How to Use This Tool
Pick the size a document was designed for and the size you are printing on. The scale-to-fit percentage is the number to type into the printer dialog.
Why the A series works the way it does
One rule generates the whole system: each size folded in half gives the next size down, with the same proportions. Only one ratio does that, and it is the square root of two — about 1.41421. A0 is then fixed by defining its area as one square metre.
The consequences are all useful:
- Enlarging one step is always 141.42% and reducing one step is always 70.71%. Those are the two buttons on every office copier.
- Two A4 sheets side by side make an A3 exactly, so booklet imposition needs no adjustment.
- Area halves each step, which is why A4 is 1/16 of a square metre and why paper weight in grams per square metre converts so easily — an 80 gsm A4 sheet weighs exactly 5 grams.
Published dimensions are rounded down to whole millimetres at each step, which is why A4 is 210 × 297 rather than the exact 210.224 × 297.302. The ratio is therefore very slightly off at small sizes, and never enough to notice.
A4 and Letter cannot both fit
Letter is 215.9 × 279.4 mm, which is 8.5 × 11 inches. Its ratio is 1.294, not 1.414, so it is a fundamentally different shape.
A4 is 5.9 mm narrower and 17.6 mm taller. Neither sheet contains the other, so:
- A4 onto Letter fits at 94.07%, limited by height.
- Letter onto A4 fits at 97.27%, limited by width.
Every "fit to page" tick box silently applies one of those. Usually nobody notices, because the shrink is small and the margins absorb it — until something depends on absolute size, like a form designed to be measured, a scale drawing, or a document with content close to the edge.
Fit against fill
Two different scales, and printer dialogs are not always clear which one they use:
- Fit uses the smaller of the two ratios, so everything is visible and one dimension has spare paper. This is what you almost always want.
- Fill uses the larger, so the sheet is covered and the excess is cropped. Useful for photographs and destructive for documents.
Neither accounts for the printer's unprintable margin. Most printers cannot mark the outer few millimetres, so a document scaled to exactly 100% of the paper will still lose its edges. Genuine edge-to-edge printing requires a borderless-capable printer, or printing oversized and trimming.
The other systems
- B series — geometric means between A sizes, so B4 sits between A3 and A4. Used for posters, passports and some books.
- C series — envelopes. A C4 envelope holds an A4 sheet flat, and C5 holds A4 folded once. That is the entire logic.
- US sizes — Letter, Legal, Tabloid and Ledger, defined in inches with no consistent ratio between them. Legal is Letter with 76.2 mm added to the height only, so it is a different shape again.
