UK Ovulation and Fertility Calculator

Knowing roughly when you ovulate each month is useful whether you are trying to conceive, trying to understand your body better, or simply curious about your own cycle. This calculator estimates your ovulation date, fertile window, and next expected period from your last period date and average cycle length, along with a projection for your next three cycles.

How to use the ovulation calculator

Enter the first day of your last period and your average cycle length, counted from the first day of one period to the first day of the next. If you are not sure of your average, 28 days is a common starting point, though anywhere from 21 to 40 days is considered a normal range. The calculator returns your estimated ovulation date, your fertile window, your next expected period, and projected ovulation dates for the following three cycles based on the same average length.

How the calculation works

Rather than assuming ovulation always happens exactly 14 days into your cycle, the calculator works backwards from your next expected period. It takes your cycle length, subtracts a standard 14-day luteal phase, the typically consistent stretch of time between ovulation and the next period, and adds that adjusted number of days to your last period's start date. This means a 32-day cycle and a 26-day cycle produce noticeably different estimated ovulation days, even though both might be entered by someone assuming “day 14” ovulation by default.

The fertile window is then set as the 5 days before ovulation through to the day of ovulation itself, 6 days in total. This reflects that sperm can survive in the body for up to around 5 days in the right conditions, while an egg is generally viable for only about 24 hours after release, meaning intercourse in the days leading up to ovulation can still result in pregnancy even if it happens before the egg is actually released.

Worked example

Take a last period starting on 1 July 2026, with an average 28-day cycle. Subtracting the 14-day luteal phase from the 28-day cycle gives an ovulation day offset of 14 days, so estimated ovulation falls on 15 July 2026. The fertile window runs from 10 July to 15 July, and the next period is expected around 29 July 2026, 28 days after the last one started.

Now try changing the cycle length to 32 days in the calculator above, keeping the same last period date. The ovulation day offset becomes 18 days instead of 14, shifting the estimated ovulation date and fertile window later by 4 days compared with the 28-day example, which shows why using your own actual average, rather than assuming 28 days for everyone, matters for accuracy.

Understanding your fertile window and cycle

A typical menstrual cycle has two phases either side of ovulation: the follicular phase, from the first day of your period to ovulation, which varies quite a lot between people and even between cycles for the same person, and the luteal phase, from ovulation to the next period, which is usually much more consistent at around 14 days. This is why cycle length differences between people (or month to month for the same person) mostly show up as differences in the first phase rather than the second, and why counting back from the expected next period tends to be a more reliable estimate than counting forward a fixed number of days from the last one.

Common mistakes people make tracking ovulation

A common mistake is assuming ovulation always happens on day 14 regardless of cycle length, which works reasonably well for a standard 28-day cycle but becomes increasingly inaccurate for shorter or longer cycles. Another is treating a single month's calculation as fixed and permanent, when in reality cycle length and ovulation timing can shift from month to month due to stress, illness, travel, and many other everyday factors, which is why this calculator projects your next 3 cycles rather than just one fixed date going forward.

It is also worth remembering that this kind of date-based estimate is a starting point, not a precise measurement. If you are actively trying to conceive, combining this estimate with an ovulation predictor kit or tracking other signs your body gives, alongside speaking to your GP if you have concerns, will generally give a clearer picture than dates alone.

Related calculators

If your estimate leads to a positive pregnancy test, our pregnancy weeks calculator tracks your current gestational week from that point, and our pregnancy due date calculator estimates your full due date and NHS milestones. Our BMI calculator can also be a useful general health reference point if you are planning a pregnancy, alongside guidance from your GP or midwife.