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Home / Health / Fertile Window Estimate

Fertile window estimate for trying to conceive

Estimate your fertile days, with the limits spelled out and sources from ACOG and the CDC. Not birth control. Updated October 23, 2026.

Important: This tool estimates fertile days for people who are trying to become pregnant. It is not birth control. Calendar-based estimates are not reliable for preventing pregnancy: the CDC lists a typical-use failure rate of 2 to 23 percent for fertility awareness methods, higher than most other methods. If you want to avoid pregnancy, talk to a clinician about options. General information, not medical advice.

Estimate your fertile window

Nothing you enter leaves your device. Use the first day of your last period (the first day of bleeding) and your usual cycle length.

First day of your last period Usual cycle length in days (21 to 35)

Estimate my fertile days

How this works, and its limits

Ovulation usually happens about 14 days before the next period starts. ACOG describes a fertile window of about six days each cycle: sperm can survive up to five days, and an egg survives about 12 to 24 hours after it is released. For the best chance, research suggests having sex every day or every other day during the window, though ACOG notes this is not required and that pressure can make sex stressful.

The tool counts back 14 days from your next expected period to estimate ovulation, then shows the five days before ovulation, the day itself and the day after. Real cycles vary from month to month, stress, illness and travel can shift ovulation, and no method is foolproof. Thin, stretchy, clear cervical mucus often appears one to two days before ovulation and can help you check the estimate. If your periods are highly irregular, you may not be ovulating regularly, and home estimates may not be useful. See an ob-gyn or a reproductive endocrinologist.

Source: ACOG, Trying to Get Pregnant? Here's When to Have Sex →

If you want to avoid pregnancy

The CDC's typical-use failure rates over one year include: implant 0.1%, LNG IUD 0.1 to 0.4%, copper IUD 0.8%, the shot 4%, the pill, patch and ring 7%, male condom 13%, and fertility awareness-based methods 2 to 23%. Methods that are not fertility-awareness based are generally more effective. A clinician or a local clinic can help you choose one.

Source: CDC, Contraception and Birth Control Methods →

See a clinician sooner if

You are 35 or older and have been trying for six months, or under 35 and trying for a year without success.
Your cycles are shorter than 21 or longer than 35 days, or very unpredictable.
You have severe pain, very heavy bleeding or other symptoms that worry you.

Timing guidance for when to seek a fertility evaluation varies by situation, so ask your own clinician.

Disclaimer: General information only, not medical advice. Estimates depend on accurate inputs and typical cycles. No result is a guarantee.

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