Cycle Length Calculator
Enter the first day of your last 3 to 6 periods to work out your average cycle length. The calculator also shows your shortest and longest cycle and whether the pattern counts as regular.
Free, no sign-up. What you enter stays in your browser: it is not saved or sent to HerFound. Reviewed .
Enter the first day of each period, 3 to 6 in a row. Any order is fine.
Enter the first day of at least 3 periods to see your average cycle length.
This is general information, not medical advice. It cannot take your own health history into account. For anything that worries you, speak to a doctor, midwife or pharmacist.
How it works
A cycle is counted from the first day of one period to the first day of the next. The calculator sorts your dates and counts the days between each pair. Three dates give 2 cycles, and six dates give 5.
Average cycle length = total days from the first date to the last date, divided by the number of cycles. Variation = longest cycle minus shortest cycle.
The pattern is called regular when every cycle is between 21 and 35 days and the variation is 7 days or less. The 21 to 35 day range is the one used by the NHS and ACOG. ACOG describes periods as irregular when cycle length varies by more than 7 to 9 days, so a variation of 8 or 9 days is shown as borderline and 10 days or more as varying.
What it cannot tell you
- A few cycles are a small sample. One unusual month, from stress, illness or travel, can change the result. More dates give a steadier average.
- It only works if no period is missing from the list. A skipped date makes one cycle look twice as long.
- It is not a diagnosis. Cycles outside 21 to 35 days are common in the first years of having periods, after pregnancy, while breastfeeding and near menopause, and with hormonal contraception.
- The NHS advises seeing a doctor if your periods are irregular, last longer than 7 days, or if you bleed between periods.
Apps that do this for you
A calculator answers once. These apps keep track over time. Compare all period tracking apps.
Clue
A period tracker: cycle tracking with predictions for your period, PMS and ovulation.
Cycles
A period tracker for iPhone: predict your periods and fertile days and share your cycle phases with a partner.
Euki
A period tracker: log your cycle and sexual health, with your entries kept on your phone.
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Common questions
To calculate your cycle length, count the days from the first day of one period to the first day of the next. If one period started on June 1 and the next on June 29, that cycle was 28 days. For an average, do this for several cycles, add them up and divide by the number of cycles.
A normal menstrual cycle is between 21 and 35 days long, according to both the NHS and ACOG. The average is about 28 days. A cycle that is always 24 days or always 33 days is still normal.
Cycle length can vary by up to about 7 days and still be regular. ACOG describes periods as irregular when the length of the cycle varies by more than 7 to 9 days. A cycle that changes by a few days from month to month is very common.
Yes, day 1 of the menstrual cycle is the first day of your period. The last day of the cycle is the day before your next period starts. Light spotting before the period starts properly is usually not counted as day 1.
See a doctor if your periods come more often than every 21 days or less often than every 35 days, or if your usual pattern changes. The NHS also advises seeing a doctor if periods last longer than 7 days or you bleed between periods. Irregular periods are often not a sign of a problem, but they are worth checking.