Ovulation calculator

The calculator needs two things: the first day of your last period and your average cycle length. A cycle is counted from the first day of one period up to the day before the next one starts, so the first day of bleeding is always day 1.

First it adds your cycle length to that day 1 to estimate when your next period will start. Then it counts back 14 days from that date to estimate your ovulation day. The 14 days stand for the luteal phase, the part of the cycle between ovulation and the next period. Finally, it marks your fertile window as the five days before ovulation plus ovulation day itself, six days in total. If you leave the cycle length at its default, the calculator assumes a 28-day cycle, which puts ovulation on day 15.

Say your last period started on 1 March and your cycles are usually 32 days long. The calculator adds 32 days to 1 March and estimates that your next period will start on 2 April. Counting back 14 days from 2 April gives an estimated ovulation day of 19 March, and the fertile window runs from 14 March to 19 March.

With the 28-day default instead, the same start date would give an estimated ovulation day of 15 March and a fertile window from 10 March to 15 March. Four extra days of cycle length move every date four days later, which shows how much the result depends on the number you enter.

Very few people have a cycle that is exactly the same length every month. It is common for cycle length to shift by a few days from one month to the next, which is why the calculator asks for an average rather than the length of a single cycle.

Because the calculator places ovulation a fixed 14 days before your next expected period, any difference between your average and the cycle you are actually having moves the estimated ovulation day by the same number of days. If this cycle turns out three days longer than your average, ovulation may also come later than the calculator suggests. Only the next period, when it arrives, shows how long the cycle really was.

If you do not know your average, count the days from the first day of one period to the first day of the next for a few cycles, then take the average. Using your own number instead of the 28-day default is the single biggest improvement you can make to the estimate.

The calculator assumes your cycles follow a steady pattern and that your luteal phase lasts 14 days. Neither is true for everyone. The luteal phase can be a little shorter or longer from person to person, and when it is, the real ovulation day sits away from the date the calculator shows, even if your cycle length is exactly right.

Cycles are also often less predictable in the first years after periods begin, in the years leading up to menopause, after pregnancy and while breastfeeding, and when starting or stopping hormonal contraception. Stress, illness, travel and changes in sleep, weight or exercise can all shift ovulation within a single cycle. If your cycles often fall outside the 20 to 45 day range the calculator accepts, or vary widely from month to month, treat the results as a rough guide only.

If you have been trying to conceive for a while, or you are worried about irregular or missing periods, speak to a healthcare professional. This calculator cannot tell you whether you ovulate or why a cycle is irregular.

A single calculation is a snapshot of the numbers you entered today. Tracking turns it into a record. When you note the first day of every period, you can see your real cycle lengths instead of guessing, notice whether they stay steady or vary, and spot changes worth mentioning to a healthcare professional.

With a few months of history you can replace the 28-day default with your own average and recalculate with more confidence. The Evecare app is built for this: you log your periods as they happen, so your estimated fertile window starts from your own cycles rather than a textbook average.

Whichever way you track, the results on this page are for information only. They cannot, and should not, be used as a conception aid or to prevent pregnancy.