Ovulation Calculator

What is an Ovulation Calculator?

Ovulation Calculator — Find Your Ovulation Date and Fertile Window

An ovulation calculator estimates the day you ovulate and the days around it when you can get pregnant, based on the first day of your last period and the length of your cycle.

Enter your details in the calculator above — no login, no signup. This ovulation date calculator shows your fertile window, your best days to conceive, your next period and a colour-coded ovulation calendar for the next few months. It also shows the lower-chance days used by the Standard Days Method, often searched for as the safe period calculator — with clear warnings about how unreliable that method is.

Nothing you type is sent to our server. The calculation runs entirely in your browser.

How to Use This Ovulation Calculator

Ovulation Formula

How Ovulation Is Calculated

Ovulation happens about 14 days before your next period starts. The second half of the cycle (the luteal phase) is fairly constant, while the first half changes from woman to woman. That is why ovulation is not always on day 14.

Worked Example

If your period starts on 5 Jan 2026 and your cycle is 28 days, you ovulate around 18 Jan 2026 (day 14). Your fertile window runs from 13 Jan to 19 Jan, and your next period is expected on 2 Feb 2026.

Ovulation Day by Cycle Length

This calculator assumes a 14-day luteal phase. In real life it can be anywhere from about 10 to 16 days, which can move ovulation by a few days either way.

Best Days to Conceive

Fertile Window — The Best Days to Get Pregnant

You can only get pregnant during a few days each cycle. The fertile window is set by how long sperm and the egg survive.

• Sperm can survive up to 5 days in the body, so intercourse before ovulation can still lead to pregnancy.
• The egg survives only 12 to 24 hours after it is released.
• The most fertile days are the two days before ovulation and the day of ovulation itself.
• Having intercourse every day or every second day during the fertile window is a good approach if you are trying to conceive.
• Ask your doctor about starting folic acid before you conceive, and about any health conditions to check first.

Once you get a positive pregnancy test, use our Pregnancy Due Date Calculator to find your due date and week of pregnancy.

Safe Period

Safe Period Calculator — How Reliable Is the Calendar Method?

The "safe period" is the idea that pregnancy is only possible around ovulation, so avoiding unprotected sex on fertile days prevents it. In practice this fails often, because ovulation is hard to predict from the calendar alone, cycles change every month, stress, illness and travel can delay ovulation, and sperm can survive up to 5 days.

The Standard Days Method

The best-studied calendar rule is the Standard Days Method. It treats days 8 to 19 of the cycle as fertile and works only for women whose cycles are always between 26 and 32 days long. Even with perfect use, at least 5 in 100 women become pregnant in a year, and more in everyday use. This calculator shows those days as "lower chance", never as safe, and only when your cycle length is between 26 and 32 days.

If You Want to Avoid Pregnancy

• Use a reliable method such as condoms, oral contraceptive pills or a copper IUD, chosen with your doctor.
• If you have had unprotected sex, emergency contraception works best when taken as soon as possible, within 72 hours for the common pill, and some types work up to 5 days. Ask a pharmacist or doctor.
• Calendar methods work poorly with irregular cycles, PCOS, breastfeeding, or after stopping hormonal contraception.

Signs of Ovulation

Signs of Ovulation and When to See a Doctor

A calendar gives you an estimate. Your body gives you clues to confirm it.

• Cervical mucus: becomes clear, slippery and stretchy, like raw egg white, just before ovulation.
• Mild pelvic pain: some women feel a one-sided twinge around ovulation.
• Body temperature: your resting temperature rises slightly after ovulation.
• LH test kits: a positive result means ovulation is likely within 24 to 36 hours.

See a doctor if your periods are very irregular or missing, if you have severe pain, or if you have not conceived after 12 months of trying (6 months if you are 35 or older). Irregular cycles are a common sign of PCOS or thyroid problems — you can check your symptoms with our PCOS Analyzer and Thyroid Health Quiz. For the research behind the fertile window, see Wilcox et al. in the New England Journal of Medicine and the ACOG guide to fertility awareness.

Common Questions

Frequently Asked Questions