Paper Size Converter — ISO and North American formats

A paper size converter is a small tool that solves a problem almost everyone runs into eventually: knowing a paper format's name — A4, Letter, Tabloid — but not its actual dimensions in the unit you need right now, whether that's millimeters for a print shop, inches for a US-made template, or pixels for a design file at a specific resolution. This page converts every standard ISO and North American paper size into all four at once, so you never have to look it up twice or do the DPI math by hand.

Arb Digital designs marketing collateral, print-ready PDFs, and web assets for clients across countries that use different paper standards — mostly ISO A-series outside North America, and Letter/Legal inside the US and Canada — and getting a size wrong at the file-setup stage is one of the most common, easily preventable production mistakes in both print and digital design.

What This Paper Size Converter Does

Pick any ISO A-series size (A0 through A10), ISO B-series size (B0 through B5), or North American size (Letter, Legal, Tabloid, Executive), and the tool instantly shows the exact dimensions in millimeters, centimeters, and inches, plus the pixel dimensions at a DPI you choose. It also shows the simplified aspect ratio, which is useful for scaling artwork or checking whether a design will crop awkwardly when moved from one paper size to another.

How to Use It

1. Select a paper size from the ISO A, ISO B, or North American groups in the dropdown.
2. Choose a DPI if you need pixel dimensions — 300 DPI is the standard for print-quality output, 72 or 96 DPI for on-screen design.
3. Click Convert Size to see millimeters, centimeters, inches, and pixels together, along with the aspect ratio.
4. Use the pixel figure directly when setting up a canvas in image editing or design software that only accepts pixel dimensions.

The Math Behind ISO Paper Sizes: The √2 Ratio

Every ISO A-series and B-series paper size shares the same defining property: a fixed aspect ratio of 1 to the square root of 2, roughly 1:1.4142. The reason this specific ratio was chosen, standardized internationally as ISO 216, is elegant: if you fold a sheet with a √2 aspect ratio exactly in half along its longer dimension, the two resulting halves each have the identical √2 aspect ratio, just at half the area. That's why A4 folded in half gives you two A5 sheets, each still shaped exactly like a smaller version of A4, and why A3 folded in half gives you A4. No other aspect ratio has this self-similar folding property, which is precisely why it was chosen as the international standard: it lets a single printing press or paper mill produce every size in the series by successive halving, with zero wasted trim and no distortion of proportions at any step.

The starting point of the whole series is A0, defined to have an area of exactly one square meter, with its width and height chosen so the √2 ratio holds. Every other size in the A series is derived by halving A0 that many times: A1 is A0 folded once, A2 is A0 folded twice, and so on down to the tiny A10. The B series uses a slightly larger base sheet at each tier, intended for posters, passports, and other formats that need a size that sits between two adjacent A sizes.

Why North America Uses a Different System

The United States and Canada never adopted ISO 216, and instead use a set of sizes — Letter (8.5 × 11 in), Legal (8.5 × 14 in), Tabloid (11 × 17 in), and Executive (7.25 × 10.5 in) — that trace back to historical paper-making conventions rather than a mathematical ratio. None of these sizes share the ISO series' fold-in-half property: Letter's aspect ratio is roughly 1:1.294, noticeably squatter than A4's 1:1.414, which is why a document designed for A4 doesn't scale cleanly onto Letter paper without either leaving extra white space or cropping content. This is a genuinely common cross-border headache for anyone producing a PDF meant to be printed correctly on both sides of the Atlantic (or Pacific): a form built for Letter will have different margins than the same form reflowed for A4, and "auto-fit to page" settings in PDF viewers often compensate by shrinking or stretching the content in ways that look slightly off.

Understanding DPI and Pixel Dimensions

DPI (dots per inch, sometimes called PPI for pixels per inch on screens) is the bridge between a physical size and a digital canvas size. It tells you how many pixels are packed into each inch of the printed or displayed output. A 96 DPI setting — the long-standing default assumption for web and screen design — means one inch of physical space corresponds to 96 pixels; a professional print job typically uses 300 DPI, because the human eye can resolve much finer detail on paper held close up than it can on a backlit screen viewed from further away. The pixel dimensions of any paper size are simply its inch dimensions multiplied by the chosen DPI — an A4 sheet (8.27 × 11.69 in) at 300 DPI becomes roughly 2481 × 3508 pixels, while the same sheet at 72 DPI is only about 595 × 842 pixels. Setting up a canvas at too low a DPI for print produces a file that looks fine on screen but prints blurry or pixelated; setting it too high for a web image wastes file size and load time without any visible benefit, since most screens can't resolve detail beyond roughly 96–150 effective DPI at normal viewing distance.

Common Real-World Uses

• Setting up a print-ready design file at the correct pixel dimensions for a chosen paper size and DPI before sending it to a printer.
• Converting a template built for one region's paper standard (say, A4) so it fits correctly when printed on the other region's default size (Letter), and vice versa.
• Choosing packaging or poster dimensions from the B series, which sits between adjacent A sizes for formats that need a size in between.
• Quick reference during software setup when a program's page-size dropdown only shows a size name, not its dimensions.
• Estimating file size and canvas requirements before starting a high-resolution scan or print job.

Common Mistakes to Avoid

• Assuming A4 and Letter are interchangeable. They're close in size but have different aspect ratios, so content can crop or leave unexpected white space when moved between them.
• Designing at screen DPI for a print job. A file built at 72 or 96 DPI will look sharp on screen but print blurry — always design at 300 DPI or higher for physical output.
• Confusing the B series with the A series. They're both ISO standards with the same √2 ratio but different base sizes — B4 and A4 are not the same dimensions.
• Forgetting bleed and margin allowances. A print shop's finished trim size is usually smaller than the raw paper size once bleed is trimmed off; this tool gives the paper size itself, not the safe print area.
• Mixing up landscape and portrait when reading dimensions. "210 x 297" and "297 x 210" describe the same sheet in different orientations — check which one your software expects.

Related Free Tools From Arb Digital

You may also find the Aspect Ratio Calculator useful for checking how a design's proportions translate across formats, the Currency Format Converter for localizing pricing on printed materials, the Timezone Converter for coordinating with print vendors abroad, and the Length Converter for general unit conversions. Browse everything in our free online tools hub.

Frequently Asked Questions

Choosing the Right Size for the Job

A4 is the default for everyday documents, letters, invoices, and reports across most of the world outside North America, and it's the safest default when you don't know which country a printed document will end up in, aside from the US, Canada, and a few other Letter-standard markets. A3 is the natural step up for posters, presentation boards, and spreadsheets that need extra width, since it's exactly double an A4 sheet with identical proportions. A5 and A6 work well for flyers, postcards, and note cards where a full A4 sheet would be needlessly large. On the North American side, Legal is mainly used for contracts and legal documents that need extra vertical length beyond Letter, while Tabloid is common for newspapers, large charts, and spreadsheets printed in landscape. Executive is less common today but still appears in some older office templates and certain government forms.

When a design has to travel between an ISO market and a North American one — a template shared between a US office and a European partner, for instance — it's worth deciding up front which size is the "master" and building margins generously enough that reflowing to the other standard doesn't clip any content. A safe practice is designing critical content (text, logos, key visuals) within the smaller of the two sizes' printable area, even if the file itself is set to the larger size, so nothing important sits in the zone that gets trimmed when the same file prints on the other region's paper.

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