Paper is so familiar that most people stop noticing it. But if you've ever sent a document to a printer only to find it cropped oddly, or wondered why a brochure feels flimsy compared to another, the answers usually come down to three things: size, weight, and thickness. Understanding these basics makes you a better buyer of print — and helps you avoid costly reprints.
The ISO A Series: Where the UK Standard Comes From
The A series of paper sizes follows an international standard (ISO 216) that dates back to a German proposal from 1922. The genius of the system is mathematical: every sheet in the series has the same aspect ratio — approximately 1:1.414, or 1:√2 — so that folding a sheet in half gives you the next size down with exactly the same proportions.
- A0 — 841 × 1,189 mm. The base size, with an area of exactly one square metre.
- A1 — 594 × 841 mm. Popular for technical drawings and large posters.
- A2 — 420 × 594 mm. Exhibition graphics, maps, and retail point-of-sale.
- A3 — 297 × 420 mm. Menus, tabloid-style newsletters, folded A4 brochures.
- A4 — 210 × 297 mm. The everyday workhorse: letters, reports, leaflets.
- A5 — 148 × 210 mm. Flyers, notepads, compliment slips.
- A6 — 105 × 148 mm. Postcards and pocket-sized cards.
The UK, along with most of Europe and the rest of the world, uses this system as standard. It's tidy, logical, and enormously practical for commercial print.
The SRA Series: A Commercial Printer's Working Sizes
If you've worked closely with a printer, you may have heard of SRA sizes — Supplementary Raw format A. These are slightly larger than A sizes and are what professional presses actually print on. The extra margin allows for bleed (the artwork that runs to the edge of the page) and is trimmed away to give you a clean, finished edge. SRA3, for instance, is 450 × 320 mm — big enough to print an A3 sheet with a comfortable bleed all round.
US Letter and the North American Difference
If you work with colleagues or clients in the United States or Canada, you'll encounter a completely different sizing system. The most common size is US Letter, at 8.5 × 11 inches (216 × 279 mm) — noticeably shorter and slightly wider than A4. This can cause real problems when sharing documents: an A4 PDF sent to an American printer may have its top and bottom clipped, or gain awkward white bands.
Other common North American sizes include Legal (216 × 356 mm), used for contracts and legal documents, and Tabloid (279 × 432 mm), the American equivalent of A3. If you're printing anything destined for a US audience — a trade show handout, a report for a North American partner — it's worth specifying the size explicitly rather than assuming.
Grammage: What Paper Weight Actually Means
Paper weight in the UK and Europe is measured in grams per square metre, usually written as gsm or g/m². This is a consistent measure because it refers to a fixed area regardless of sheet size.
- 60–90 gsm — Lightweight, used for internal documents and newsprint.
- 90–120 gsm — Standard office and copy paper sits around 80–100 gsm.
- 130–170 gsm — Heavier leaflets, quality flyers, annual report pages.
- 200–350 gsm — Business cards, covers, greetings cards, and packaging.
In the US, paper weight is measured differently — in pounds, based on the weight of 500 sheets at a base size that varies by paper type. This makes direct comparison tricky: US 20 lb bond paper is roughly equivalent to 75 gsm. If you're specifying print across borders, always confirm in gsm.
Microns and Caliper: When Thickness Matters
Weight and thickness are related but not identical. Two papers can share the same gsm yet feel quite different in the hand, because caliper — measured in microns (thousandths of a millimetre) — depends on how the paper is manufactured and coated. A coated gloss paper at 150 gsm will be thinner and denser than an uncoated paper at the same weight, because coating fills the surface rather than adding bulk. Thickness matters when you're producing anything that has to fit a specific envelope, pass through a letter box, or sit inside a printed folder without bulging.
If in any doubt, ask your printer for a paper specification sheet — any good trade printer will supply one, and it tells you both the gsm and the caliper.
Choosing the Right Spec for Your Job
The right paper size and weight is rarely just about aesthetics — it affects postage costs, envelope compatibility, how a brochure is perceived, and whether a document prints correctly for an international audience. Getting these details right before you go to press saves time and money.
If you'd like straightforward advice on paper specifications for your next project, visit Ratcliff & Roper Print Group — we've been helping businesses get print right since 1946.