How does the ovulation calculator work?

It estimates ovulation as roughly 14 days before your next period. From the first day of your last period and your average cycle length, it works out your likely ovulation day, your fertile window (the 5 days before ovulation plus ovulation day), and your most fertile days.

When am I most fertile?

You are most fertile on the day of ovulation and the day before it. Sperm can survive up to 5 days, so having intercourse in the days leading up to ovulation also gives a good chance of conceiving.

How accurate is an ovulation calculator?

It is a helpful guide for regular cycles, but ovulation can shift from month to month. For more precise tracking, combine it with ovulation predictor kits, basal body temperature, or cervical mucus changes.

Can I use it if my cycles are irregular?

Switch to Variable length mode and enter your shortest and longest recent cycles — you'll see a possible ovulation window rather than a single date. Calendar predictions are still less reliable with irregular cycles, so confirm with LH kits or cervical mucus. Persistently irregular cycles are worth discussing with your doctor.

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Ovulation Calculator 🥚

Find your most fertile days and fertile window so you can time things with confidence.

First day of your last period

Average cycle length (days)

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How to find your fertile window

Ovulation is when an ovary releases an egg. It usually happens about 14 days before your next period starts. The egg lives for around 24 hours, but because sperm can survive for up to 5 days, your fertile window is about six days long — the five days before ovulation plus ovulation day itself.

To use the calculator, enter the first day of your last period and your average cycle length (the number of days from the start of one period to the start of the next). Most people have a cycle of 28 days, but anywhere from 21 to 35 days is common.

Next steps

Already trying? Once you have a positive test, use our due date calculator to find out when your baby is due, then follow along with our week-by-week pregnancy guide.

Ovulation calculator FAQs

How does the ovulation calculator work?: It estimates ovulation as roughly 14 days before your next period. From the first day of your last period and your average cycle length, it works out your likely ovulation day, your fertile window (the 5 days before ovulation plus ovulation day), and your most fertile days.
When am I most fertile?: You are most fertile on the day of ovulation and the day before it. Sperm can survive up to 5 days, so having intercourse in the days leading up to ovulation also gives a good chance of conceiving.
How accurate is an ovulation calculator?: It is a helpful guide for regular cycles, but ovulation can shift from month to month. For more precise tracking, combine it with ovulation predictor kits, basal body temperature, or cervical mucus changes.
Can I use it if my cycles are irregular?: Switch to Variable length mode and enter your shortest and longest recent cycles — you'll see a possible ovulation window rather than a single date. Calendar predictions are still less reliable with irregular cycles, so confirm with LH kits or cervical mucus. Persistently irregular cycles are worth discussing with your doctor.