Ovulation Calculator
Enter the first day of your last period and your cycle length to see your next three ovulation dates and fertile windows on a calendar.
How it works
- Next period = first day of last period + average cycle length
- Ovulation day = next period - 14 days
- Fertile window = 5 days before ovulation through the day after ovulation
Already pregnant? Use the due date calculator to see how far along you are.
These are estimates based on averages, not medical advice or a diagnosis. The dates you enter never leave your device.
Frequently asked questions
How is the ovulation date calculated?
Ovulation usually happens about 14 days before your next period starts. So the estimated ovulation day is the first day of your last period plus your average cycle length, minus 14 days. With a 28-day cycle that is around day 14; with a 32-day cycle, around day 18.
When is my fertile window?
Sperm can survive in the body for up to 5 days, and an egg lives for about a day after ovulation. That makes the roughly 6 days from 5 days before ovulation through the day after ovulation the time when pregnancy is most likely, with the highest chances in the 2 days before ovulation and on ovulation day itself.
Can I use this calculator as birth control?
No. Cycles shift easily with stress, illness and other factors, and calendar-based counting alone has a high failure rate as contraception. Use a reliable birth control method. If you are trying to conceive, ovulation test kits or tracking basal body temperature can confirm the estimate.
What if my cycles are irregular?
Enter the average of your last few cycles, but treat the whole span between the ovulation dates for your shortest and longest cycles as potentially fertile. If your cycles are often shorter than 21 days or longer than 35 days, it is worth checking in with a gynecologist.