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Finding your fertile window
Ovulation is the day an ovary releases an egg, and it usually falls about 14 days before your next period starts — not 14 days after the last one, which is where most estimates go wrong for anyone whose cycle is not exactly 28 days. This ovulation calculator works backwards from your cycle length and gives both the likely ovulation date and the fertile window around it.
How to use the Ovulation Calculator
- Enter your LMP Date — the first day of your last period.
- Enter your usual Cycle Length (days); 28 is the average but 21 to 35 is all normal.
- Read the estimated ovulation date.
- Note the fertile window — the days before ovulation matter most.
Formula and a worked example
Ovulation ≈ LMP + (cycle length − 14) days
Fertile window ≈ ovulation − 5 days to ovulation + 1 day
The luteal phase — ovulation to the next period — is fairly consistent at about 14 days for most people. The follicular phase before it is what varies, which is why cycle length changes the ovulation date but not the 14-day gap.
Worked example
With an LMP of 1 March and a 32-day cycle, ovulation falls around 19 March (1 March + 18 days), and the fertile window runs roughly 14 to 20 March. Assuming a 28-day cycle would have put it on 15 March — four days early.
Frequently asked questions
How many days am I fertile?
About six: the five days before ovulation plus the day itself. Sperm survive several days, but an egg is viable for only around 24 hours.
What if my cycles are irregular?
Calendar estimates become unreliable. Ovulation predictor kits, basal body temperature tracking or cervical mucus changes are more dependable.
Is this a reliable form of contraception?
No. Calendar methods have a high failure rate because ovulation timing shifts with stress, illness and travel. Do not rely on it to avoid pregnancy.
When should I take a pregnancy test?
From the first day of a missed period. Testing earlier often gives a false negative. Our due date calculator takes over from there.
Related calculators
Other tools people use alongside the ovulation calculator.