Fertile Window Calculator

See your full fertile window and day-by-day conception chances based on your last period and cycle length.

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First Day of Your Last Period

Select Date

Select Month

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Average Cycle Length (days)

Typically between 21 and 35 days. Count from the first day of one period to the first day of the next.

Luteal Phase Length (days)

The time from ovulation to your next period. 14 days is average if you're unsure.

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Free Fertile Window Calculator

This free fertile window calculator shows you the full span of days each month when pregnancy is most likely, based on the first day of your last period and your average cycle length. Instead of a single date, you get a day-by-day breakdown of your conception chances.

Your fertile window typically spans the five days before ovulation through the day of ovulation itself, since sperm can survive in the body for up to five days while the egg remains viable for about 24 hours after release. Conception chances rise gradually through the window and peak in the one to two days before ovulation.

How to Use the Fertile Window Calculator

Enter the first day of your last period.
Enter your average cycle length and luteal phase length (use 14 days if unsure).
Click Calculate My Fertile Window.
View your full fertile window with a day-by-day fertility level, plus your estimated ovulation date and next period.

Features

Shows your entire fertile window, not just a single date.
Day-by-day fertility level so you know exactly when chances are highest.
Estimates your ovulation date and next expected period.
Mobile-friendly with print, share, copy, and download options.

Frequently Asked Questions

What is the fertile window?

The fertile window is the span of days in each cycle when pregnancy is most likely, generally the five days before ovulation through the day of ovulation itself.

Which days in my fertile window give the best chance of conceiving?

Conception chances rise the closer you get to ovulation and peak in the one to two days right before it, then drop sharply after ovulation.

How is my fertile window calculated?

We estimate ovulation as 14 days before your next period (your luteal phase length), based on the first day of your last period and your average cycle length, then count back five days to mark the start of your fertile window.

Is this calculator accurate for irregular cycles?

This calculator gives a general estimate based on average cycle length. If your cycles are irregular, ovulation predictor kits or tracking basal body temperature may give more reliable results. Consult your doctor if you have concerns.

Related Calculators

Want a single estimated ovulation date instead? Try our ovulation calculator. Trying to figure out when you conceived? Use the conception date calculator. Already know you're expecting? Use the pregnancy due date calculator. Want to know how far along you are? Check the pregnancy week calculator.

Last Updated: August 2026