Period Calculator
Enter the first day of your last period and your usual cycle length to see when your next period is likely to start. The calculator also lists the start and end dates of your next 6 periods.
Free, no sign-up. What you enter stays in your browser: it is not saved or sent to HerFound. Reviewed .
First day of one period to the first day of the next. Use 28 if you are not sure.
How many days you usually bleed.
Enter the first day of your last period to see when your next period is due.
This is general information, not medical advice. It cannot take your own health history into account. For anything that worries you, speak to a doctor, midwife or pharmacist.
How it works
The calculator uses calendar counting: next period = first day of your last period + your usual cycle length. A cycle runs from the first day of one period to the day before the next one starts.
Each later period is one more cycle length after the one before it. The end date of a period is its start date plus your usual period length, minus one day, because the first day of bleeding counts as day 1.
If you do not know your cycle length, 28 days is the average. The NHS describes cycles from 21 to 35 days as normal, and periods as lasting 2 to 7 days, usually about 5.
What it cannot tell you
- It is a prediction from averages. Most people's cycles change by a few days from month to month, so a period that comes a little early or late is common.
- The dates further ahead are less certain, because each small change in a cycle moves every date after it.
- It does not work well if your periods are irregular, if you use hormonal contraception, or if you are pregnant, breastfeeding or close to menopause.
- It cannot tell you why a period is late or missing. If you have missed a period and could be pregnant, take a pregnancy test. If your periods stop or change a lot, speak to a doctor.
Apps that do this for you
A calculator answers once. These apps keep track over time. Compare all period tracking apps.
Clue
A period tracker: cycle tracking with predictions for your period, PMS and ovulation.
Cycles
A period tracker for iPhone: predict your periods and fertile days and share your cycle phases with a partner.
Euki
A period tracker: log your cycle and sexual health, with your entries kept on your phone.
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Common questions
To calculate your next period, add your usual cycle length to the first day of your last period. With a 28-day cycle and a last period that started on November 1, the next period is expected on November 29. Count the first day of bleeding as day 1.
There are about 28 days between the start of one period and the start of the next, on average. The NHS says anything from 21 to 35 days is normal. The gap is counted from the first day of one period to the first day of the next, not from the day bleeding stops.
A period usually lasts between 2 and 7 days, and about 5 days is typical. The bleeding is often heaviest in the first 2 days. The NHS advises seeing a doctor if your periods last longer than 7 days.
A period that is a few days late is common and is usually not a sign of a problem. Stress, illness, travel and weight changes can all delay a period. If your period is late and you could be pregnant, take a pregnancy test, and speak to a doctor if you miss 3 periods in a row.
Yes, a period calculator can be wrong by several days. It assumes every cycle is the same length, and real cycles vary. It is most useful for planning ahead when your periods are regular.