Period Calculator

Formula

How to use it

1. Enter the first day of your most recent period (the first day of real bleeding, not spotting).
2. Enter your average cycle length — ideally averaged over several months — and how many days your period usually lasts.
3. Choose how many cycles to show, then read the dates and the table.

Worked examples

Cycles vary, so treat the dates as a guide

A cycle is counted from the first day of one period to the day before the next. In adults, the FIGO definition puts normal cycles at 24 to 38 days, with periods lasting up to 8 days; the NHS gives 28 days as the average. Cycles also vary from month to month, and the further ahead the calendar runs, the more any difference adds up. The list below gives the points at which a doctor should take a look.

The ovulation estimate assumes about 14 days between ovulation and the next period. That is the standard calendar assumption, but the real gap varies between people and cycles, so ovulation can come several days earlier or later than shown. Ovulation tests or tracking body signs show what is actually happening in a given cycle.

This calculator is not a method of contraception. A calendar estimate cannot tell you on which days you can or cannot get pregnant. If you want to use fertility awareness to avoid pregnancy, the NHS advises learning it from a trained teacher.

When to talk to a doctor

The NHS suggests seeing a GP if your periods suddenly become irregular and you are under 45, come more often than every 21 days or less often than every 35 days, last longer than 7 days, vary by 20 days or more between your shortest and longest cycle, or are irregular while you are trying to get pregnant. See one too if your periods stop for more than three months and you are not pregnant, or are heavy or painful enough to affect daily life. Irregular cycles are common in the first years after periods start and in the years before menopause.

See a doctor too about bleeding between periods or after sex, any bleeding after menopause, severe pain, or heavy bleeding: soaking a pad or tampon every 1 to 2 hours, passing clots larger than about 2.5 cm (1 in), or feeling tired and breathless with heavy periods.

If a period is late and you could be pregnant, take a pregnancy test.

This is an estimate for general information, not medical advice. Talk to a doctor or other qualified clinician before making health decisions based on it.

Sources

Average cycle of 28 days, common range about 21 to 40 days, periods lasting 2 to 7 days: NHS, “Periods” (nhs.uk).

When to see a GP about cycle length, duration, variation and bleeding: NHS, “Periods”, “Irregular periods”, “Heavy periods”, “Stopped or missed periods” and “Bleeding after the menopause” (nhs.uk).

Normal limits in adults (frequency 24–38 days, duration up to 8 days): Munro MG, Critchley HOD, Fraser IS. “The two FIGO systems for normal and abnormal uterine bleeding symptoms and classification of causes of abnormal uterine bleeding in the reproductive years: 2018 revisions.” International Journal of Gynecology & Obstetrics 2018;143:393–408.

Wider normal range in adolescents: ACOG Committee Opinion No. 651, “Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign” (2015).

Six-day fertile window ending on ovulation day: Wilcox AJ, Weinberg CR, Baird DD, New England Journal of Medicine 1995;333:1517–1521.

Fertility awareness should be learned from a trained teacher: NHS, “Natural family planning (fertility awareness)” (nhs.uk).