A4 has 0.8124 times the area of US Legal. Legal is the long American format — the same 8.5 inch width as Letter but three inches taller — and it is the size a European document is least likely to fit.
About the units
US Legal is 8.5 × 14 inches, or 215.9 × 355.6 mm. It shares its width with Letter and adds three inches of height, giving an aspect ratio of 1.647 against A4's 1.414 — a noticeably more elongated page. The format has no ISO equivalent; the nearest is not A4 but a custom size, which is why documents laid out for Legal are awkward to reproduce anywhere outside North America. Its use has declined sharply since the 1980s, and several American federal agencies have actively discouraged it, but it survives in contracts, court filings and some financial documents.
The exact factor
A4 is 62,370 mm² and Legal is 76,774 mm², giving a ratio of 0.8124. The shape difference matters more than the area: fitting Legal content onto A4 requires reducing to about 84 per cent to preserve the width, which shrinks the text noticeably, or cropping 59 mm from the bottom. Neither option is good, which is why documents intended for international distribution are rarely laid out in Legal.
Where you meet this conversion
American legal and financial documents. Contracts, deeds, mortgage papers and court filings use it, largely from tradition and because the extra length suits documents with long signature blocks or multi-column tables. Anyone receiving a Legal-format PDF outside North America meets the problem immediately: printers stocked only with A4 either scale it down or crop it. Scanning services and document management systems have to handle the format explicitly, since automatic paper detection often fails on it.