A3 has 2.0679 times the area of US Letter. The pair matters because A3 is the nearest ISO equivalent to the American Tabloid format, and the two do not match any better than A4 and Letter do.
About the units
A3 is 297 × 420 mm, exactly twice A4 by area. US Tabloid, sometimes called Ledger when rotated, is 11 × 17 inches or 279.4 × 431.8 mm, exactly twice Letter. So each system has its own doubling, and the two doubled sizes are as mismatched as the originals — A3 is 18 mm narrower and 12 mm shorter than Tabloid. Comparing A3 directly to Letter rather than to Tabloid gives the ratio above, which is simply twice the A4-to-Letter ratio.
The exact factor
A3 is 124,740 mm² and Letter is 60,322 mm², giving 2.0679 — exactly double the 1.0339 that separates A4 from Letter, since A3 is exactly double A4. That relationship is worth noticing because it means the mismatch between the two paper systems is scale-invariant: it is the same 3.4 per cent area difference at every step of both series, never converging and never getting worse.
Where you meet this conversion
Large-format printing and presentation across markets. Posters, plan drawings and spreadsheets laid out on A3 have to be reproduced on Tabloid in North America, and the scaling issue repeats at every size in the series. Photocopiers in international offices carry both A3 and Tabloid trays for exactly this reason. Imposing two Letter pages on one A3 sheet leaves 3.4 per cent of unused area, which is enough to shift the fold line if a booklet is being produced.