Period Calculator
When your next periods are due, for the next six cycles.
Good to know. Only about one cycle in eight is exactly 28 days. Most vary by a few days, and that is normal.
Not medical advice. Only your midwife or doctor can date a pregnancy.
How it works
In plain words
Add your cycle length to the first day of your last period to get the next one, and keep adding it for the periods after. Ovulation is usually about 14 days before a period starts.
The formula
next period = last period + cycle length period ends = start + period length − 1 ovulation ≈ next period − 14 days
Worked example
A period that started on 1 March 2026, in a 28-day cycle, means the next one is due on Sunday, March 29, 2026, and likely lasts until 2 April.
Sources
- Bull et al., Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles, npj Digital Medicine, 2019 About 13% of cycles are exactly 28 days; most vary from cycle to cycle.
- ACOG, Heavy and Abnormal Periods Normal cycle length of 21 to 35 days, and when to see a doctor.
Sources checked September 30, 2026. 4 automated tests check its answers on every change.
About the period calculator
Knowing when your next period is due helps you plan, notice changes and tell a doctor about your cycle. Enter the first day of your last period and your usual cycle length, and the calculator shows when the next six are likely to start and end.
A cycle is counted from the first day of one period to the day before the next. 28 days is the textbook average, but anything from 21 to 35 days is normal for adults, and teenagers’ cycles can be longer. Most people’s cycles vary by a few days from month to month, so treat the dates as a guide, and use the average of your last few cycles for the best estimate.
A period is usually considered late when it is more than about a week past the expected day. Stress, illness, travel, weight change, exercise and breastfeeding can all delay it. If you might be pregnant, a home pregnancy test is reliable from the first day of a missed period.
See a doctor if your cycles are regularly shorter than 21 days or longer than 35, if periods stop for three months or more, or if bleeding is very heavy or lasts more than seven days. This calculator can’t be used as contraception.
Questions people ask
How do I calculate when my next period is due?
Add your usual cycle length to the first day of your last period. With a 28-day cycle starting on 1 March, the next is due on 29 March.
When is a period late?
Usually when it is more than about a week past the expected date. A few days either way is normal.
What is a normal cycle length?
21 to 35 days for adults, counted from the first day of one period to the day before the next.