A, B and C paper sizes: what the numbers actually mean

One ratio, one square metre

ISO 216 defines the A series so that every sheet has the same aspect ratio, 1:√2 (about 1:1.414). A0 has an area of exactly one square metre, which fixes its sides at 841 × 1189 mm after rounding to whole millimetres. Every subsequent size is the previous sheet cut in half across the long side, so A1 is half of A0, A4 is half of A3, and A4 is one sixteenth of A0.

The ratio is the reason a folded document keeps its proportions. Fold A4 once and you get A5; fold it twice and you get A6. Put two A4 sheets side by side and you get A3. That is also why a PDF scaled from A4 to A3 can enlarge without changing its margins proportionally.

What B, C and K series are for

The B series uses the same √2 ratio but a different starting area. B0 is 1000 × 1414 mm, so B sizes sit between neighbouring A sizes and are useful for posters, books and covers that need a little more bleed area. The C series is built for envelopes: a C4 envelope holds an A4 sheet flat, C5 holds a folded A4, and C6 holds a folded A5.

Chinese K-sheet sizes (开纸) come from a different tradition. 16K is about 195 × 270 mm or 210 × 285 mm depending on the trim, 8K is roughly twice that, and 32K half of 16K. They are common in books, notebooks and school paper, but they are not ISO sizes, so a printer may ask which K convention the job uses.

Letter, Legal and photo paper

US office paper does not use ISO 216. Letter is 8.5 × 11 in (215.9 × 279.4 mm), slightly wider and shorter than A4. Legal is 8.5 × 14 in (215.9 × 355.6 mm). If a US client sends a Letter PDF and you print it on A4 without scaling, the right edge is clipped; use “fit to page” or ask for an A4 export.

Photo paper is usually sold by R sizes or inch names. 4R is 102 × 152 mm (4 × 6 in), 5R is 127 × 178 mm, and 6R is 152 × 203 mm. The same picture at 300 DPI needs 1200 × 1800 px for 4R, so a phone photo is usually sufficient, while a cropped image may not be.

Print setup: bleed, safe margin and resolution

A print file needs more than the finished size. Add 3 mm bleed on each edge for a standard commercial job, keep text and logos at least 3–5 mm inside the trim, and place crop marks outside the bleed. For a 210 × 297 mm A4 page, the document is 216 × 303 mm with 3 mm bleed.

Resolution is measured in DPI at the final printed size. 300 DPI is the normal target for photos and text; large posters can be 150 DPI because they are viewed from farther away. Paper weight (gsm) is separate from size: 80 gsm is ordinary office paper, 120–170 gsm is a leaflet or poster, and 250–350 gsm is a business card or cover.

How to read the numbers in this guide

Values here are nominal. A manufacturer may allow a tolerance of a few tenths of a millimetre on a small part, or several millimetres on furniture and clothing, and an office may measure a photo print differently. Treat the table as a starting point, then confirm with the item or the authority that will receive it.

Conversions are rounded for display, but the calculation uses the full factor. Converting back can therefore return 0.999 or 1.001 instead of 1. When a decision depends on a threshold, work from the original unit rather than the rounded display value.

The guides are edited independently of advertising. An ad never changes a value or the order of a table. A correction is checked against the public standard and the data file, then the site is rebuilt, so the guide, the calculator and the category page show the same number.

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