Paper & Envelope Sizes

The ISO A series rests on a single idea: every sheet has the proportion 1:√2, which is the only ratio that survives being halved. Fold A4 across its short edge and you get two A5 of exactly the same shape. That is why artwork scales between sizes without reframing, and why the enlargement factor between adjacent sizes is always √2 — about 141% going up, 71% coming down.

A0 is defined as one square metre in area, and everything else follows by halving. The figures here are the standard's own values rather than computed ones, because ISO 216 rounds each dimension down to a whole millimetre at every step and a size derived by repeatedly halving A0 drifts away from the official number.

The B series sits geometrically between adjacent A sizes, for when A4 is too small and A3 too large — B5 is a common book trim for that reason. The C series is sized to hold the A series: C4 takes an unfolded A4, C5 takes it folded once, C6 folded twice. That relationship is the whole point of having three series.

US sizes follow no system at all. Letter, Legal and Tabloid have no consistent ratio, do not halve into each other, and cannot be scaled between without reframing. This is not a criticism so much as a warning: a layout that relies on the A-series halving property has no US equivalent, and artwork moving between the two needs redrawing rather than scaling.

A C5 envelope is 162 × 229mm and A5 paper is 148 × 210mm, so the arithmetic says A5 fits C5 with 14mm and 19mm to spare. That is true and it is also the easy case. The one that catches people is folding: A4 folded in three is roughly 99 × 210mm, which fits DL at 110 × 220mm — but only just, and only if the folds are accurate.

The number that matters is clearance, and it depends on how the envelope gets filled. A single sheet inserted by hand needs very little. A dozen sheets plus a reply card, machine-inserted at speed, needs several millimetres on every edge or the inserter jams and the job stops. A chart giving nominal sizes cannot tell you which situation you are in, which is why the fit tool here reports the actual slack rather than a yes or no.

The other thing a size chart hides is that thickness makes an envelope effectively smaller. A fat enclosure bows the paper, and a bowed sheet needs more room than a flat one of the same dimensions. If the contents are more than a few sheets, go up a size rather than trusting the measurement.