What is ovulation?

Ovulation is the moment when one of your ovaries releases a mature egg. It happens once in each menstrual cycle, roughly in the middle of the cycle. After the egg is released, it travels into the fallopian tube, where it can be fertilized by sperm. If it is not fertilized, the egg breaks down, and the lining of the uterus is shed about two weeks later as your period.

An ovulation calculator uses the dates of your cycle to estimate when this will happen. It cannot see your hormones or your ovaries, so the answer is always an estimate. It is still a helpful starting point if you are planning a pregnancy, learning how your cycle works, or trying to understand why you feel different at different times of the month.

How this calculator works

Your menstrual cycle is counted from the first day of one period to the first day of the next. The calculator needs two things: the first day of your last period, and the usual length of your cycle. The typical cycle is 28 days, but many healthy cycles are shorter or longer.

• Next period: first day of your last period + cycle length.
• Ovulation: 14 days before your next period.
• Fertile window: the five days before ovulation, plus the day of ovulation itself.

The 14-day rule works because the second half of the cycle, after ovulation, is more steady from person to person than the first half. Health sources commonly give a range of about 10 to 16 days between ovulation and the next period, and 14 days is a common middle point.

How to use this calculator

1. Pick the first day of your last period. Day 1 is the first day of real bleeding, not light spotting.
2. Enter your average cycle length. If your cycles vary, use the average of the last few months.
3. Press Calculate. The result also updates as you change the numbers.
4. Read your estimated ovulation date, your fertile window and your next period date. The table shows the next three cycles as well.

If you are not sure about your cycle length, count from the first day of one period to the day before the next one starts, and repeat this for a few months.

A simple example

Suppose your last period started on June 1 and your cycle is 28 days long. Your next period is expected on June 29. Counting back 14 days gives an estimated ovulation date of June 15. The fertile window runs from June 10 to June 15.

Now suppose the same start date but a 32-day cycle. The next period is expected on July 3, and ovulation is estimated for June 19, which is cycle day 19 counting June 1 as day 1. The fertile window runs from June 14 to June 19. A longer cycle moves ovulation later, because the extra days come before ovulation, not after it.

Ovulation day by cycle length

This table shows how the estimate changes when the cycle is shorter or longer. Day 1 is the first day of your period.

Cycle length	Ovulation (cycle day)	Fertile window (cycle days)
24 days	Day 11	Days 6 to 11
26 days	Day 13	Days 8 to 13
28 days	Day 15	Days 10 to 15
30 days	Day 17	Days 12 to 17
32 days	Day 19	Days 14 to 19
35 days	Day 22	Days 17 to 22

What is the fertile window?

You can only become pregnant on a limited number of days in each cycle. An egg survives for only about 12 to 24 hours after it is released. Sperm can survive in the body for up to five days, and some sources say up to seven. Because sperm can wait for the egg, sex in the days before ovulation can lead to pregnancy. That is why the fertile window is about six days long: the five days before ovulation and the day of ovulation.

Many sources say the last two or three days of the window are the most fertile. If you are trying to conceive, health organizations such as Tommy’s suggest having sex every two to three days throughout the cycle, with a little extra attention to the days around ovulation. For most people, that is all that is needed.

Why the result is only an estimate

A large study published in the BMJ tracked 696 cycles from 213 women using hormone tests. It found that ovulation happened as early as day 8 and as late as day 60 of the cycle. Only about 30 percent of the women had a fertile window that fell entirely between days 10 and 17, the days often quoted in older guides. The study’s authors concluded that predicting the fertile window is unreliable.

Stress, illness, travel, sleep changes, breastfeeding, weight change and many other things can move ovulation. A calculator based on dates cannot know about these. Treat the result as a good guess that you can check with other signs.

Signs that can help you check

• Cervical mucus: around ovulation it often becomes clear, slippery and stretchy, a bit like raw egg white.
• Ovulation test kits: these detect a rise in luteinizing hormone (LH) in urine, which usually comes about 24 to 36 hours before ovulation.
• Mild pain on one side: some people feel a brief twinge in the lower abdomen at ovulation.
• Basal body temperature: your resting temperature rises slightly after ovulation. This confirms ovulation after it has happened rather than predicting it.

Tracking these signs for a few months, along with your period dates, gives a much clearer picture than any single estimate.

If your cycles are irregular

Cycles that are shorter than 21 days or longer than 35 days are usually called irregular. When your cycle length changes a lot from month to month, the date of ovulation changes too, and a calculator becomes much less reliable. Using an ovulation test kit can help in that case. If you also find it hard to become pregnant, it is a good idea to talk to a doctor, because irregular ovulation can have causes that are easy to treat.

When to talk to a doctor

• Your periods are missing, very irregular, or have stopped.
• You have been trying to conceive for about a year without success, or for six months if you are 35 or older.
• You have very heavy bleeding, severe pain or bleeding between periods.
• You have a medical condition or take medicines and want to plan a pregnancy.

A doctor or nurse can look at your whole health picture. This page does not give medical advice and cannot diagnose any condition.

Frequently asked questions

How is my ovulation date calculated?

This calculator counts back 14 days from the date your next period is expected. Your expected period is the first day of your last period plus your cycle length. The fertile window is the five days before ovulation plus the day of ovulation.

How many days after my period do I ovulate?

It depends on your cycle length. Counting the first day of your period as day 1, this calculator puts ovulation on day 15 of a 28-day cycle, day 19 of a 32-day cycle and day 11 of a 24-day cycle.

Can I get pregnant outside the fertile window?

The chance is low, but it is not zero. Ovulation can come earlier or later than predicted, and sperm can survive for several days, so no calendar method is exact.

Can I use this calculator as birth control?

No. It gives an estimate only. Ovulation timing changes from cycle to cycle, so this tool should not be used to prevent pregnancy.

Is my information saved?

No. All calculations happen in your browser. The dates and numbers you enter are never sent to our server or stored.

Helpful resources

• Your menstrual cycle from the U.S. Office on Women’s Health
• Ovulation calculator and fertile window guidance from the charity Tommy’s
• The timing of the fertile window in the menstrual cycle, a prospective study in the BMJ
• Our BMI Calculator, Age Calculator and Days Between Dates Calculator

This calculator is for general information only. It is not medical advice, it cannot diagnose any condition, and it must not be used as a method of birth control. Talk to a qualified health professional about your health. See our Terms & Disclaimer.