Compare the dimensions and proportions
Neither sheet contains the other at actual size. A4 is the taller, narrower rectangle, with about 3.3943% more nominal area. Letter's 8.5 × 11 inch dimensions convert to millimetres at exactly 25.4 mm per inch.
| Measure | A4 | US Letter |
|---|---|---|
| Millimetres | 210 × 297 mm | 215.9 × 279.4 mm |
| Centimetres | 21 × 29.7 cm | 21.59 × 27.94 cm |
| Inches | 8.2677 × 11.6929 in | 8.5 × 11 in exactly |
| Height ÷ width | 1.414286 | 1.294118 |
| Nominal area | 623.7 cm² | 603.2246 cm² |
| Image at 300 PPI | 2480 × 3508 px | 2550 × 3300 px |
The nominal A4 ratio is close to √2, with the small difference caused by whole-millimetre standard dimensions. Letter has a different ratio. Enlarging one uniformly cannot make both edges coincide with the other.
What percentage fits one onto the other?
For a uniform geometric fit, divide the target width by the source width, do the same for height, and take the smaller ratio. These results use the whole sheet, with no printer margin. The original proportions stay intact.
| Direction | Fit percentage | Scaled source | Total spare space |
|---|---|---|---|
| A4 onto Letter | 94.0741% | 197.5556 × 279.4 mm | 18.3444 mm across the width |
| Letter onto A4 | 97.2673% | 210 × 271.7647 mm | 25.2353 mm along the height |
A4 onto Letter is limited by height: 279.4 ÷ 297 = 0.94074074. The scaled page is 197.5556 × 279.4 mm. If centred, its 18.3444 mm of spare width is split into approximately 9.1722 mm on each side.
Letter onto A4 is limited by width: 210 ÷ 215.9 = 0.97267253. Its spare height is approximately 12.6176 mm at the top and bottom when centred. A printer's Fit command normally uses its smaller printable area, so the real percentage can be lower.
What happens at 100%?
At actual size, the source page keeps its physical dimensions. An A4 page on Letter extends 17.6 mm beyond the available height; a Letter page on A4 extends 5.9 mm beyond the available width. Content in those regions can be clipped.
| Direction | Overflow before printer margins | If centred |
|---|---|---|
| A4 onto Letter | 17.6 mm total height | 8.8 mm above and below |
| Letter onto A4 | 5.9 mm total width | 2.95 mm at each side |
Those edge amounts assume centred placement. A top-left aligned page puts its overflow at the bottom or right instead. Existing document margins may keep important content away from the overflowing boundary, but they do not change the page size.
Choose the document size your printer actually holds whenever exact measurements matter. Otherwise decide explicitly between preserving the whole page with a smaller scale and retaining 100% with possible clipping. Changing the page size in an editable document can also reflow its text; that is a different operation from uniformly scaling an existing PDF.
Check a measured print
The print scale calculator compares the nominal formats. For actual output, follow the actual-size printing guide and use the 100 mm calibration sheet. A correctly sized source file and a measured physical print are different checks.
The outlined plates are scaled illustrations, not physical-size templates. All displayed percentages and spare-space values are rounded; the calculation uses unrounded input dimensions. Neither a generous printer margin nor a borderless mode is assumed.
Sources and workings
- Adobe's A-series dimensions and the trimmed-format scope of ISO 216 establish the A4 reference.
- Adobe's document-format reference lists Letter as 8.5 × 11 inches.
- Adobe's PDF page-sizing instructions distinguish Fit, Actual size and Custom Scale.
Areas, ratios, percentages, spare-space measurements, clipping examples and diagrams are our calculations from those nominal sizes. Image dimensions are physical inches multiplied by 300, rounded to whole pixels; they describe PPI, not the printer's dot resolution.