Written by the Cycella editorial team · Last reviewed October 11, 2026
Published October 11, 2026

How to count your cycle and work out your next period

The short answer

Day 1 of your cycle is the first day of full period bleeding. Your cycle length is the number of days from that day 1 to the day before your next day 1. To estimate your next period, add your usual cycle length to your last day 1. That is all you need.

Which day is cycle day 1?

Count the first day you have real flow, meaning bleeding that needs a pad, cup or tampon. Then:

• Light spotting before the flow: do not count it. Day 1 is the first day of proper flow.
• Starts at night: there is no official rule, so pick one and keep to it. The simple rule is to count the date you first noticed real bleeding. Our guide on what counts as day 1 of your period has more.
• Bleeding in the morning: that morning is day 1.
• The last day of bleeding does not matter for cycle length. It only tells you how long the period lasted (usually 2 to 7 days).

Pick the rule and stick with it every month. Consistency matters more than perfect precision.

How to calculate cycle length

Cycle length = number of days from day 1 of one period to the day before day 1 of the next.

A simple way: count the days between the two start dates. If a period started on 1 March and the next on 29 March, the gap is 28 days, so the cycle was 28 days.

• Adults usually have cycles of 21 to 35 days (NHS)
• Teens in the first years can have 21 to 45 days
• The average is about 28, but average is not a rule

A worked example

Say your last four periods began on these dates:

Average cycle length: (28 + 28 + 30) / 3 = about 29 days.

Your last day 1 was 26 September. Add 29 days: 25 October is the estimated next period. Because cycles shift by a few days, it is wiser to think of it as a window of about 24 to 27 October than a single day.

The formula

Next period = day 1 of your last period + your average cycle length.

• Use at least three cycles to find your average, if you can
• If your cycles vary, use the shortest and longest for an early and a late date
• Ovulation is usually about 14 days before the next period, so for a 29-day cycle it falls around day 15

Our next period calculator does the sums, and the Nepali (BS) calculator lets you enter dates in Bikram Sambat and shows results in BS and AD. Both run in your browser.

When the formula does not fit

Predictions are only as steady as your cycles. Expect a wider window if:

• you are a teen in the first few years of periods, see irregular periods in teens
• you are close to menopause
• you have recently had stress, illness or travel
• you have just started or stopped hormonal contraception
• your cycles are regularly shorter than 21 days or longer than 35 days

For more on how often periods come, see how often do periods come.

When to see a doctor or health worker

A forecast does not replace a check-up. See a doctor or health worker if:

• cycles are regularly shorter than 21 days or longer than 35 days
• your period is late and not explained by pregnancy, see why is my period late
• you go more than 90 days without a period
• bleeding is very heavy, or you bleed between periods

Keep the dates, and the maths is easy

The hard part is remembering day 1 each month. Writing it down as it happens takes seconds. Cycella (for ages 13 and up) lets you log day 1 in a few taps, with no account, and keeps data on your phone. It is free on Google Play for Android. A paper note or phone calendar works too.