Printable month calendar, worked out for the page

What this works out

A month calendar looks like a fixed thing and is not. The same sheet of paper, the same margins and the same month can come out as four rows or as six, depending only on which day the 1st happens to fall on and which day you decide the week starts on. That row count then decides how tall every day box is, which is the difference between a calendar you can write in and one you cannot.

This works the whole layout out before any of it reaches a printer: how many rows the grid needs, which column the 1st sits in, how many blank cells sit either side of the month, how large each box comes out at that row count, and the ISO week number against every date.

The method

Two numbers decide everything. The first is how many blank cells precede the 1st, which is the distance from the opening column round to the weekday the month begins on. The second is the length of the month. Add them and you have the number of cells the grid must hold; divide by seven and round up and you have the rows.

That is why a 28-day February — four weeks exactly — so often still needs five rows. February 2026 begins on a Sunday, and a grid whose first column is Monday has to leave six cells blank before it can put the 1st down. Six blanks plus 28 days is 34 cells, which does not fit in 35 with any room to spare but does not fit in 28 either. Move the first column to Sunday and the blanks vanish, the month fits in four rows, and every box on the sheet grows a quarter taller.

No Date object is used for any of the date arithmetic, which is a deliberate choice rather than a preference. Date is a local-timezone object, and the standard way of walking a calendar with it — mutating a date by setDate in a loop — is how off-by-one-day calendar bugs get written, because a machine west of Greenwich renders the instant on the previous day. The weekday and the ISO week number here come instead from Howard Hinnant’s days_from_civil, which is exact integer arithmetic over the proleptic Gregorian calendar with no leap-year branch in it at all: shifting the year to begin in March puts February, the only irregular month, at the end where its extra day disturbs nothing after it. A Date appears in exactly one place, as the argument Intl.DateTimeFormat needs to produce a name, and every formatter here is pinned to UTC so the machine’s own timezone cannot move a name onto the wrong day.

ISO week numbers deserve their own note, because the definition is not the intuitive one. Weeks run Monday to Sunday, and week 1 is the week containing the year’s first Thursday — so a week belongs to whichever calendar year holds its Thursday, and a date near either end of December or January can carry a number from the neighbouring year. Those weeks are not counted twice, which is the whole point of the rule, but it does mean the week number against 1 January is often 52 or 53 rather than 1. They are also unaffected by the first-column setting: ISO weeks are Monday-based by definition, so a Sunday-start grid shows week numbers whose weeks began a day before each row does.

The page geometry is the straightforward part. The printable area is the sheet less twice the margin, less the space reserved above the grid for the heading and the weekday labels. Divide the width by seven columns — eight if the week-number column is on — and the remaining height by the number of rows. Every length is held as a whole number of micrometres until that final division, so subtracting a margin twice cannot leave a floating-point residue behind.

Before you read on

February 2026 has exactly 28 days — four weeks to the day. How many rows does a grid whose first column is Monday need?

• AFour. Twenty-eight days is four rows of seven.

  Only if the month starts on the first column, and this one does not.

• BFive.

  Yes: six blank cells before the 1st, then 28 days, is 34 cells.

• CIt depends on the year — 2026 is not a leap year.

  Leap years change the length. What decides the rows here is the weekday it starts on.

Five. February 2026 begins on a Sunday, so a Monday-first grid has to leave six cells blank before it can put the 1st down: 6 + 28 = 34 cells, which needs five rows of seven. Move the first column to Sunday and the blanks vanish, the month fits in four rows, and every box on the sheet grows from 32.80 mm to 41.00 mm tall — a quarter taller, on identical paper, from one control that looks like a preference.

1. blanks = (0 for Sunday - 1 for Monday + 7) mod 7 = 6

   The distance from the opening column round to the weekday the month begins on. The + 7 is there so the modulo never sees a negative.

2. cells = 6 + 28 = 34

   Blanks plus the length of the month. This is the only place the two numbers meet.

3. rows = ceil(34 / 7) = 5

   And one cell left blank after the 28th, since 5 x 7 is 35.

4. box height = (210 - 24 - 22) / 5 = 32.80 mm

   A4 landscape less twice a 12 mm margin, less 22 mm reserved above the grid, divided by the rows. Everything is held in whole micrometres until this division.

