Period Calculator
Period calculator: enter the first day of your last period and your usual cycle length to predict your next 6 periods, with a calendar and fertile days.
Written by HealthSensibly Editorial TeamReviewed by the HealthSensibly editorial teamUpdated
Predicting when your next period starts
Add your cycle length to the date your latest period started. A cycle runs from the first day of bleeding to the first day of the following period; for most adults that is somewhere from 21 to 35 days, with 28 days being just an average. Say your last period began on 3 October and you have a 30-day cycle — your next one would be expected around 2 November.
What counts as a typical cycle?
| Usual range | |
|---|---|
| Cycle length (adults) | 21–35 days |
| Cycle length (teens) | 21–45 days |
| Days of bleeding | 2–7 days |
| Change from one cycle to the next | up to 7–9 days is common |
Reasons a period may come late
- A possible pregnancy — do a test once you are more than a few days overdue
- Stress, being unwell, travelling or disrupted sleep
- Losing or gaining weight, or very hard training
- Conditions such as PCOS or a thyroid disorder
- Breastfeeding, or coming off hormonal contraception
- The run-up to menopause (most often in your 40s)
Speak to a doctor if you go 3 months without a period (and are not pregnant), if periods arrive more often than every 21 days, bleed for more than 7 days, or feel very painful or unusually heavy. Cycles all over the place? Look at our PCOS diet plan, and if you hope to conceive, try the ovulation calculator.
Common questions
Will the predicted dates be right every month?
They work well when your cycle is regular. If it shifts by more than a week from one month to the next, use the dates as a loose guide and enter your newest period start each month.
My cycle is 35 days long — is that a problem?
For adults, anything from 21 to 35 days is within the normal range. If your cycles are longer than this or vary a lot, mention it to your doctor.
At what point is a late period worth checking?
Running up to roughly 7 days late is fairly common. Beyond that, do a pregnancy test; if it is negative and you still have no period, see a doctor.
Does the calculator also show fertile days?
Yes, it gives an estimated fertile window for each cycle, but this is only a guide and should not be used as contraception.
References: ACOG: Menstruation in girls and adolescents · NHS: Periods
Checked against the sources listed above. This is general health information and not medical advice — please ask your doctor or a registered dietitian what suits you.
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