Fertility Window Estimator

Regularity follows FIGO (2018): a difference of 7 days or less between your shortest and longest cycle is regular. A typical cycle is 24–38 days; cycles outside that range are flagged separately.

Common reasons include stress, travel, big changes in weight or exercise, breastfeeding, coming off hormonal contraception, thyroid conditions and polycystic ovary syndrome (PCOS). Cycles also become less predictable in the late 30s and 40s. A printed cycle summary from this tool is a useful thing to bring to an appointment.

The method combines two facts. Ovulation usually happens 12–16 days before a period, and sperm can survive up to about 5 days. Subtracting 18 from your shortest cycle finds the earliest day that sex could still lead to pregnancy in a short month. Subtracting 11 from your longest cycle finds the latest likely fertile day in a long month. The window in between covers both your early and late cycles.

Example: cycles from 26 to 32 days give a window from day 8 (26 − 18) to day 21 (32 − 11).

• Count from day one: the first day of full flow, not spotting.
• Add every cycle, including unusual ones. They're exactly what this method needs to see.
• Six or more cycles give a window that reflects your real range. With only three, an unusual month can skew the result.
• Keep it going: your history is saved on this device, so just add each new period when it starts.

The calendar method gives a wide, safe window. To narrow it down each month, combine it with ovulation predictor kits (start testing a couple of days before your window opens), cervical mucus observations, or basal body temperature charting. Many people find that after a few months of combined tracking they recognise their own pattern.

Some variation is normal. Stress, travel, illness and changes in sleep or exercise can all shift ovulation. Cycles also tend to become less predictable after the late 30s and while breastfeeding. Persistent irregularity can be linked with conditions such as PCOS or thyroid problems, which are very treatable. If your cycles are often shorter than 24 or longer than 38 days, vary by more than 20 days, or stop for three months, book a check-up. The printable cycle summary from this page is a helpful record to bring, whether you see a GP, a polyclinic doctor or a gynaecologist.

Both tools estimate your fertile days, but they answer slightly different questions. The Ovulation Calculator assumes one typical cycle length and pinpoints a single ovulation day and a 7-day window. It's ideal if your cycles are regular. The Fertility Window Estimator looks at the full spread of your past cycles and gives a window wide enough to cover both your shortest and longest months. It suits cycles that vary, at the cost of a longer window.

If your cycles vary by 7 days or less, the Ovulation Calculator is usually enough. If they vary more, use this estimator, and consider ovulation tests to find the exact days each month.

Suppose your last six cycles were 27, 30, 26, 31, 29 and 28 days long. Your shortest is 26 days and your longest 31. First fertile day: 26 − 18 = day 8. Last fertile day: 31 − 11 = day 20. So your fertile window runs from day 8 to day 20 of each cycle, counting the first day of your period as day 1. Your cycles vary by 5 days, which is regular by the FIGO definition.