Ovulation Calculator
Estimate your fertile window from your last period and cycle length.
Enter values
days
—
—
Step by step
The formula
ovulation ≈ (last period + cycle length) − 14 days
The luteal phase after ovulation is fairly consistent at about 14 days, so counting back from the expected next period predicts ovulation better than counting forward from the last one. The fertile window opens earlier because sperm can survive several days.
All calculations run locally in your browser and update instantly as you type.
Example calculations
Common questions this calculator answers — select one to load its values.
Related calculators
Frequently asked questions
How accurate is this?+
It is an estimate based on a regular cycle. Real ovulation shifts with cycle variation, stress and health, so treat it as a guide, not a medical certainty.
All calculations run locally in your browser. Nothing you type is sent to a server.