Ovulation calculator and fertile window

From the first day of your last period and your cycle length, we estimate when you'll ovulate, your fertile window and when a pregnancy test will be accurate.

Updated September 14, 2026

First day of your last period

Average cycle length

days

Days from the start of one period to the start of the next. Anywhere from 21 to 35 is typical.

This is a calendar-method estimate: it assumes you ovulate 14 days before your next period, which can shift from cycle to cycle. Don't use it as birth control. If your cycles are irregular, ovulation tests or basal body temperature tracking are more reliable.

Your fertile days

Enter the first day of your last period to see your fertile days.

How this ovulation calculator works

It uses the calendar method, which rests on two facts:

You ovulate about 14 days before your next period. Cycle length mostly varies in the days before ovulation, not after.
An egg lives about a day, but sperm can survive up to five. So your fertile window is the 5 days before ovulation, ovulation day and the day after.

Example: period on September 1

On a 28-day cycle:

Next period: September 29.
Estimated ovulation: September 15.
Fertile window: September 10 to 16.

Cycle length	Ovulation	Fertile window
24 days	September 11	September 6–12
28 days	September 15	September 10–16
32 days	September 19	September 14–20

It also shows your next few cycles. On a 28-day cycle, the following ovulation dates would be October 13 and November 10.

Ways to pinpoint ovulation

Ovulation can shift by a few days from month to month, so if you’re trying to conceive, these signs help confirm it:

Ovulation predictor kits detect the LH surge that comes 24 to 36 hours before ovulation.
Cervical mucus turns clear, slippery and stretchy, like raw egg white, on your most fertile days.
Basal body temperature rises slightly after ovulation. Tracked over a few cycles, it shows your pattern.

When to take a pregnancy test

Home tests are most reliable from the day your period is due. If you test earlier and get a negative, test again in a few days. If it’s positive, find out how far along you are with the due date calculator.

Limits

The calendar method isn’t reliable with irregular cycles, after giving birth, while breastfeeding or after stopping hormonal birth control, and it should never be used as contraception. For questions about your fertility, talk to your OB-GYN or midwife.

Frequently asked questions

How do you calculate your fertile window?

Ovulation happens about 14 days before your next period. Your fertile window is the 5 days before ovulation, ovulation day and the day after, because sperm can survive up to five days. If your last period started September 1 and your cycle is 28 days, your fertile window is September 10 to 16.

When do I ovulate on a 28-day cycle?

Around day 14, counting the first day of your period as day 1. With a period that started September 1, estimated ovulation is September 15.

What if my cycle is longer or shorter than 28 days?

Ovulation moves with it. With a period on September 1, a 32-day cycle puts ovulation around September 19, and a 24-day cycle around September 11.

When can I take a pregnancy test?

Home urine tests are most accurate from the day your period is due. In the 28-day example, that's September 29. Testing earlier makes a false negative more likely.

When should I see a doctor about getting pregnant?

ACOG recommends an evaluation after 12 months of trying if you're under 35, after 6 months if you're 35 or older, and sooner if your periods are irregular or you have a known condition.