Where volume conversions actually come up
Volume is where the US/UK gallon split trips people up: 1 US gallon = 3.785 L, 1 UK (imperial) gallon = 4.546 L — a 20% difference. Recipes moving between US cups (240 ml) and metric ml produce different results if you use the wrong equivalent. Fluid volumes for medication, laboratory reagents, automotive coolant / oil (many Japanese and European cars ship spec sheets in litres, US mechanics work in quarts) all need honest conversion.