Paper Size Converter
A4, A3, Letter, Legal, Tabloid dims, area, and scale.
What it does
Paper sizes look arbitrary until the two systems split apart. ISO A sizes halve and double at a fixed 1 to 1.4142 aspect, so A3 is exactly two A4 sheets. US sizes stand apart: Letter at 8.5 by 11 inches with Legal and Tabloid nearby, none matching A4.
The tool above converts between five standards: A4, A3, Letter, Legal, and Tabloid. Pick source and target and it prints target dimensions, the area scale percent, and the target area. A4 to A3 reads 200 %; Letter to A4 reads 103.4 %.
Standards pin every figure. ISO 216 defines the A and B series, Cornell library tables list Letter at 215.9 by 279.4 mm with Legal and Tabloid beside, and NIST confirms the inch link its SI charts use. One square inch is exactly 645.16 mm2, so US areas follow without rounding.
Cross system printing needs the scale line. A4 documents on Letter stock lose margins or shrink to 96.7 %, while Letter on A4 gains to 103.4 %. Set the driver scale from the printed percent instead of eyeballing fit to page.
How to use
- Pick the source paper size.
- Pick the target paper size.
- Read the target dimensions first.
- Read the scale percent for print scaling.
- Read the target area for postage and stock.
- Round scaling in the printer driver, not mid chain.
Formula
- from
- Source sheet from the five standards.
- to
- Target sheet with its exact dims.
- 645.16
- Exact inch to mm2 factor used for US sizes.
ISO 216 defines the A series with 1 to sqrt(2) sides; US sizes come from Cornell and NIST tables at 25.4 mm per inch exactly.
Worked example
Example: A4 to A3
Defaults: A4 to A3.
- Inputs: from A4 to A3.
- A4 area: 210 x 297 = 62,370 mm2.
- A3 area: 297 x 420 = 124,740 mm2.
- Scale: 124,740 / 62,370 x 100 = 200 %.
- Tool shows 297 x 420 mm, 200% area, 124,740 mm2.
Reference table
| Size | Dimensions | Area |
|---|---|---|
| A4 | 210 x 297 mm | 62,370 mm2 |
| A3 | 297 x 420 mm | 124,740 mm2 |
| US Letter | 8.5 x 11 in | 60,322 mm2 |
| US Legal | 8.5 x 14 in | 76,774 mm2 |
| US Tabloid | 11 x 17 in | 120,645 mm2 |
Limits
- Five standards only. B, C, and RA sizes excluded.
- Nominal dims; trims and bleeds sit outside.
- Scale is area based; linear scale is its square root.
- Driver and printer margins still apply.
Common mistakes
- Calling A4 and Letter the same size.
- Scaling linearly by the area percent.
- Rounding dims mid chain across systems.
- Ignoring printer margins on cross printing.
- Mixing Tabloid with Ledger grain direction.
Try it yourself
Quick checks. Answers sit tucked under each question.
Letter to A4?
A4 area 62,370 mm2. 62,370 / 60,322 x 100 = 103.4 %. Target dims 210 x 297 mm.
A4 to Letter?
60,322 / 62,370 x 100 = 96.7 %. Target dims 8.5 x 11 in.
Questions
What are A4 and A3 in mm?
A4 is 210 x 297 mm (62,370 mm2). A3 is 297 x 420 mm (124,740 mm2), exactly double.
What are Letter and Legal in mm?
Letter is 8.5 x 11 in (215.9 x 279.4 mm, 60,322 mm2). Legal is 8.5 x 14 in (76,774 mm2).
How does the A series scale?
Each ISO A step doubles area at a 1 to 1.4142 aspect. A4 to A3 is 200 %; A3 to A4 is 50 %.
Is A4 the same as Letter?
A4 is about 3.4 % larger than Letter (62,370 vs 60,322 mm2). Printing across systems needs margin checks.
How is scale percent figured?
Area target over area source times 100. Letter to A4: 62,370 / 60,322 x 100 = 103.4 %.
Are the inch to mm factors exact?
Yes, at 25.4 mm per inch exactly. Letter: 8.5 x 25.4 = 215.9 mm wide.
Sources
- ISO 216:2007 (A and B paper series standard)
- Cornell Library: International Standard Paper Sizes
- NIST: Metric SI Prefixes (inch-mm link)