ISO Paper Sizes: A4, A3, A5 — Every Dimension You Need

The ISO 216 paper standard covers 99% of the world outside North America. The elegant part: every size is derived from a single sheet (A0) with an area of exactly 1 m², using one mathematical property — a width-to-height ratio of 1:√2. That one ratio ensures that folding any sheet in half always produces the next size down with identical proportions.

The A-Series: All Sizes in All Units

Size

mm (W × H)

cm

inches

Area

A0

841 × 1189

84.1 × 118.9

33.1“ × 46.8“

1.000 m²

A1

594 × 841

59.4 × 84.1

23.4“ × 33.1“

0.500 m²

A2

420 × 594

42.0 × 59.4

16.5“ × 23.4“

0.250 m²

A3

297 × 420

29.7 × 42.0

11.7“ × 16.5“

0.125 m²

A4

210 × 297

21.0 × 29.7

8.27“ × 11.69“

0.0625 m²

A5

148 × 210

14.8 × 21.0

5.83“ × 8.27“

0.0313 m²

A6

105 × 148

10.5 × 14.8

4.13“ × 5.83“

0.0156 m²

A7

74 × 105

7.4 × 10.5

2.91“ × 4.13“

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Pixel Dimensions at Common DPI Values

Screens use 72 or 96 PPI (pixels per inch). Print typically requires 150 PPI minimum, 300 PPI for quality work. A4 at 300 PPI:

DPI

A4 pixels (portrait)

A3 pixels (portrait)

A5 pixels (portrait)

72

595 × 842

842 × 1191

420 × 595

96

794 × 1123

1123 × 1587

559 × 794

150

1240 × 1754

1754 × 2480

874 × 1240

300

2480 × 3508

3508 × 4961

1748 × 2480

Formula: pixels = mm × (DPI / 25.4)

Why √2 Makes the System Work

Take a sheet with dimensions 1 × √2. Fold it in half along the long edge: the new sheet is √2/2 × 1. Simplify: multiply both dimensions by √2/√2 to get 1 × √2. Same ratio. The sheet is self-similar under halving.

No other aspect ratio has this property. If a sheet is 1 × 1.5, folding in half gives 0.75 × 1 — a different ratio. The √2 rectangle is the unique fixed point of the “fold in half” operation.

A0: The Reference Sheet

A0 has an area of exactly 1 m². Its dimensions (841 × 1189 mm) are found by solving:

• w × h = 1,000,000 mm²
• h / w = √2

Solving: w = √(1,000,000 / √2) ≈ 840.9 mm, rounded to 841. Each subsequent size halves the longer dimension and swaps width and height, with values rounded down to the nearest mm.

B-Series: Between the A-Sizes

B-series sizes are the geometric mean between consecutive A-series sizes. B1 is between A1 and A0. B-series is used for books, posters, and passports. B5 (176 × 250 mm) is a common book format; B4 (250 × 353 mm) is used for some newspapers and large documents.

C-Series: Envelopes

C-series sizes are designed specifically to hold A-series paper. The geometric mean between A-series and B-series, they’re sized so paper fits without folding:

• C4 (229 × 324 mm) — holds A4 flat
• C5 (162 × 229 mm) — holds A4 folded once (to A5)
• C6 (114 × 162 mm) — holds A4 folded twice (to A6)
• DL (110 × 220 mm) — holds A4 folded in thirds; the standard business letter envelope

US Paper Sizes: Letter, Legal, Tabloid

The US uses a separate system not based on √2:

Size

Inches

mm

Notes

Letter

8.5 × 11“

216 × 279

Standard US document — narrower and shorter than A4

Legal

8.5 × 14“

216 × 356

Legal documents, contracts

Tabloid / Ledger

11 × 17“

279 × 432

Two Letter sheets side by side

US Letter (8.5 × 11“) and A4 (210 × 297 mm = 8.27 × 11.69“) are close but not interchangeable. A4 is taller and narrower. Documents designed for one may need margins adjusted for the other.

Frequently Asked Questions

What are the exact dimensions of A4 paper in inches?

A4 paper measures 8.27" × 11.69" (or approximately 8.3" × 11.7"). In millimeters, it's 210 × 297 mm. This is the standard paper size used for documents, letters, and printing across most of the world outside North America.

Why does A3 paper have the same proportions as A4?

Both sizes share the same 1:√2 width-to-height ratio because of how the ISO 216 system works. A3 is created by folding A2 in half, and A4 by folding A3 in half. The mathematical property of the √2 rectangle means each fold produces a smaller sheet with identical proportions—no other aspect ratio has this property.

What pixel dimensions do I need for A4 printing at high quality?

For professional print quality, use 300 DPI, which gives A4 dimensions of 2480 × 3508 pixels in portrait orientation. If you're designing for screen only, 96 DPI is sufficient (794 × 1123 pixels). Use the formula: pixels = mm × (DPI ÷ 25.4) to calculate any size at any resolution.

What's the difference between A-series, B-series, and C-series paper sizes?

A-series (A4, A3, A5) is the general-purpose standard with A0 defined as exactly 1 m². B-series sizes sit geometrically between A-sizes and are used for books and posters—B5 is a common book format. C-series is specifically designed for envelopes that hold A-series paper flat without folding, like C4 which holds A4 sheets.