Period Calculator
Enter the first day of your last period, your cycle length and how long your period lasts to see when your next period should start and end, which day of your cycle you are on, and an estimated period calendar for the coming months.
- Expected last dayMonday 23.11.2026
- Cycle day11
- Cycle length (FIGO)Normal (24–38 days)
- Period length (FIGO)Normal (up to 8 days)
Estimated period calendar
How it is calculated
The International Federation of Gynecology and Obstetrics (FIGO) defines normal menstruation from the 5th–95th percentiles of large population studies (FIGO AUB System 1). At ages 18–45 a normal cycle is 24–38 days; a cycle shorter than 24 days is frequent and one longer than 38 days infrequent menstruation. A period lasting more than 8 days is prolonged. Regularity depends on age: the gap between the shortest and longest cycle is at most 7 days at ages 26–41 and at most 9 days at 18–25 and 42–45. In the first year after the first period (menarche) cycles usually range from 21 to 45 days, and FIGO notes that a person’s own regular cycle may not settle until the sixth year after menarche. The tool repeats the average cycle you enter every month; since real cycles vary by about these amounts, the dates are estimates. This page is for information only, is not medical advice and cannot be used to prevent pregnancy.
Start of period k = First day of last period + k × Cycle length; End = Start + Period length − 1Cycle length: days from the first day of one period to the first day of the next. Period length: the number of bleeding days. Day 1 of the cycle is the day bleeding starts; today is cycle day (today − last period + 1).- Add the cycle length to the first day of the last period: the expected start of the next one.
- Add the period length minus one to that start: the expected last day of bleeding.
- Keep adding the cycle length for the following cycles.
- If the expected day has passed, the difference is how late the period is.
Example
If the last period started on 18.10.2026, with a 28-day cycle and a 5-day period: the next starts on 18.10.2026 + 28 days = Sunday 15.11.2026 and ends on 15.11.2026 + 4 days = 19.11.2026; the one after runs 13.12.2026–17.12.2026. On 28.10.2026 you are on cycle day 11.
Frequently asked questions
How do I work out my cycle length?
Count the days from the first day of one period to the first day of the next, counting the first day but not the new period’s first day. Entering the average of several cycles gives a better estimate.
How long is a normal menstrual cycle?
Under FIGO, 24–38 days at ages 18–45, with bleeding lasting no more than 8 days. In the first year after the first period, cycles of 21–45 days are common too.
When should I see a doctor?
Under FIGO’s definitions of abnormal uterine bleeding, these should be checked: a cycle shorter than 24 or longer than 38 days, cycles that differ by more than 7–9 days depending on your age, a period lasting more than 8 days, heavy bleeding that affects your daily life, bleeding between periods, and no period for 6 months in a row. Also see your doctor if your period is late and you could be pregnant.
Are there “safe days” on the calendar?
This tool only estimates period dates; it is not a method of contraception. Ask your doctor or a family planning service about birth control. For an estimate of ovulation, use the ovulation calculator.
What does the tool show if my period is late?
If today’s date is past the expected day, the result shows that day and how many days late the period is. Once your period comes, enter its first day as the new last period.
Source
- Munro MG et al. (FIGO), The FIGO ovulatory disorders classification system, Int J Gynaecol Obstet 2022;159(1):1–20, doi:10.1002/ijgo.14331 — section 6.1.1 (normal cycle frequency, regularity by age, cycles after menarche, secondary amenorrhoea) and glossary (prolonged, heavy and intermenstrual bleeding)
- Munro MG, Critchley HOD, Fraser IS (FIGO), 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, Int J Gynaecol Obstet 2018;143(3):393–408, doi:10.1002/ijgo.12666 — full text, University of Helsinki repository; Fig. 1 footnote (cycle variation 18–25 y ≤9 days, 26–41 y ≤7 days, 42–45 y ≤9 days)
- Munro MG, Critchley HOD, Fraser IS (FIGO), FIGO AUB Systems 1 and 2: 2018 revisions, Int J Gynaecol Obstet 2018;143(3):393–408, doi:10.1002/ijgo.12666 (original source of the limits)