Quick answer: A4 is 210 × 297 mm; US Letter is 215.9 × 279.4 mm. Letter is 5.9 mm wider, while A4 is 17.6 mm taller. They are not the same size and do not share the same aspect ratio.
Measurement note: dimensions are stated width × height in portrait orientation. Turn the sheet through 90 degrees for landscape use. Millimetres are the controlling values for ISO sizes; inch figures are rounded conversions.
Size comparison
| Format | Dimensions | Practical difference |
|---|---|---|
| A4 | 210 × 297 mm | Taller and slightly narrower |
| US Letter | 215.9 × 279.4 mm | Shorter and slightly wider |
What happens when printing Letter on A4
At 100%, the Letter page fits within A4’s height but can exceed the available width once printer margins are included. Automatic fitting may reduce the document slightly.
What happens when printing A4 on Letter
A4 is taller than Letter. At 100%, content near the top or bottom can be clipped. Fitting reduces the page and creates side margins.
Conversion strategy
For editable documents, change the page size and reflow the content. For fixed PDFs, choose fit only when scale is not critical. Never scale forms, labels or technical drawings without checking requirements.
Frequently asked questions
Can one format be substituted for the other?
Sometimes, but not automatically. Check the printer, envelope, binding, postal category and whether the design may be scaled or reflowed. Exact-size forms and technical drawings should not be substituted without approval.
Should the document be redesigned or simply scaled?
Editable documents are usually better reflowed to the new page dimensions. Finished artwork can be scaled proportionally, but this may create unequal margins or require cropping.
Worked conversion method
When converting A4 vs US Letter, first decide whether the content must remain at its original scale. If scale is critical, place the old page at 100% on the new sheet and accept margins or controlled cropping. If scale is not critical, calculate a proportional fit from the limiting dimension, then inspect every page. For editable documents, changing the page size and allowing text to reflow usually gives a better result than shrinking a finished page.
- Duplicate the original file and preserve an unchanged master.
- Set the new page dimensions explicitly rather than relying on a local printer default.
- Compare the old and new aspect ratios.
- Rebuild margins, columns and image crops where necessary.
- Check page breaks, tables, signatures, footnotes and form fields.
- Export a PDF and print one page at actual size.
Do not use automatic “fit” while converting A4 vs US Letter when the item is a label, sewing or cutting template, plan, calibration chart or form whose boxes must align with preprinted stock. A small percentage change can invalidate measurements even when the page looks normal.
How to specify the size without ambiguity
For A4 vs US Letter, a reliable print specification states more than the familiar format name. Give the finished width and height, measurement unit, orientation, number of pages or panels, paper weight, surface, colour process, bleed, finishing and quantity. This prevents regional names and printer defaults from silently changing this particular job.
- Finished size: write width × height and identify portrait or landscape.
- Artwork size: distinguish the trimmed page from any larger bleed canvas or parent sheet.
- Stock: specify grammage or basis weight, coating, colour and recycled or certified requirements.
- Finishing: state folds, scoring, binding, drilling, corner rounding, lamination or mounting.
- Tolerance: ask the supplier what cutting, folding and registration variation should be expected.
When ordering A4 vs US Letter, a short name alone leaves important decisions unstated. A specification such as “finished size as stated above; portrait; 3 mm bleed; 170 gsm uncoated white stock; full colour both sides; scored where shown” is harder to misinterpret. When the format has regional variants, include the governing standard or exact dimensions every time.
Make a full-size physical proof
Screens do not communicate the physical scale of A4 vs US Letter reliably. Before a large order, print a representative page at 100%, trim it to the stated size and use it as the finished item will be used. Measure one horizontal and one vertical feature with a ruler. Fold it, place it in its envelope or holder where relevant, view it from the intended distance and ask someone unfamiliar with the design to find the main information.
A proof of A4 vs US Letter should check more than spelling. Look for text that is technically present but difficult to read, lines that sit too close to the trim, images whose important details enter a fold or binding area, barcodes or QR codes that do not scan, and stocks that crack or show through. For colour-critical work, a calibrated contract proof or press proof is more informative than an office print.
Sources and standards
The measurements and production guidance in this guide were checked against the following standards bodies, postal operators and print-platform specifications. Standards can be revised or withdrawn, so check the linked record when a contractual or regulated specification matters.