Cycle Length Calculator

Free Cycle Length Calculator

Use this free cycle length calculator to work out your average menstrual cycle length from your last four period start dates. It also shows your shortest and longest cycle, the variation between them, and whether your cycles are considered regular.

Your cycle length is counted from the first day of one period to the first day of the next. Tracking several cycles rather than just one gives a far more accurate picture of your average, since occasional short or long cycles are normal and don't necessarily mean something is wrong.

How to Use the Cycle Length Calculator

• Enter the start date of your most recent period as Date 1.
• Enter your three previous period start dates as Date 2, 3, and 4, in order from most recent to oldest.
• Click Calculate Cycle Length.
• View your average, shortest, and longest cycle length, plus a regularity assessment.

Features

• Calculates the length of each individual cycle between your entered dates.
• Shows your average, shortest, and longest cycle length.
• Flags whether your cycles are regular or irregular.
• Mobile-friendly with print, share, copy, and download options.

Frequently Asked Questions

How is cycle length calculated?

Cycle length is the number of days from the first day of one period to the first day of the next period. This calculator computes that for each pair of consecutive dates you enter.

What counts as a regular cycle?

Most clinicians consider cycles regular if the length varies by 7 days or less from cycle to cycle. Larger swings may still be normal for some people, but can be worth discussing with a doctor.

Why do I need to enter 4 dates instead of 2?

Two dates only give you one cycle length, which may not represent your average. Four dates give three cycle lengths, letting you see your average and how much your cycles naturally vary.

Is this calculator free?

Yes, this cycle length calculator is completely free to use with no sign-up required.

Method and Sources

Cycle length is counted from the first day of one period to the first day of the next. This calculator measures each of the three cycles between the four dates you enter, then shows the average, shortest, and longest cycle and the variation between them.

Cycles between about 24 and 38 days are generally considered typical for adults. If your periods come more often than every 24 days, less often than every 38 days, or suddenly become irregular, it is worth talking to a healthcare professional.

Sources:

• U.S. Office on Women's Health — Your Menstrual Cycle
• Mayo Clinic — Menstrual Cycle: What's Typical, What's Not

This calculator is for general information only and is not medical advice. Read our Calculation Methodology and medical disclaimer.

Related Calculators

Want a forecast of your next periods and fertile window? Try our menstrual cycle calculator. Focused specifically on ovulation? Use the ovulation calculator. Need a day-by-day fertility breakdown? Check the fertile window calculator. Trying to figure out when you conceived? Use the conception date calculator. Already expecting? Try our pregnancy due date calculator. Want to check whether your luteal phase is short, normal, or long? Try the luteal phase calculator.