What is the Ovulation Calculator?
Estimate your ovulation day, fertile window and next period from your cycle.
Enter first day of last period and cycle length and the Ovulation Calculator returns estimated ovulation day, fertile window, next period expected and earliest reliable pregnancy test. Results update as you type, and nothing you enter leaves your device.
How it is calculated
Cycles vary, so these dates are estimates and should not be relied on for contraception. This is a general estimate for adults and not medical advice. Talk to a doctor or dietitian about your own health.
How to use it
- Enter first day of last period.
- Enter cycle length in days.
- Read estimated ovulation day in the results panel. It updates as you type.
- Change any value to compare scenarios, or use Share to send the result as a link.
Worked example
Common questions
What formula does the Ovulation Calculator use?
It uses: Ovulation ≈ next period − 14 days; Fertile window = 5 days before ovulation to 1 day after.
What do I need to use the Ovulation Calculator?
You need first day of last period and cycle length. Sensible starting values are filled in so you can see how it works straight away.
Is the Ovulation Calculator free, and is my data stored?
It is free with no sign-up. The calculation runs in your browser, so the values you enter are not sent to or stored on our server.