so the fertile window is the five days before ovulation, the ovulation day and the day after.
Can I use it to avoid pregnancy?
No. Calendar estimates fail often because ovulation moves with stress, illness, travel and medication. To avoid pregnancy, use a contraceptive method chosen with a doctor.
My cycle is irregular. Does it work?
The estimate is for regular cycles. If the length changes by more than about seven days from one cycle to the next, the dates are not reliable; ovulation tests or tracking with a doctor work better.
Is my data saved?
No. The calculation runs in your browser. The dates only go into the link if you click Copy link.
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