One decilitre is 0.4226752838 US cups, and conversely one US cup is 2.366 decilitres. This is the single most useful conversion for anyone cooking Nordic recipes with American equipment, or the reverse.
About the units
The decilitre is a tenth of a litre, 100 ml. It is barely used in English but it is the backbone of Scandinavian recipe writing: Swedish, Norwegian and Danish recipes specify flour, sugar, milk and liquids in deciliters as a matter of course, and Nordic measuring jugs are graduated in them. The US cup is 236.588 ml, defined through the US gallon. The two units are close enough — 2.37 dl to the cup — that they are often treated as interchangeable in casual conversion, and far enough apart that the error compounds badly across a recipe with several cup measures.
The exact factor
The exact relationship is 1 cup = 2.365882365 dl, so 1 dl = 0.422675283773 cups. The convenient approximations run both ways: a cup is very nearly two and a third decilitres, and a decilitre is a little under half a cup. Treating a cup as 2.5 dl — a common shortcut, because that is the metric cup — overstates by 5.7 per cent, which across four cups of flour is an extra half decilitre. Going the other way, treating 2 dl as a cup understates by 15 per cent.
Where you meet this conversion
Nordic baking is the main case, and it is a demanding one because Scandinavian recipes lean heavily on volume for dry ingredients. A Swedish bun recipe specifying 12 dl of flour becomes 5.07 US cups, a number no American measuring set can hit precisely. The practical answer used by experienced cooks in both directions is to convert to grams instead: Nordic recipes increasingly print both, and weighing removes the packing-density problem entirely. Liquids convert cleanly and can be done with the volume figure directly.