How to use
Enter the first day of your last period and your usual cycle length to see your estimated ovulation date, your fertile window — the days when you’re most likely to conceive — and when your next period is due.
Below is a calendar for the next six cycles, assuming your cycle length stays the same — handy for planning ahead.
How it’s calculated
The second half of the cycle, from ovulation to your period, lasts about 14 days for most women, so ovulation is expected 14 days before your next period:
ovulation = first day of cycle + cycle length − 14
Sperm can survive up to 5 days and an egg about a day. That gives the fertile window: the 5 days before ovulation, ovulation day and the day after.
Example: a 28-day cycle that started on March 1. Ovulation is around March 15, the fertile window runs March 10–16, and the next period is due March 29.
FAQ
How accurate is this? It’s an estimate. Ovulation can shift by several days even in a regular cycle because of stress, illness or travel. Ovulation tests, basal body temperature tracking and ultrasound monitoring give a more precise picture.
Can I use this to avoid pregnancy? No. The calendar method is one of the least reliable forms of birth control.
What if my cycle is irregular? Calendar estimates are especially uncertain with irregular cycles. Calculating your shortest and longest cycles does not establish reliable boundaries for your fertile window. Do not use this calculator for contraception.
If you’re already pregnant, the due date calculator shows how far along you are.