Period calculator: when is my next period?
Enter the first day of your last period and how long your cycle usually lasts. You'll see your next three periods on a calendar and, if your period is late, by how many days.
A period calculator counts forward from the first day of your last period by the length of your cycle. It's an estimate: cycles shift by a few days for many reasons. Use it to plan ahead, to see when a period is late and to notice changes worth raising with a doctor. If your cycles vary, choose "My cycle varies" and enter your shortest and longest cycle to get a window instead of a single day.
How the period calculator works
The calculator adds your average cycle length to the first day of your last period to find the day your next period should start, then repeats it for the following cycles. A cycle is counted from the first day of one period to the first day of the next. Most cycles last 21 to 35 days, and a period usually lasts 2 to 7 days (NHS, Periods; ACOG, Abnormal Uterine Bleeding). If your cycles vary, enter your shortest and longest cycle instead: the calculator then shows a window rather than a single day.
The dates are estimates. Stress, illness, weight change, a lot of exercise, breastfeeding and some hormonal contraceptives can all move a period (NHS, Missed or late periods). If your period is late, the calculator counts the days and shows when a home pregnancy test is most reliable. It is not a method of contraception: calendar methods are among the least effective ways to avoid pregnancy (WHO, Family Planning: A Global Handbook for Providers). This tool is for general information and is not medical advice.
Common questions
Add your cycle length to the first day of your last period: if your cycles usually last 28 days and your last period started on October 1, the next one should start around October 29. The calculator does this for you and repeats it for the next three cycles. The more regular your cycles, the closer the estimate.
Most cycles last 21 to 35 days, 28 on average, and a period usually lasts 2 to 7 days (NHS). In the first years after a first period, cycles of up to 45 days are common (ACOG). Talk to a doctor if your cycles are often shorter than 21 or longer than 35 days, or if their length changes by more than 7 to 9 days from one cycle to the next (ACOG).
Most home tests work from the first day of a missed period, and the result is most reliable a week or more after it (NHS; FDA). If the test is negative and your period still hasn't come, test again a few days later, and see a doctor if the second test is negative too (NHS). Our when-to-take-a-test calculator gives you the dates.
Stress, sudden weight loss or being overweight, a lot of exercise, some hormonal contraceptives, breastfeeding, polycystic ovary syndrome and the start of the menopause can all delay or stop periods (NHS). See a doctor if you've missed three periods in a row or your periods have become irregular (NHS).
No. It predicts periods, not the days you can get pregnant, and ovulation can shift from cycle to cycle. With the calendar method as commonly used, about 15 in 100 women become pregnant within a year (WHO). Talk to a health professional about contraception. If you're trying to conceive, our ovulation calculator estimates your most fertile days.
Read more
The dates and health statements on this page follow these sources.
- NHS — Periods
- NHS — Missed or late periods
- ACOG — Abnormal Uterine Bleeding
- ACOG Committee Opinion 651 — Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign
- WHO — Family Planning: A Global Handbook for Providers (2022)
- NHS — Doing a pregnancy test
- FDA — Home Use Tests: Pregnancy
Medical Disclaimer
This tool is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or qualified health provider with any questions you may have regarding a medical condition.
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