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Period Calculator

Predict your next periods: the next six start and end dates from your last period and cycle length, with a window for irregular cycles.

Knowing when your next period is likely to start helps you plan holidays, events and sport, keep supplies at hand, and notice when something changes. This calculator predicts your next six periods from the first day of your last period and your usual cycle length, and shows how many days your period usually lasts.

Cycles are rarely exactly the same every month, so you can also say how much yours usually varies. The calculator then shows a window for each period — the earliest and latest it is likely to start — which widens a little for periods further ahead. You also see which day of your cycle you are on today. Your dates are private: they are not stored, sent anywhere, or added to the page address.

How to use Period Calculator

  1. Enter the first day of your last period.
  2. Enter your average cycle length and how many days your period lasts.
  3. Enter how many days your cycle usually varies.
  4. Read your next period date, today's cycle day and the next six periods.

Features

  • Next six periods with start and end dates.
  • Earliest–latest window for irregular cycles.
  • Today's cycle day.
  • Days until the next period, or days late.
  • Private: dates are not stored, sent or added to the address.
  • Runs entirely in your browser.

How the dates are predicted

A cycle runs from the first day of one period to the first day of the next. The calculator adds your average cycle length to the first day of your last period to find the next start date, and repeats that for six cycles. Each period's end date is its start plus the number of days your period lasts. If your last recorded period was a few cycles ago, it moves forward to the current cycle automatically. Because small differences add up, the window for later periods is wider: the variation you enter is applied once for the next period, twice for the one after, and at most three times.

What is a typical cycle?

Adult cycles vary from person to person and from month to month. These ranges are widely used by gynaecologists as a guide to what is common.

MeasureCommon range
Cycle length21–35 days (average about 28)
Period length2–7 days
Variation between cyclesup to about 7–9 days
Teenagers in the first years21–45 days, often irregular

Tracking to improve the prediction

The prediction is only as good as the average you enter. Write down the first day of each period for three to six months, work out the number of days between them, and use the average as your cycle length and the spread as your variation. Stress, illness, travel, changes in weight or exercise, breastfeeding and hormonal contraception can all shift a cycle. If you use hormonal contraception, the bleeding you have is usually a withdrawal bleed set by the pill schedule rather than a natural period.

When to see a doctor

See a doctor or nurse if your periods are often closer than 21 days or further apart than 35 days, last more than 7 days, are very heavy or painful, stop for three months or more without pregnancy, or if you bleed between periods or after sex. These predictions are estimates and must not be used as contraception. To estimate fertile days, use the Ovulation Calculator, keeping in mind that it is also an estimate.

Frequently asked questions

How do I calculate my next period?

Add your average cycle length to the first day of your last period. The calculator does this for the next six cycles.

What if my periods are irregular?

Enter how many days your cycle usually varies; each prediction then shows an earliest and latest start date.

What is a normal cycle length?

21 to 35 days for most adults, with 28 days as the average.

Can I use this as contraception?

No. Predictions are estimates and are not a method of contraception.

Is my information private?

Yes. Dates stay in your browser and are not stored, sent or added to the page address.

My period is late — what should I do?

Cycles vary, but if it is more than a week late and you could be pregnant, take a pregnancy test; see a doctor if periods stop for three months.

Related tools

Ovulation CalculatorFind your most fertile days, estimated ovulation date and next period for the next six cycles, with a calendar of your current cycle.Pregnancy Due Date CalculatorEstimate your baby's due date from your last period, conception date or IVF transfer, with how many weeks pregnant you are and key dates.Gestational Age CalculatorHow many weeks pregnant are you? Gestational age in weeks and days on any date, from your last period, due date or conception date.Date Difference CalculatorCount the days between two dates in years, months, weeks and days, or add and subtract days, weeks, months and years from a date.Pregnancy Weight Gain CalculatorRecommended pregnancy weight gain from your pre-pregnancy BMI, the healthy range for your current week, and how your gain compares.Sleep CalculatorHow much sleep do you get, and is it enough for your age? Plus the best bedtimes and wake-up times for full 90-minute sleep cycles.