Ovulation & Fertility Calculator
Estimate your fertile window each cycle.
An estimate based on cycle averages. Actual ovulation varies — this is not a substitute for medical advice or contraception.
About the Ovulation Calculator
Free ovulation calculator. Enter the first day of your last period and your cycle length to estimate your fertile window, ovulation day and next period.
Everything is calculated instantly in your browser — nothing you type is uploaded or stored. Bookmark Calcooly for the next time you need it.
How to use the Ovulation Calculator
- 1Enter the first day of your last period.
- 2Set your average cycle length.
- 3See your estimated fertile window, ovulation day and next period.
Frequently asked questions
How is the fertile window estimated?+
It centres on ovulation (about 14 days before your next period) and spans the days leading up to and just after it.
Is this reliable for contraception?+
No. It's an estimate based on averages and shouldn't be used as a method of birth control.
Is the result exact?+
It's a reliable estimate for general information. For important financial, medical or legal decisions, check with a qualified professional.