Women's Health tool
Ovulation Calculator
Estimate your ovulation day and fertile window for the next three cycles from your last period and cycle length. For regular cycles. Free, nothing stored.
Nothing you enter leaves your device.
Inputs
Results
Enter values to see your results.
Simple steps
How it works
- 01
Enter the first day of your last period.
- 02
Enter your average cycle length.
- 03
Select Calculate to see the estimated ovulation day, fertile window and next period for three cycles.
Good to know
Frequently asked questions
How is ovulation estimated?
Ovulation usually happens about 14 days before the next period starts. The calculator finds your next period from your cycle length and counts back 14 days.
What is the fertile window?
The five days before ovulation and the day of ovulation. Sperm can live up to five days, so these are the days when pregnancy is possible.
How reliable is it?
It is an estimate for regular cycles. Ovulation can move by several days from cycle to cycle. Ovulation tests or tracking body temperature give a more precise picture.
Can I use it as contraception?
No. Do not use these estimates to prevent pregnancy.
My cycles are irregular. Can I still use it?
The dates will be less reliable. Enter your average cycle length and read the results as a rough guide, or talk to a doctor.
What cycle lengths can I enter?
From 21 to 45 days.
Is anything I enter stored or sent?
No. The calculation runs in your browser. Nothing you type is saved or sent to a server.
How it is calculated
Next period = first day of last period + cycle length. Ovulation = next period − 14 days. Fertile window = ovulation − 5 days to ovulation day, after Wilcox and colleagues (New England Journal of Medicine, 1995). The same steps repeat for three cycles.
Sources
Last reviewed: October 14, 2026