A4 vs Letter: Paper Sizes in Every Unit

You’re setting up a print template, designing a flyer, or exporting a PDF, and you need to know exactly how big A4 is in pixels at 300 DPI — or whether Letter and A4 are close enough that a template designed for one will roughly work for the other (they’re not, and the difference matters more than people expect). Paper sizes seem like they should be a settled, universal thing, and then you remember the US uses a completely different standard than most of the rest of the world.

The mix-up between A4 and Letter causes real problems: a document formatted for one prints with unexpected margins or gets cropped on the other, and it’s an easy mistake to make when collaborating internationally or reusing a template from a different region.

What paper size differences actually involve

A4, the international standard (part of the ISO 216 series), measures 210 × 297 mm — roughly 8.27 × 11.69 inches. US Letter, the standard in the United States and a few other countries, measures 8.5 × 11 inches — 215.9 × 279.4 mm. A4 is narrower and taller than Letter; Letter is wider and shorter than A4. The difference is small in absolute terms but large enough that a document designed for one will have wrong margins, or get cropped, when printed on the other without adjustment.

Beyond A4 and Letter, there’s a whole family of related sizes worth knowing: Legal (used for contracts, longer than Letter), A3 and A5 (double and half of A4, respectively, part of the same ISO 216 doubling system), and Tabloid (double Letter). Converting any of these into pixels for digital design requires knowing your target resolution (DPI), since pixel dimensions scale directly with DPI.

Why people get stuck here

• A4 and Letter aren’t interchangeable. They’re close in size but different enough that templates and layouts designed for one won’t cleanly fit the other.
• Pixel dimensions depend on DPI. “A4 in pixels” isn’t a single fixed number — it changes depending on whether you’re targeting screen resolution or print resolution (72 DPI vs. 300 DPI, for instance).
• Regional defaults catch people off guard. Software defaulting to Letter when you expected A4 (or vice versa) is a common source of unexpected margins on a first print.
• The ISO 216 doubling system is unintuitive at first. A3 is double A4, A5 is half A4 — useful once you know it, confusing if you don’t realize the sizes are systematically related.

What a good paper size guide looks like

Covers dimensions in every unit that matters

Millimeters for the ISO 216 sizes, inches for US sizes, and pixels for digital design — all shown together, since different contexts call for different units.

Lets you set a custom DPI for pixel conversions

Pixel dimensions should scale with whatever DPI you’re targeting, not be locked to a single assumed resolution.

Shows a scaled visual comparison

Seeing A4 and Letter drawn to scale next to each other makes the size difference intuitive in a way that comparing two lists of numbers doesn’t.

Common mistakes to avoid

• Assuming a template designed for A4 will print cleanly on Letter paper, or vice versa, without checking margins.
• Using a single fixed “paper size in pixels” number without accounting for your actual target DPI.
• Forgetting that Legal is a different size from Letter (both US-region sizes, but Legal is longer) — an easy mix-up.
• Not double-checking a print job’s paper size setting before sending a large batch to a physical printer.

How to do it with the Paper Size Guide

Online Tool Store’s Paper Size Guide looks up common paper sizes in every relevant unit entirely in your browser.

1. Browse or search for the paper size you need — A4, Letter, Legal, A3, A5, and more.
2. See its dimensions in millimeters, inches, and pixels at once.
3. Adjust the DPI setting to get pixel dimensions for your specific target resolution.
4. Use the scaled visual preview to compare sizes side by side.

Because every unit is shown together, there’s no need to look up a conversion separately or guess at a DPI-dependent pixel figure.

Frequently asked questions

Why does the US use Letter instead of A4?

It’s largely historical — the US, along with a handful of other countries, adopted its own paper size standards before the ISO 216 system (which includes A4) became the international default most other countries use today. There’s no technical reason Letter is better or worse than A4, just a difference in which standard became entrenched regionally.

How many pixels is A4 at 300 DPI?

At 300 DPI, A4 (210 × 297 mm, or 8.27 × 11.69 inches) works out to roughly 2480 × 3508 pixels. The pixel count scales directly with DPI, so the same physical page size gives a different pixel count at 72 DPI (screen resolution) versus 300 DPI (print resolution).

What’s the relationship between A4, A3, and A5?

They’re part of the same ISO 216 system, where each size is exactly half the area of the size before it when folded along its longer dimension — A3 is double the area of A4, and A5 is half the area of A4. This is why A-series paper always has the same aspect ratio regardless of size.

Final thought

Paper size differences are small enough to ignore until they aren’t — the moment you’re designing something meant to print correctly in more than one region, checking A4 against Letter explicitly saves you from a document that looks fine on screen and wrong on paper.

Try the free Paper Size Guide