4–4–5 calendar

The 4–4–5 calendar is a method of managing accounting periods, and is a common calendar structure for some industries such as retail and manufacturing. It divides a year into four quarters of 13 weeks, each grouped into two 4-week "months" and one 5-week "month". The longer "month" may be set as the first (5–4–4), second (4–5–4), or third (4–4–5) unit.

Its major advantage over a regular calendar is that each period is the same length and ends on the same day of the week, which is useful for planning manufacturing or work shifts.

A disadvantage is that comparisons or trend analysis by "month" are flawed, as one month is 25% longer than the other two (whereas comparisons between weeks or to the same "month" in the previous year are still useful).

Another disadvantage is that the 4–4–5 calendar has only 364 days (7 days x 52 weeks), meaning a 53rd week must be added every five or six years: this can make year-on-year comparison difficult.