Cycle length calculator
Your cycle length is the number of days from the first day of one period to the first day of the next. Enter two or more period start dates and the calculator counts the days between each pair, averages them, and shows your shortest and longest cycle and how much one cycle typically differs from the next. For most adults, cycles of 21 to 35 days are typical, and a few days of change from month to month is common.
Nothing you enter leaves your browser: the calculation runs on your device and nothing is stored.
Worked example
Your last five periods started on 5 June, 3 July, 2 August, 29 August and 28 September 2026.
| Cycle | From | To | Length |
|---|---|---|---|
| 1 | 5 June | 3 July | 28 days |
| 2 | 3 July | 2 August | 30 days |
| 3 | 2 August | 29 August | 27 days |
| 4 | 29 August | 28 September | 30 days |
- Average: (28 + 30 + 27 + 30) ÷ 4 = 28.8 days, about 29 days.
- Shortest 27 days, longest 30 days, all inside the typical 21–35 day range.
- Variation: the differences between consecutive cycles are 2, 3, 3 days, so the average change from one cycle to the next is (2 + 3 + 3) ÷ 3 = 2.7 days.
- Next period: 28 September 2026 + 29 days = around 27 October 2026.
How the calculation works
The calculator sorts your dates, counts the days between each start date and the next (that is one cycle), and averages those cycle lengths. Your shortest and longest cycle show the range you have actually had. The variation is the mean absolute difference between consecutive cycles, the same measure the Lunation app uses: for each pair of back-to-back cycles it takes the difference in days, ignoring whether it got longer or shorter, and averages those differences. It needs at least three dates (two cycles).
If two dates are less than 10 days apart, they are probably from the same period, and the calculator points that out. If any cycle is shorter than 21 days or longer than 35, it adds a neutral note: one unusual cycle now and then is common, and a regular pattern outside that range is worth mentioning to a doctor.
How accurate is this?
The arithmetic is exact; what it tells you depends on the dates. A few recent cycles describe your current pattern better than many old ones, because cycle length changes with age: in Bull and colleagues’ analysis of 612,613 cycles, average cycle length fell by about 0.18 days per year of age between 25 and 45, and one person’s cycles typically varied with a standard deviation of about 2.6 days.
ACOG and the NHS describe 21 to 35 days as the typical range for adults, and the NICHD notes that in teens cycles range from 21 to 45 days. The NHS says irregular periods are not usually a sign of a problem, but suggests speaking to a GP if your periods are irregular or your usual pattern changes.
Frequently asked questions
What is a typical cycle length?
For most adults, a cycle of 21 to 35 days is typical, according to ACOG and the NHS. In the first years after periods start, cycles of 21 to 45 days are common. Few people have exactly 28 days every time.
How many period dates should I enter?
Two start dates give one cycle, which is enough for a first estimate. Four to six recent dates give a steadier average and a meaningful variation figure. The Lunation app averages your last five completed cycles.
What does the variation number mean?
It is the average difference in days between one cycle and the next. A variation of 2 means your cycles usually differ from the previous one by about two days, which is common. It tells you how wide a window to expect around a predicted period.
Which day should I count as the start of my period?
Day 1 is the first day of bleeding, so enter that date for each period. Light spotting before your flow starts is usually not counted.
When is it worth talking to a doctor about cycle length?
If your cycles are often shorter than 21 days or longer than 35, if the gap between periods changes a lot from month to month, or if your periods last longer than 7 days, it is worth mentioning to a doctor. The NHS also suggests seeing a GP if your usual pattern changes.
Sources
- Abnormal Uterine Bleeding (FAQ): what is a normal menstrual cycle?. American College of Obstetricians and Gynecologists (ACOG).
- Irregular periods. NHS.
- About Menstruation. Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD, NIH).
- Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (Bull et al., 2019). npj Digital Medicine.
Written by the Lunation editorial team.
Related guides
- How Long Is a Normal Menstrual Cycle?Most cycles last 21 to 35 days, and few are exactly 28. What large studies show about cycle length by age, how much variation is typical, and when to ask.
- How to Predict Your Next PeriodA simple method to estimate your next period: average your recent cycles, add it to your last start date, and plan for a window of a few days.
- What Are the Phases of the Menstrual Cycle?The four phases of the menstrual cycle explained: typical days, the hormones behind each one, and why real cycles rarely follow the 28-day textbook.
More calculators
- Period calculatorEstimate your next six periods, with start and end dates, from your last period, cycle length and period length.
- Ovulation calculatorEstimate your ovulation day (about 14 days before your next period) and six-day fertile window for the next three cycles.
- Due date calculatorEstimate a due date, current pregnancy week and trimester from your last period (280 days) or conception date (266 days).
Medical disclaimer. This calculator gives general wellness estimates, not medical advice. It does not diagnose or treat any condition and is not a method of contraception. If something about your cycle or your health worries you, talk to a doctor or midwife.