Fertility · Cycle
Cycle length average and regularity.
Type the length of each of your recent cycles, in days, and see your mean, range and standard deviation.
Inputs
Result
Enter your numbers.
Cycle Length Average and Regularity Calculator: how it works
Your average cycle length is the sum of your logged cycles divided by how many you logged. The calculator also gives the median, shortest, longest, range and sample standard deviation. For example, cycles of 28, 30, 27, 33, 29 and 31 days average 29.7 days with a range of 6 days and a standard deviation of 2.16 days.
A cycle is counted from the first day of one period to the day before the next period starts. The standard deviation uses the sample formula (divide by n - 1), so it needs at least two cycles. The NHS says the average gap between periods is around 28 days and that periods are irregular if the gap is less than 21 days or more than 35 days, so the tool counts how many of your cycles fall outside 21 to 35 days. It does not apply any other regularity cut-off, because this page did not find a single published standard for a standard-deviation threshold.
If you add the first day of your last period, the tool projects your next period from the average and shows the range from your shortest to your longest cycle. That is a planning estimate only; periods can arrive outside that span.
Worked example. Cycles of 28, 30, 27, 33, 29 and 31 days total 178 days, so the mean is 178 / 6 = 29.7 days. The sorted middle two values are 29 and 30, so the median is 29.5. The range is 33 - 27 = 6 days and the sample standard deviation is 2.16 days. None is outside 21 to 35 days. If your last period began on 20 September 2026, the average-based next period is 20 October 2026 (20 September plus 30 days, the average rounded to the nearest day).
Frequently asked questions
What is a normal menstrual cycle length?
The NHS says the average gap between periods is around 28 days, and that periods are irregular if the gap is under 21 days or over 35 days.
How is cycle length counted?
From the first day of bleeding of one period to the day before the first day of the next. A 28 day cycle means the next period starts on day 29.
How many cycles do I need to enter?
The tool works from 2 cycles, but averages from more cycles are steadier. Use your last 6 or more if you have them.
What does the standard deviation tell me?
It shows how far your cycle lengths typically sit from your average. A smaller number means your cycles are closer to the same length. This page does not set a threshold for what counts as regular.
When should I see a clinician about my cycle?
The NHS advises speaking to a GP if your periods are irregular, your usual pattern changes, your periods last longer than 7 days, or you have bleeding between periods.
Sources
Checked
Runs entirely in your browser. Nothing you type is sent or stored.
Not medical advice
Not medical advice. This tool does not diagnose anything and cannot tell you whether a pregnancy is healthy or whether a cycle is normal for you. See your clinician for heavy or unusual bleeding, pain, dizziness, or any symptom that worries you, and get urgent care for severe pain or very heavy bleeding.