Why does A4 measure 210 × 297 mm and not something rounder?

Because the ratio is fixed at √2 : 1, so the sides come out irrational and get rounded to whole millimetres. The rounding is small enough that the halving rule still works in practice.

Can I fold an A4 sheet in half and get a standard size?

Yes — fold across the long side and you get A5 at 148 × 210 mm. Folding across the short side gives a 105 × 210 mm scrap that matches no ISO size.

Is A3 twice as big as A4?

Twice the area, yes; twice each side, no. A3 is 297 × 420 mm, which is one halving step up from A4 rather than a doubling of both dimensions.

Which envelope fits a flat A4 sheet?

A C4 envelope at 229 × 324 mm. If the A4 is folded once, use C5 at 162 × 229 mm instead.

Exact ISO sheet sizes

Morvatek — ISO paper sizes | morvatek.com

Every ISO sheet in the A, B and C series shares one aspect ratio: root two to one, about 1.414 to 1. Cut any sheet in half across its long side and the two halves keep that same shape, one step smaller in the table.

One ratio, three series

ISO 216 fixes a single aspect ratio — root two to one — and applies it across the A, B and C series, so every sheet size shares the same shape.

Halving as the sizing rule

Each step down a series halves the area: the long side of one size becomes the short side of the next, which is why folding a sheet in half always lands on a named size.

From sheet to envelope

The C series exists to wrap A sheets, and because envelopes share the root-2 proportion, each C size maps cleanly onto one A size, flat or folded.

Cut any ISO sheet across its long side and the two halves keep the same shape — that is the entire trick behind the standard.

What does each A size measure?

The A series is a single ladder of sizes from A0 down to A10, each half the area of the one above.

The A series is defined by ISO 216 and starts at A0, 841 × 1189 mm. Halving that sheet across its long side gives A1 at 594 × 841 mm, halving again gives A2 at 420 × 594 mm, and the ladder continues down through A3 297 × 420 mm and A4 210 × 297 mm to A5 148 × 210 mm.

A4 is the size most people meet first: 210 × 297 mm, or 8.3 × 11.7 inches. Two A4 sheets side by side make A3, and two A3 sheets make A2, so the same doubling works going up as halving does going down.

The series does not stop at A5. A6 (105 × 148 mm) suits postcards, A7 (74 × 105 mm) suits small notes, and the ladder runs all the way to A10 at 26 × 37 mm. Every one of these sizes keeps the root-2 proportion.

A0 — 841 × 1189 mm
A1 — 594 × 841 mm
A2 — 420 × 594 mm
A3 — 297 × 420 mm
A4 — 210 × 297 mm (8.3 × 11.7 in)
A5 — 148 × 210 mm

Formats at a glance

Each card lists one ISO size with its dimensions and the kind of printed item it typically carries.

The key terms of this guide, drawn to one scale

Dimension table

Selected ISO sizes in millimetres and inches
Size	Series	Width × height (mm)	Width × height (in)	Typical use
A0	A	841 × 1189	33.1 × 46.8	Large technical drawings
A1	A	594 × 841	23.4 × 33.1	Posters and plans
A3	A	297 × 420	11.7 × 16.5	Spreads and spreadsheets
A4	A	210 × 297	8.3 × 11.7	Standard office document
A5	A	148 × 210	5.8 × 8.3	Notebooks and programmes
B0	B	1000 × 1414	39.4 × 55.7	Bridging format above A0
B4	B	250 × 353	9.8 × 13.9	Between A4 and A3
C4	C	229 × 324	9.0 × 12.8	Envelope for a flat A4

Selected ISO sizes in millimetres and inches

Common questions about ISO sizes

Why does A4 measure 210 × 297 mm and not something rounder?: Because the ratio is fixed at √2 : 1, so the sides come out irrational and get rounded to whole millimetres. The rounding is small enough that the halving rule still works in practice.
Can I fold an A4 sheet in half and get a standard size?: Yes — fold across the long side and you get A5 at 148 × 210 mm. Folding across the short side gives a 105 × 210 mm scrap that matches no ISO size.
Is A3 twice as big as A4?: Twice the area, yes; twice each side, no. A3 is 297 × 420 mm, which is one halving step up from A4 rather than a doubling of both dimensions.
Which envelope fits a flat A4 sheet?: A C4 envelope at 229 × 324 mm. If the A4 is folded once, use C5 at 162 × 229 mm instead.