Period Calculator NHS

This independent UK period calculator estimates your next period dates from the first day of your last period and your usual cycle length. It is a planning tool, not an NHS diagnostic or contraception tool.

What is a period calculator?

A period calculator projects future menstrual dates using your recent cycle pattern. NHS guidance defines the menstrual cycle as the time from the first day of one period to the day before the next period, with the first day of bleeding counted as cycle day 1.

How to use it

1. Enter the first day of your most recent period.
2. Enter your usual cycle length, counted from one period start to the next.
3. Enter how many days your period usually lasts.
4. Select Calculate to see the next estimated period and two later start dates.

Formula and methodology

The basic estimate is: next period start = last period start + usual cycle length. Later dates repeat the same interval. This is a calendar estimate, not a biological certainty.

The NHS says the average cycle is around 28 days and regular cycles from 21 to 35 days can be normal. Periods commonly last about 2 to 7 days. Cycle timing varies between people and can also vary from month to month.

Worked example

If a period started on 1 September and the usual cycle is 28 days, the next estimated start is 29 September. With a usual five-day period, the estimated bleeding window is 29 September to 3 October.

Can this predict ovulation?

This calculator focuses on period dates. The NHS explains that ovulation is difficult to pinpoint and commonly occurs around 10 to 16 days before the next period. Do not use calendar estimates as your only method of contraception. For fertility timing, use our dedicated ovulation calculator.

When period predictions may be less reliable

Predictions can be less useful when cycles are irregular, during puberty or around menopause, after pregnancy, while breastfeeding, or when hormonal contraception or health changes affect bleeding patterns. NHS advice says to speak to a GP if your periods are irregular or your normal pattern changes.

Authoritative sources

• NHS: periods and fertility in the menstrual cycle
• NHS: irregular periods
• NHS: periods overview

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FAQs

How do I calculate my next period?

Add your usual cycle length to the first day of your last period. The result is only an estimate because cycle length can change.

Is a 28-day cycle required?

No. The NHS says 28 days is the average, while regular cycles from 21 to 35 days can be normal.

What counts as day 1?

Day 1 is the first day of your period.

Can I use this calculator to avoid pregnancy?

No. Calendar predictions should not be relied on as the only method of contraception because ovulation and cycle timing can vary.

Medical disclaimer: This calculator provides general calendar estimates only. It does not diagnose pregnancy, fertility problems or menstrual conditions and does not replace advice from a GP or other qualified healthcare professional.