The one-pixel error hiding in every A4 conversion

A lot of print production errors begin with a sentence that is very nearly true: “A4 is 8.27 × 11.69 inches.” It is close enough for almost everything, and it is not the definition — and once you start multiplying from it, the difference stops being theoretical.

Convert from the definition, not from the conversion

ISO 216 defines A4 as exactly 210 × 297 millimetres. The inch figure is derived, and it is derived by rounding.

Convert from the definition and you get:

210 mm ÷ 25.4 × 300 = 2480.3 px  →  2480

Convert from the rounded inches and you get:

8.27 in × 300 = 2481 px

One pixel. On a single flyer, nothing — you would never see it, and no printer would query it. The problem is that this is exactly the kind of error that does not stay one pixel.

Tile that A4 across a step-and-repeat and the drift accumulates once per tile. Impose eight A4 pages onto a B1 sheet and the cumulative offset is enough to walk your crop marks off register. Butt two panels of a step-and-repeat backdrop together and the seam does not line up. Each individual number was “close enough”; the stack of them was not.

The rule is simple and worth making a habit: for any ISO size, convert from the millimetres. For US sizes, do the opposite — Letter is defined as 8.5 × 11 inches, so inches are the authoritative figure there and the millimetres are the derived one. Convert from whichever unit the standard actually specifies, and there is no drift to accumulate.

Bleed is not a rounding error, and it is the one people forget

The second number missing from most conversions is bleed, and unlike the pixel above, it is large enough to ruin a job on its own.

Anything printed to the edge of the sheet needs artwork extending past the trim line, because guillotines and folders do not cut to the micron. The standard allowance is 3 mm on every side, about 1/8 inch. In pixels at 300 DPI:

3 mm ÷ 25.4 × 300 = 35.4 px  →  35 px per edge

So an A4 flyer with bleed is not 2480 × 3508. It is 2551 × 3579 — 71 pixels wider and taller, because bleed applies to both edges of each axis.

Miss it and one of two things happens. Either the printer rejects the file and you lose a day, or it gets accepted and trimmed, and you get a hairline of white along one edge where the blade drifted a millimetre outside your artwork. The second outcome is worse, because you pay for the run first.

While you are adding bleed, pull the important content inwards too. Keep type and logos at least 3–5 mm inside the trim line, for the same reason bleed exists: the cut position is approximate, and anything close to the edge looks like a mistake even when it is technically inside.

What this means in practice

Three things, in order:

1. Start from the standard’s own unit. Millimetres for ISO, inches for US sizes.
2. Add bleed before converting, not after. Work out the bleed size in the physical unit, then convert once.
3. Round at the end, exactly once. Every intermediate rounding is another chance for drift.

The inches to pixels converter and the pixels to mm converter will handle either direction, and the print size calculator carries the common paper and poster sizes with their pixel dimensions already worked out — from the defining measurements, so the numbers agree with the ones above. If you just want the finished figures to check against, 300 DPI in Pixels lists them.