Know your window

Ovulation calculator

Estimate your fertile days with a simple, private calculation.

Your cycle

First day of your last period

Average cycle length

28 days

Calculated privately in your browser

Your fertile window and ovulation day will appear here after you calculate.

Based on ACOG & NHS guidelines

100% private — data stays local

Reviewed against the official NHS calculator

How your fertile window is estimated

We estimate ovulation at around 14 days before your next expected period, then show the five days before and the day after — the six days with the highest chance of conception, following ACOG guidance. Cycles can vary, so this is a guide rather than a diagnosis.