Ovulation Calculator

The short answer

• Ovulation usually happens about 14 days before your next period; the NHS gives a range of 10 to 16 days.
• For a 28-day cycle that is around day 15, counting the first day of your period as day 1.
• Your fertile window is the 5 days before ovulation plus ovulation day, because sperm can survive for up to 7 days.
• Cycles change from month to month, so the dates are estimates. They are not a safe way to avoid pregnancy.

How the menstrual cycle works

A cycle starts on the first day of a period (day 1) and ends the day before the next period. The NHS says the average is about 28 days, and anything from 21 to 35 days is normal. In each cycle, hormones ripen an egg in one of the ovaries. Ovulation is when that egg is released into the fallopian tube, where it can be fertilised.

If the egg is not fertilised, the lining of the womb is shed about two weeks later: that is your period, and the next cycle begins.

When ovulation happens

Ovulation is easier to count backwards from the next period than forwards from the last one. The part of the cycle after ovulation is fairly steady, while the part before it varies. So the calculator takes your cycle length and subtracts the luteal phase (14 days unless you change it).

The luteal phase

The luteal phase runs from ovulation to the next period. The calculator uses 14 days, but some people have 11 or 12 days and others 16. If ovulation tests or temperature charts show yours, enter it under More options. With a 28-day cycle and an 11-day luteal phase, ovulation moves three days later: for a period that started on 1 October 2026, from 15 October to 18 October.

The fertile window

An egg lives for a short time after ovulation, but sperm can survive in the body for up to 7 days. So sex in the days before ovulation can lead to pregnancy. The calculator shows the 5 days before ovulation and ovulation day itself: the days when pregnancy is most likely. The chances are highest in the two or three days up to and including ovulation.

A worked example

The next two cycles follow on every 28 days:

Longer and shorter cycles

With a longer cycle, ovulation comes later. A 32-day cycle starting on 1 October 2026 gives ovulation around 19 October and a fertile window from 14 October. With a short cycle, ovulation can come soon after the period ends, which is why the NHS notes that you can sometimes get pregnant just after a period.

Irregular cycles

If your cycle length changes by more than a few days, one average gives a false sense of precision. Turn on "My cycles are irregular" and enter your shortest and longest recent cycles. The calculator then shows a wider window: from the earliest fertile day in your shortest cycle to the latest ovulation in your longest.

Irregular cycles are common in the teenage years, around the menopause, after pregnancy, while breastfeeding and with conditions such as polycystic ovary syndrome (PCOS). Ovulation tests help most here.

Signs of ovulation

Some people notice signs around ovulation:

• cervical mucus becoming clearer, wetter and stretchy, like raw egg white;
• a small rise in body temperature after ovulation (measured first thing each morning);
• a mild ache on one side of the lower tummy;
• a higher sex drive.

These signs vary and are easy to miss. Together with dates, they give a clearer picture.

Ovulation tests and tracking

Ovulation predictor kits from pharmacies test your urine for the surge in luteinising hormone (LH) that comes a day or two before ovulation. Start testing a few days before your expected fertile window; the calculator’s dates tell you when. Temperature charts confirm ovulation after it has happened, which helps you learn your own pattern over a few months, including your luteal phase.

How often to have sex

The NHS suggests regular unprotected sex every 2 or 3 days through the cycle. That way there are usually healthy sperm waiting when ovulation happens, without the stress of timing it to the day. The calculator is useful for knowing roughly when you are most fertile, not for scheduling.

How long it usually takes

It is normal not to conceive straight away. The NHS says more than 8 out of 10 couples, where the woman is under 40, will conceive naturally within a year of regular unprotected sex. Age is the biggest factor: fertility falls gradually from the early thirties.

When to take a pregnancy test

Most home pregnancy tests work from the first day of a missed period. The calculator shows when your next period is due. Testing earlier can give a negative result even if you are pregnant; if your period still does not come, test again a few days later.

From ovulation to due date

If you conceive, your due date is about 266 days after ovulation, or 280 days after the first day of your last period for a 28-day cycle. A period on 1 October 2026 would give a due date of 8 July 2027. The pregnancy due date calculator shows the full timeline, from scans to full term, and the results table above links to it with your dates filled in.

Why this is not contraception

If you do not want to get pregnant, talk to a GP, pharmacist or sexual health clinic about contraception.

After stopping contraception

After you stop hormonal contraception, it can take a few cycles for periods to settle. You can get pregnant straight away, even before your first period. Until your cycles are regular, ovulation is hard to predict from dates.

Getting ready for pregnancy

• The NHS recommends taking a daily folic acid supplement from before you conceive until 12 weeks of pregnancy.
• Stopping smoking and cutting out alcohol helps fertility and a healthy pregnancy.
• A healthy weight helps: check yours with the BMI calculator.
• Plan the money side too: the cost of a baby calculator and the maternity pay calculator show what to expect in the UK.

When to see a GP

1. After a year of tryingSee a GP

   If you have not conceived with regular unprotected sex.

2. SoonerIf you are 36 or over

   Or if you already know of a possible fertility problem.

3. Any timeIf periods are very irregular, painful or stop

   Or are under 21 or over 35 days apart.

Outside the UK, speak to your doctor or gynaecologist.

Common myths

• "Everyone ovulates on day 14." Only with a 28-day cycle and a 14-day luteal phase, and even then not every month.
• "You can’t get pregnant during your period." Unlikely, but possible with a short cycle, because sperm can survive into your fertile window.
• "Apps know exactly when you ovulate." Apps that use dates only make the same estimate as this calculator.

Keeping a cycle diary

The calculator is only as good as the cycle length you give it. Write down the first day of each period for three to six months, in a diary, a calendar or an app. Count the days between the starts to get each cycle length, then use the average. Note the shortest and longest too: if they differ by more than a few days, use the irregular option.

A diary also helps a GP if you ever need to discuss your periods or fertility. Note any bleeding between periods, very heavy periods or bad pain, as these are worth mentioning even if your cycles are regular.

For partners: what helps

Fertility is shared. The NHS advice on regular sex every 2 or 3 days applies to both partners, and so does the advice on lifestyle: not smoking, keeping alcohol within the guidelines and a healthy weight all help sperm as well as eggs. Trying for a baby can be stressful, especially when it takes longer than hoped. Talking openly, and not treating the fertile window as a deadline, helps many couples. If you have been trying for a year, both partners should see a GP, as tests are usually offered to both.

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