Period Cycle Length Calculator
Measure your exact cycle intervals from consecutive period start dates to understand whether your cycle is normal, short, long, or variable.
Enter Consecutive Period Dates
Enter at least 2 period start dates (most recent first).
How to Interpret Your Cycle Length
- Short Cycles (< 21 days): May indicate rapid follicular phases or luteal phase defect.
- Normal Cycles (21–35 days): Standard ovulatory biological interval.
- Long Cycles (> 35 days): Frequently associated with PCOS or delayed ovulation.
When to See a Doctor
Online calculators provide general estimates based on biological averages. Consult a healthcare provider (such as an OB/GYN or general practitioner) if you notice:
- Your periods suddenly become irregular after being consistent
- Your cycle is consistently shorter than 21 days or longer than 35 days
- Your period bleeding lasts longer than 7 days or is unusually heavy (soaking through a pad/tampon every hour)
- You experience severe, debilitating pelvic pain or cramping that disrupts daily life
- You experience bleeding between periods or after intercourse
- You are under 35 and have been trying to conceive for 12 months (or 6 months if 35+)
- Your period has stopped completely for 90 days or more (and you are not pregnant)
Frequently Asked Questions
Evidence-based answers to common questions about your cycle and fertility.
How do you accurately measure cycle length?
Count the number of calendar days starting from the first day of menstrual bleeding (Cycle Day 1) up to and including the day before your next period begins.
What is a normal cycle length?
A normal adult menstrual cycle is between 21 and 35 days long (average: 28 days). For adolescents, 21 to 45 days is standard.
Accurate Period Calculator
Enter 3 to 13 past period start dates to calculate your weighted average cycle length and personalized prediction window with high accuracy.
Period Calculator for Irregular Periods
Predict your next period range and fertile window using your shortest and longest cycle lengths. Designed for irregular cycles and PCOS.
Menstrual Cycle Calculator
Explore a complete breakdown of all four menstrual cycle phases, estimated hormone rhythms, and next period dates with interactive charts.
Medical & Scientific Sources
- American College of Obstetricians and Gynecologists (ACOG)(2020). "Using the Menstrual Cycle as a Vital Sign"