Workday counts for a month: why length and starting weekday decide the total

Every month has 20, 21, 22 or 23 weekdays. There is no fifth possibility, and which of the four you get depends on exactly two things: how long the month is and which day of the week it starts on.

That sounds like a small range. It is a 15 percent swing between the shortest and the longest. A daily rate derived from a monthly figure therefore moves from month to month. A plan that assumes "about 21 days" is wrong more often than it is right.

Only two things vary, and both are properties of the Gregorian calendar rather than of anyone's working practices: the lengths of the months and the day the year starts on. That is also why the whole pattern repeats: there are only fourteen possible year shapes, which is what a perpetual calendar tabulates.

What to take away

The rule, and the table it produces

Four weeks are 28 days and always contain exactly 20 weekdays, whatever the month starts on. A 28-day February is therefore always 20. A longer month adds one, two or three days beyond those four weeks, and each of those extra days is a weekday or not depending on the start.

Days 29, 30 and 31 fall on the same weekdays as days 1, 2 and 3. So the count is 20 plus however many of the month's first (length minus 28) days are weekdays.

A 31-day month starting on a Monday, Tuesday or Wednesday has 23 weekdays. The same month starting on a Friday has 21. Two years apart, the same calendar month can differ by two working days with no closure involved at all.

Applied to one year

The rule applied to 2027, which is not a leap year and whose months start where they start. This is arithmetic, not a holiday list: nothing below is closed.

The twelve months add to 261, which is the year's weekday count, and March and December each carry three more weekdays than February. A monthly cost spread evenly per weekday is 15 percent higher per day in February than in March.

From weekdays to working days

Subtract every closure that lands on a weekday. A closure on a Saturday or Sunday takes nothing off, unless the calendar you are following substitutes it onto a weekday, in which case it takes one.

November 2027 has 22 weekdays. In the United States, Veterans Day falls on Thursday, November 11, and Thanksgiving on Thursday, November 25. Both are federal holidays on weekdays, so a US federal closing calendar takes the month to 20 working days.

December 2027 has 23 weekdays, but Christmas Day falls on a Saturday and New Year's Day 2028 also falls on a Saturday. The federal calendar observes each on the Friday before, December 24 and December 31, so December's count falls to 21.

In Canada the list is different: Thanksgiving falls in October, and Remembrance Day in November is a statutory day off in some provinces and not in others.

Which closures apply is not a fact about the month: it is a fact about whose calendar governs, the receiving body's for a deadline, the bank's for a payment, the employer's for staffing. Take the list from that body's own published calendar and note the date you took it.

Two offices in the same city can have different counts for the same month, and both are right for their own purposes. The layers hidden inside the phrase are set out under adding business days, and they apply to a monthly count unchanged.

Reproducing it in a spreadsheet

For a month given by a year in A2 and a month number in B2:

=NETWORKDAYS(DATE(A2,B2,1), EOMONTH(DATE(A2,B2,1),0), Closures)

EOMONTH(date, 0) returns the last day of the same month, so the range covers the whole month whatever its length. NETWORKDAYS counts both endpoints, which is what a whole-month count needs.

Closures is a range of dates you supply. Leave it out to get the raw weekday count from the table above.

For a working week other than Saturday and Sunday, use NETWORKDAYS.INTL and pass a weekend code or a seven-character string of ones and zeros for Monday through Sunday.

By hand, without a function: find the weekday of the 1st, read the row for the month's length, then subtract the closures that fall on weekdays.

Where the number is used, and where it misleads

Daily rates. A monthly figure divided by working days gives a per-day cost that changes every month. If the per-day figure has to be stable, the divisor has to be fixed by policy rather than taken from the calendar, and that fixed figure should be written down where the rate is defined.

Capacity. A team's monthly capacity is working days times people times hours, minus leave. The working-day term alone moves by up to three days between months, before anyone books a holiday.

Fixed divisors. Many payroll and billing systems use one constant for every month rather than the actual count. Whether yours does, and which constant, is a setting inside that system and a policy of the organization that runs it. It is not something a calendar can tell you, and it is not something to infer from an average.

Averages. 261 weekdays over twelve months is 21.75 a month; 260 is 21.67. An average is useful for a yearly budget and useless for any single month, because no month actually has 21.75 days.

A quarter is not three average months

The four possible monthly answers do not average out inside a quarter, because the months that make up a quarter are consecutive and their start days are related rather than random.

The four quarters of 2027, from the monthly figures above:

They sum to 261, which is the year. The spread across the four is two days, which is smaller than the spread across individual months but still enough to move a quarterly per-day rate by three percent. A model that treats quarters as equal will over-recover in the first and under-recover in the third, every time, in that year.

The figures are not transferable to another year. They are the year's own months added up, and a different year starts on a different day.

Common questions

Why did last year's March have fewer working days than this year's?

Because it started on a different weekday. A 31-day month gains or loses up to two weekdays purely on where the 1st falls, and the closures in it may also have moved.

Does a month starting on a weekend always have fewer weekdays?

For 29 and 30-day months, yes. A 31-day month starting on a Sunday has 22, the same as one starting on a Thursday, because its extra days are Sunday, Monday and Tuesday and two of those are weekdays.

Can I use the yearly count divided by twelve?

For a budget across the whole year, yes. For any individual month, no. The monthly figure sits anywhere from 20 to 23 and the average lands on none of them. If you need the per-month figures, the pillar on counting between two dates covers the endpoint rules that decide whether the 1st and the last day are both in.

How do closures on weekends affect the count?

They do not, unless the calendar substitutes the observance onto a weekday. Whether it does is in the published list.

A count that runs backward through a month, from a fixed end date, needs that same list and is less forgiving of a wrong one. That side is covered under subtracting business days.