While most of the world uses the ISO 216 system (A4, A3, B5...), three countries — the United States, Canada, and Mexico — still rely on their own traditional paper sizes. These are defined by the ANSI/ASME Y14.1 standard (published in 1996) and various architectural series.
The Origin of Letter Size
The most important US paper size, Letter, measures 8.5 × 11 inches (216 × 279 mm). Its origins are surprisingly humble. Historians trace it to 17th- and 18th-century American colonial papermaking traditions, where standard writing paper measured roughly 8.5 × 11 inches. When the typewriter became common in the late 19th century, Letter was already the de facto standard — and it stuck.
ANSI Sizes Table
| ANSI Name | Common Name | Inches | Millimetres |
|---|---|---|---|
| ANSI A | Letter | 8.5 × 11 in | 216 × 279 mm |
| ANSI B | Tabloid | 11 × 17 in | 279 × 432 mm |
| ANSI C | — | 17 × 22 in | 432 × 559 mm |
| ANSI D | — | 22 × 34 in | 559 × 864 mm |
| ANSI E | — | 34 × 44 in | 864 × 1118 mm |
Why the US Never Switched
After World War II, ISO 216 spread across Europe and Asia. The US, Canada, and Mexico chose to keep their traditional sizes. The main reason was infrastructure — existing filing cabinets, binders, printer trays, and postal equipment were all sized for Letter. Switching would have required retooling entire industries.
Practical Consequences Today
Because Letter and A4 are not the same shape, international documents often need resizing:
- A4 printed on Letter: usually scaled to 94–97% to avoid clipping the sides
- Letter printed on A4: usually scaled to 94–97% to avoid clipping the top/bottom
US Envelope Numbering System
| Number | Inches | Millimetres | Common Use |
|---|---|---|---|
| #6¾ | 3.625 × 6.5 in | 92 × 165 mm | Personal mail |
| #9 | 3.875 × 8.875 in | 98 × 225 mm | Business reply |
| #10 | 4.125 × 9.5 in | 104 × 241 mm | Standard business |
| #11 | 4.5 × 10.375 in | 114 × 264 mm | Larger mailings |
Frequently Asked Questions
What is US Letter in mm?
US Letter is 216 × 279 mm.
What is US Legal in mm?
US Legal is 216 × 356 mm.
Is Letter the same as A4?
No. A4 is 210 × 297 mm; Letter is 216 × 279 mm.
Why is it called "Tabloid"?
The name comes from 19th-century London where a "tabloid" was a compressed pill.
Conclusion
US paper sizes reflect a distinct papermaking tradition that predates ISO 216. Understanding the differences between Letter and A4 is essential for international work.