How to print something at its true size

Three chances to get the size wrong

A page laid out in millimetres can still print at the wrong size. That is true of graph paper, calendars and printable ruled paper alike, because there are three independent places where scaling happens:

1. The page itself — the CSS that sizes the content.
2. The print dialog — scale percentage, fit-to-page, margins, paper size.
3. The printer driver — its own shrink-to-fit, applied to avoid the unprintable margin.

Only the first is under a web page’s control. The other two are the usual culprits when a grid comes out slightly small, and “slightly” is the tell: a uniform 94% or 97% is a scaling setting, not a layout bug.

The CSS inch is exactly 96 pixels

CSS defines absolute units by fiat:

1in  = 96px      exactly
1cm  = 96/2.54   = 37.795...px
1mm  = 1/10 cm
1pt  = 1/72 in

These are definitions, not measurements. On screen the relationship to a physical inch is nominal — a CSS inch on a dense phone display is nothing like an inch of glass, and that is intended, because a phone is held closer than a monitor.

When printing, the mapping becomes real. The browser lays the page out in absolute units and hands the printer a page in physical dimensions, so a 5mm grid square is genuinely 5 mm on paper — provided nothing downstream rescales it.

Which is why a print-at-true-size layout should be authored in mm or in rather than in pixels, and why the same page can be correct on paper while looking arbitrary on screen.

Before you read on

You print 5 mm graph paper and measure the squares at 4.7 mm. What went wrong?

• AThe CSS millimetre is not a real millimetre.

  On paper it is — CSS fixes 1cm as 96/2.54px and the browser maps that to physical size when printing.

• BThe print dialog scaled the page down.

  Almost always this, and 4.7/5 is 94% — a scale percentage, not a layout bug.

• CThe printer cannot render fine lines accurately.

Scaling, almost always. 4.7 against 5 is 94% — a uniform reduction, which is the signature of a fit-to-page setting rather than a layout error. Most printers cannot print to the edge of the sheet, so the driver shrinks the whole page to keep content out of the unprintable margin. Set the scale to 100% and turn off shrink-to-fit.

The margin you cannot have

Nearly every consumer printer has an unprintable border, typically a few millimetres, because the paper feed has to grip somewhere. Content placed there is clipped.

Worse, many drivers respond to content in that region by scaling the entire page down to fit — which is exactly the failure that turns 5 mm squares into 4.7 mm ones. The page was right; the driver rescued it from being clipped and ruined the measurement in the process.

The reliable approach is to design inside a margin rather than to the edge, and to set the print dialog’s scale explicitly to 100%.

A4 and Letter are not interchangeable

	Width	Height
A4	210 mm	297 mm
US Letter	216 mm	279 mm

Letter is wider and shorter. A layout built for A4 and printed on Letter is either scaled — losing true size again — or clipped at the bottom.

The ISO A series has a property worth knowing: every size has the same aspect ratio, the square root of two, so halving an A3 gives exactly two A4 sheets with no waste. The Letter family has no such relationship, which is why “print two to a page” behaves differently between them.

So set the paper size in the tool and in the print dialog. When they disagree, the driver resolves it by scaling, silently.

A checklist for true size

• Choose the correct paper size in the tool before printing.
• Set the print dialog scale to 100%, not “fit to page”.
• Turn off any “shrink oversized pages” or “auto-rotate and centre” option.
• Print one sheet and measure it with a ruler before printing thirty.
• If it is uniformly small, the cause is scaling, not the layout.

The measurement step is the one people skip and the only one that actually answers the question.