40-day cycle calendar

With a 40-day cycle, each period is estimated to start 40 days after the last one began. That is about 9.1 periods a year, 10 starting in 2027 if the first is on Jan 4.

Where 40 days sits

A 40-day cycle is longer than the usual adult range given by the NHS (21 to 35 days) and the US Office on Women's Health (24 to 38 days). The Office on Women's Health notes that cycles longer than 38 days are common for a few years after a first period. If your cycles are usually this long, have changed, or it worries you, talk with a clinician.

Sources: NHS, Periods (last reviewed 5 January 2023); Office on Women's Health, Your menstrual cycle (updated 13 January 2025).

Through the week and the month

40 days is 5 weeks and 5 days, so each estimated start falls 2 days earlier in the week than the one before (a Monday start is followed by a Saturday).

A 40-day cycle is longer than the average calendar month (30.4 days), so the start date moves about 9.6 days later in the month each time, and some months have no period start at all. With a first start on Jan 4, 2027, no period would start in April and August.

Example: the next six estimated periods

If the last period started on Monday, January 4, 2027:

Each estimate has a likely range of 3 days either side for the next period, widening by 3 days per cycle up to 9. Every date here is an estimate from a typical cycle length. It is not birth control and it is not medical advice.

Work out your own dates with a 40-day cycle

The estimated middle of the cycle

The calendar method places ovulation about 14 days before the next period, which for a 40-day cycle is around day 26. That is a rough estimate: the second half of the cycle varies from person to person and month to month, and a calendar estimate cannot confirm anything. It is not birth control.

If your cycle is not always 40 days

Most people's cycles vary by a few days. A typical length is a starting point; the estimate improves when it comes from your own recent cycles. Isla uses the middle value of your last six logged cycles and shows a range when they differ by more than a week.