Ovulation & Fertility Calculator
Trying to conceive? This ovulation calculator shows your fertile window, your likely ovulation day and when your next period is due, from the first day of your last period and your cycle length.
Add your last period to see your fertile window
This tool is for general planning only and is not medical advice, contraception, or a guarantee of conception. If you have questions about fertility, your GP is a good place to start.
How it works
- 1
Add your last period
Enter the first day of your most recent period.
- 2
Set your cycle
Adjust your average cycle length and, if you know it, your luteal phase.
- 3
See your window
We show the days you're most likely to conceive, your ovulation day and next period.
Common questions
How is the fertile window calculated?
Ovulation usually happens about 14 days before your next period, so we count back your luteal phase from your predicted next period. Your fertile window covers the five days before ovulation plus ovulation day, because sperm can survive for several days.
When am I most likely to conceive?
The days leading up to and including ovulation are the most fertile. Enter your details above to see your personal window.
How accurate is it?
It's a useful guide based on an average cycle, but every body is different and cycles vary from month to month. For a closer picture, many people also track ovulation signs or use ovulation tests.
Can I use this as contraception?
No. This is a planning tool for people trying to conceive and should never be relied on to avoid pregnancy. Speak to your GP or a sexual health service about contraception.