Estimate your fertile window and ovulation day from your last period and cycle length.
Understanding the ovulation calculation
Ovulation typically happens about 14 days before your next period. Enter the first day of your last period and your average cycle length, and this calculator estimates your ovulation day, fertile window and next period.
Cycles vary, so treat this as a general estimate — it is not a contraception method.
How to fill it in
- Enter the first day of your last period.
- Enter your average cycle length.
- Read the estimated ovulation and fertile window.
Formula reference
ovulation ≈ last period + (cycle length − 14) days
fertile window ≈ 5 days before to 1 day after
Try it with these numbers
As an example, with a cycle length of 28, the calculator returns a estimated ovulation of Aug 15. Underneath, Fertile window comes out at Aug 10 – Aug 16 and Next period at Aug 29. Change any field and every figure updates as you type.
Common questions
When is the fertile window?
Roughly the five days before ovulation plus ovulation day, when conception is most likely.
How accurate is this?
It’s an estimate based on average timing. Real ovulation varies, so tracking methods give a clearer picture.
Can I use it to avoid pregnancy?
No — it isn’t reliable for contraception. Speak with a healthcare provider about suitable methods.