A worked example

February 2026, week starting Monday, names in English (United Kingdom), A4 landscape, a 12 mm margin, and 22 mm reserved above the grid:

heading            February 2026
the 1st falls on   Sunday, in column 7
days               28
blank cells        6 before, 1 after
rows               5
printable area     273.00 x 186.00 mm
grid area          273.00 x 164.00 mm   (186 less the 22 mm heading)
day box            39.00 x 32.80 mm
ISO weeks          5 through 9

The arithmetic checks by hand in both directions:

Figure	Arithmetic	Result
Blank cells before the 1st	(0 for Sunday − 1 for Monday + 7) mod 7	6
Cells in the grid	6 + 28	34
Rows	ceil(34 ÷ 7)	5
Blank cells after the 28th	5 × 7 − 34	1
Box width	(297 − 24) ÷ 7	39.00 mm
Box height	(210 − 24 − 22) ÷ 5	32.80 mm

Both divisions come out exact here, which is a coincidence of A4 and these margins rather than anything the tool arranges.

Change one control and the sheet changes shape. With the week starting on Sunday instead, the same month has no leading blanks, fits in four rows, and the box height goes from 32.80 mm to 41.00 mm — a quarter taller, on identical paper. August 2026 is the case in the other direction: 31 days beginning on a Saturday is five blanks plus 31, which is 36 cells and six rows, and the boxes come down to 27.33 mm.

These are the same numbers asserted in this tool’s test file, so if the formula ever changes without this page changing with it, the build fails. The weekday and ISO week arithmetic is checked there a second way as well, against separate Date-based implementations, on every month from 1600 to 2400 and on every single day of every month across a full leap cycle.

What it does not do

It does not draw the calendar. It gives you the geometry and the complete date table — every day with its row, its column, its weekday name and its ISO week number — which downloads as a CSV, and that is enough to lay the sheet out exactly in a drawing program, a spreadsheet or a stylesheet. It is not a ready-made page to send to a printer.

It also models one calendar and not the many things people put on one. There are no public holidays, no moon phases, no week-of-month numbering, no leading or trailing days from the neighbouring months greyed into the blank cells, and no provision for the narrower weekend or week-number columns that a printed calendar often uses. Every box is the same size, which is the one layout that can be stated exactly rather than by a convention nobody agrees on.

And it knows nothing about your printer. The paper sizes are trimmed sizes, so the hardware border a desktop printer physically cannot reach is not deducted, and neither is the scaling a print dialogue applies unless you turn it off. Both of those live on the far side of this page, and both will quietly make the printed sheet disagree with every millimetre above.

It also counts nothing for you. For the number of working days in a span, rather than a grid to write them into, that is arithmetic on two dates and not a layout problem.

daysInMonth = 29 for February in a leap year, 28 otherwise
              30 for April, June, September, November
              31 for the rest

leap year   = year mod 4 = 0  and  (year mod 100 != 0  or  year mod 400 = 0)

firstColumn = 0 for a Sunday start, 1 for Monday, 6 for Saturday
              weekdays indexed Sunday = 0 … Saturday = 6

leadingBlanks  = (weekdayOfThe1st - firstColumn + 7) mod 7
cells          = leadingBlanks + daysInMonth
rows           = ceil(cells / 7)
trailingBlanks = rows x 7 - cells

for the dth day:
  offset = leadingBlanks + d - 1
  row    = floor(offset / 7) + 1
  column = (offset mod 7) + 1

ISO 8601 weeks run Monday to Sunday, and week 1 is the week holding the
year's first Thursday:

  isoWeekday = 1 for Monday … 7 for Sunday
  thursday   = dayNumber + (4 - isoWeekday)
  weekYear   = the calendar year that thursday falls in
  isoWeek    = floor((thursday - 1 January of weekYear) / 7) + 1

columns   = 8 when a week-number column is shown, otherwise 7
boxWidth  = (pageWidth  - 2 x margin) / columns
boxHeight = (pageHeight - 2 x margin - headingSpace) / rows

Every length is held as a whole number of micrometres up to that last
division. Every day number is integer arithmetic throughout.