How accurate is this estimate?

It's a standard estimate based on typical cycle patterns, useful for general planning, but actual ovulation can vary — ovulation predictor kits or tracking basal body temperature give more precise real-time information.

What if my cycle length varies month to month?

Use your average cycle length for the best estimate, and consider that the fertile window may shift earlier or later than predicted in months that run shorter or longer than usual.

Ovulation Calculator

Estimate your ovulation date and fertile window from the first day of your last period and your typical cycle length.

First day of last periodAverage cycle length

days

Enter the first day of your last period.

How It Works

Estimating ovulation

Ovulation typically occurs about 14 days before your next period starts, regardless of cycle length — so a longer cycle pushes ovulation later, not earlier.

The fertile window

The fertile window spans roughly the 5 days before ovulation through 1 day after, since sperm can survive several days while the egg survives about 24 hours after release.

Example Calculation

For a 28-day cycle starting January 1, ovulation is estimated around January 15, with a fertile window from January 10 through January 16.

Common Mistakes

Assuming every cycle is exactly 28 days — using your actual average cycle length gives a much better estimate.
Treating the estimate as precise — actual ovulation timing varies cycle to cycle, even for people with regular cycles.

Frequently Asked Questions

How accurate is this estimate?: It's a standard estimate based on typical cycle patterns, useful for general planning, but actual ovulation can vary — ovulation predictor kits or tracking basal body temperature give more precise real-time information.
What if my cycle length varies month to month?: Use your average cycle length for the best estimate, and consider that the fertile window may shift earlier or later than predicted in months that run shorter or longer than usual.