How to find your cycle length
Count from the first day of one period to the day before the next period starts. Track 3–6 months and take the average. A normal adult cycle is 21 to 35 days, and a period usually lasts 2–7 days.
When to see a doctor
- Periods that stop for 3 months or more (and you are not pregnant)
- Cycles shorter than 21 days or longer than 35 days
- Very heavy bleeding or periods longer than 7 days
- Severe pain that stops daily activities
Frequently asked questions
Why is my period late?
Stress, travel, illness, weight changes, PCOS, thyroid problems and pregnancy can all delay a period.
Is a 35-day cycle normal?
Yes, cycles from 21 to 35 days are normal for adults.
Sources
- American College of Obstetricians and Gynecologists. Methods for estimating the due date (Committee Opinion 700), 2017. Link
Medical disclaimer: This calculator gives estimates for general information only. It is not a diagnosis or a substitute for advice from a doctor or registered dietitian. Please consult a healthcare professional before making changes to your diet, exercise or medicines, especially if you are pregnant, have a medical condition or take regular medication.
Last updated: 20 October 2026. Read our editorial policy.
