Ovulation Calculator

What your result means

Your result shows the days you are most likely to get pregnant. Day 1 is the first day of your period. The table shows the estimated days for common cycle lengths.

• Ovulation date: the day an egg is most likely to be released.
• Fertile window: the 5 days before ovulation, ovulation day and the day after. This matches the ACOG (opens in new tab) definition: you can get pregnant from sex anywhere in that span.
• Most fertile days: the 2 days before ovulation and ovulation day. Your chance of getting pregnant is highest here.
• Next period: also the day from which most home pregnancy tests work, if your period has not started (NHS (opens in new tab)).

You may read that ovulation happens on “day 14”. That is a rough average. The calculator counts back 14 days from your next period, which puts it on day 15 of a 28-day cycle. Real ovulation varies by a few days from cycle to cycle.

How ovulation is calculated

The calculator uses the calendar method. After ovulation, the luteal phase (the time until your next period) lasts about 14 days in most people. The first half of the cycle is the part that changes in length. So the best guess is to find your next period and count back.

• Next period = first day of last period + cycle length
• Ovulation ≈ next period − 14 days
• Fertile window = ovulation − 5 days to ovulation + 1 day
• Most fertile days = ovulation − 2 days to ovulation day

Cycle length is the number of days from the first day of one period to the first day of the next. Regular cycles from 21 to 35 days are normal (NHS (opens in new tab)). The window starts 5 days before ovulation because sperm usually survive about 3 days inside the body and sometimes up to 5. It ends the day after ovulation because an egg survives only about 24 hours once released (ACOG (opens in new tab)).

Your chances on each day

A landmark US study tracked 221 women trying to conceive, using daily urine hormone tests to pin down the ovulation day. Every pregnancy came from sex in a 6-day window ending on ovulation day. The chance of conceiving from sex on a single day rose from about 10% five days before ovulation to about 33% on ovulation day itself (Wilcox and colleagues, NEJM 1995 (opens in new tab)). That is why the calculator marks the 2 days before ovulation and ovulation day as the most fertile.

How to use the ovulation calculator

1. Enter the first day of your last period (the first day of full bleeding, not spotting).
2. Enter your average cycle length, from 21 to 45 days. Leave it at 28 if you are not sure.
3. Read your ovulation date, fertile window, most fertile days and next period.
4. Check the table for your next 3 cycles to plan ahead.

To find your average cycle length, note the start date of each period for 3 to 6 months, count the days between them and take the average. A period tracking app can do this for you.

Worked example

Your last period started on Thursday 1 October 2026, and your cycle is 30 days.

• Next period: 1 October + 30 days = Saturday 31 October 2026.
• Ovulation: 31 October − 14 days = Saturday 17 October 2026 (day 17).
• Fertile window: Monday 12 October to Sunday 18 October 2026.
• Most fertile days: 15, 16 and 17 October 2026.
• Pregnancy test: from 31 October 2026, if your period has not started.

The next two cycles follow the same pattern. If your period starts on 31 October, ovulation is about 16 November, with a fertile window of 11 to 17 November. If the following period starts on 30 November, ovulation is about 16 December, with a fertile window of 11 to 17 December.

The calculator always shows the first fertile window that has not ended yet. So if you enter these details after 18 October, it skips the October cycle and starts from the November one. The date box accepts a last period from the past 120 days.

Signs of ovulation

A calendar gives an estimate. Your body gives clues that are often more accurate, especially if your cycles vary. Using two or more signs together works best.

• LH test strips: these urine tests detect the surge of luteinising hormone (LH) that triggers ovulation. A positive test means ovulation is likely within the next day or two, so it falls in your most fertile days.
• Cervical mucus: near ovulation it becomes clear, wet, slippery and stretchy, like raw egg white.
• Basal body temperature (BBT): your resting temperature rises slightly, by less than half a degree Celsius, after ovulation and stays up until your period. It confirms ovulation after it has happened.
• Other signs: some people feel mild one-sided lower tummy pain, breast tenderness or a higher sex drive.

If your cycles are irregular

If your cycle length changes from month to month, one calculation will not be enough. Instead, look at your shortest and longest cycles from the last 6 months and run the calculator for each.

For example, if your cycles range from 26 to 34 days, the 26-day result puts the start of your fertile window on day 8. The 34-day result puts the end on day 22. So your possible fertile days run from about day 8 to day 22. That is a wide range, which is why LH test strips or mucus tracking help a lot here.

Irregular or missed periods are a main sign of polycystic ovary syndrome (PCOS). WHO estimates PCOS affects 10% to 13% of women of reproductive age and that up to 70% of them are undiagnosed. It is also one of the most common causes of infertility (WHO (opens in new tab)). If your cycles are often longer than 35 days, see a gynaecologist.

Tips for getting pregnant

Having sex every 2 to 3 days through the cycle covers the fertile window without needing to track it exactly. If you prefer to time it, aim for the most fertile days. Take a folic acid supplement while trying, as advised by your doctor, and keep a healthy weight. You can check yours with the BMI calculator. Once you are pregnant, use the due date calculator to find your due date.

Limitations

• The calculator assumes a regular cycle and a 14-day luteal phase. Real ovulation can vary by several days, even in regular cycles.
• It is less reliable with irregular periods, PCOS, thyroid problems, after stopping hormonal contraception, while breastfeeding or near menopause.
• It is not a form of birth control. Even full fertility awareness methods, which track temperature and mucus daily, lead to 12 to 24 pregnancies per 100 women in a year of typical use (ACOG (opens in new tab)). A calendar alone is less reliable.
• It cannot tell you whether you actually ovulate.

See a doctor if your periods are very irregular, very painful or absent for 3 months or more. ACOG (opens in new tab) advises a fertility check if you have not got pregnant after 1 year of regular unprotected sex, after 6 months if you are older than 35, and straight away if you are older than 40.

Key facts

• Pregnancy resulted only from sex during a 6-day window ending on the day of ovulation, with the chance per day rising from about 10% to 33% (Wilcox et al., NEJM 1995 (opens in new tab)).
• Ovulation usually happens 10 to 16 days before the next period starts (NHS (opens in new tab)).
• An egg survives about 24 hours after ovulation; sperm survive about 3 days and sometimes up to 5 (ACOG (opens in new tab)).
• PCOS affects an estimated 10% to 13% of women of reproductive age and is a leading cause of irregular periods and infertility (WHO (opens in new tab)).
• Fertility awareness works best when several signs, such as temperature and cervical mucus, are tracked, and it does not protect against sexually transmitted infections (NHS (opens in new tab)).
• A fertility check is advised after 1 year of trying without pregnancy, after 6 months if older than 35, and right away if older than 40 (ACOG (opens in new tab)).