Most of the world uses the ISO 216 A-series (A4, A3, A5, and so on) for everyday paper, while the US and Canada use their own standard sizes like Letter, Legal, and Tabloid. This calculator looks up the exact dimensions of both systems and computes the scale factor when you need to resize a document or design between two sizes.
How the A-series is built
The ISO 216 standard defines A0 as a rectangle with an area of exactly 1 square meter and a width-to-height ratio of 1:√2 (about 1:1.41421). Every smaller size in the series — A1, A2, A3, and so on down to A10 — is created by cutting the previous sheet in half across its longer edge. Because the 1:√2 ratio is preserved through that halving, every A-series size looks like a scaled copy of every other one; nothing gets stretched or distorted when you go up or down a size.
This is why two sheets of A5 laid side by side are exactly the same shape and area as one sheet of A4, and why an A4 page shrunk to fit on A5 paper looks identical, just smaller.
US sizes and why they don't quite match
US Letter (8.5 × 11 in), Legal (8.5 × 14 in), and Tabloid (11 × 17 in) predate the ISO 216 standard and were never designed around the √2 ratio. Letter's aspect ratio is about 1:1.294, noticeably squatter than A4's 1:1.414. That's the reason documents formatted for one size don't scale cleanly onto the other — margins and line breaks can shift even though the two page sizes look similar at a glance.
Tabloid, at exactly double Letter's long edge, is the closest the US system gets to the A-series' clean-doubling behavior — two Letter sheets side by side match one Tabloid sheet.
Using the scale factor for resizing
When you enlarge or shrink a design from one paper size to another, you need both a linear scale factor (how much longer or shorter each edge becomes) and an area scale factor (how much total surface area changes) to plan things like print resolution, ink coverage, or how many smaller sheets fit inside a larger one. This calculator computes both directly from each size's real millimeter dimensions, so it stays accurate even when comparing across systems — for example scaling artwork from A4 to Tabloid, where the simple √2 rule doesn't apply.