Hong Life CalculatorsUnderstanding the B paper size chart — concept and principle
B sizes sit between the familiar A sizes and are used for books, posters, passports, envelopes and, in Korea and Japan, for school notebooks and copy paper. The confusing part is that two different B series exist: the international ISO B series and the slightly larger JIS B series used in Japan and adopted in Korea. This chart shows, for each size from B0 to B10, the ISO dimensions in millimetres and inches, and the JIS/KS dimensions where they are commonly used.
How it is calculated
The chart is a lookup table, but every entry follows simple rules:
- ISO B0 = 1000 × 1414 mm, i.e. a short side of 1 m and a ratio of 1 : √2
- Each next size is the previous one cut in half across its long side: B1 = 707 × 1000, B2 = 500 × 707, and so on, rounded down to whole millimetres
- Inches = mm ÷ 25.4
- JIS B0 = 1030 × 1456 mm (area 1.5 m²), halved the same way
The principle
All ISO 216 sizes have sides in the ratio 1 : √2 ≈ 1 : 1.414. Only this ratio survives folding in half: if a sheet is 1 × √2, half of it is √2/2 × 1, which is the same shape again. A0 has an area of exactly 1 m². ISO B sizes are the geometric mean of two neighbouring A sizes, so B0 has an area of √2 m² and every Bn lies halfway (on a multiplicative scale) between An and A(n−1). The JIS series keeps the same 1 : √2 shape but starts from an area of 1.5 m², exactly 1.5 times the A series, so JIS B is always a little bigger than ISO B.
Worked example
Default input: B5.
- Geometric mean of A5 (148 × 210) and A4 (210 × 297): √(148 × 210) ≈ 176.3 and √(210 × 297) ≈ 249.7, giving 176 × 250 mm.
- In inches: 176 ÷ 25.4 ≈ 6.93 and 250 ÷ 25.4 ≈ 9.84, so 6.93 × 9.84 in.
- JIS/KS B5: half of JIS B4 (257 × 364) is 182 × 257 mm.
How to read the result
If a Korean or Japanese product says "B5", it almost always means JIS B5 (182 × 257 mm), the typical school notebook. ISO B5 (176 × 250 mm) is more common for books and envelopes elsewhere. B5 sits between A5 and A4; B4 sits between A4 and A3, which is why B4 was popular for documents a little bigger than A4. When printing, a "reduce A3 to B4" setting means a scale of about 86% (JIS) rather than the 71% from A3 to A4.
Common mistakes
- Assuming every B5 is the same; ISO and JIS B5 differ by 6–7 mm per side.
- Mixing up B paper with C envelope sizes, which are another series designed to hold A sheets.
- Expecting a printer tray labelled "B5" to accept both ISO and JIS sizes.
- Rounding mm to inches too early; layouts can shift by a few millimetres.
Frequently asked questions
What size is a Korean B5 notebook?
JIS/KS B5, 182 × 257 mm.
Is B5 bigger than A5?
Yes. ISO B5 is 176 × 250 mm and A5 is 148 × 210 mm; B5 is smaller than A4.
What size is B4?
ISO B4 is 250 × 353 mm; JIS/KS B4 is 257 × 364 mm.
Why does paper use the √2 ratio?
Because halving a √2 sheet produces the same shape, so every size can be scaled to the next with no wasted margins.
Sources: ISO 216, Writing paper and certain classes of printed matter — Trimmed sizes — A and B series · JIS P 0138, Processed paper sizes · KS M ISO 216