Converter Library

Centimetres to inches, and the reverse. It comes up constantly when comparing product listings between European and US shops, reading furniture dimensions, checking screen sizes, and cutting material to length. A rough rule: divide centimetres by 2.5 to get inches, or multiply inches by 2.5 to get centimetres. The exact factor is 2.54, so the rounding error is under 2%.

The US adopted the customary system before the metric system existed as an international standard (the SI system was codified in 1960). Changing entrenched systems costs money across industry, labelling, infrastructure, and everyday culture; the benefits accrue slowly while the costs are immediate. The UK went through a partial metrication from the 1970s onwards but kept miles for road distances, pints for draught beer, and stones for body weight, which is why UK measurements are sometimes a mix of both systems.

Yes, the pound-mass (lb) is the same in both countries: exactly 0.45359237 kg. Where confusion arises is with the fluid ounce and the gallon. A US fluid ounce is 29.574 mL; a UK (imperial) fluid ounce is 28.413 mL. A US gallon is 3.785 L; a UK gallon is 4.546 L. So a recipe or fuel figure in US gallons and one in UK gallons are not interchangeable. For everything except liquid volume, US and UK imperial units are identical.

Three rules cover most everyday cases. For length: 1 inch is about 2.5 cm (actual: 2.54); 1 foot is about 30 cm. For weight: 1 kg is about 2.2 lb; 1 stone (UK) is about 6.35 kg. For temperature: double the Celsius and add 30 to get an approximate Fahrenheit (works well between 0°C and 30°C). These give errors under 5%, which is close enough for shopping, packing, and cooking.

Shoe sizes are not a simple multiplication; they are based on different measuring conventions. EU sizes follow the Paris point (2/3 of a centimetre); UK sizes use a barleycorn (roughly 1/3 of an inch); US sizes run about one size above UK for men and 1.5 to 2 sizes above UK for women. There is no universal formula that works across all brands, so a size chart is more reliable than arithmetic. The shoe size converter on this site uses the standard Mondopoint-derived mappings for EU, US, UK, Japan, and Korea.

These are legacy holdovers from industries that standardised before or independently of the 1960 SI metrication wave. Screen diagonal sizes became an established marketing figure in inches during the US television era, and no manufacturer has since changed that internationally. Tyre sizing uses a hybrid: section width and aspect ratio are metric (for example, 205/55), but the rim diameter is in inches. Aviation altitude is measured in feet across most of the world outside China and Russia, because the original ICAO conventions were set when US and UK aviation dominated global standards. The practical result is that anyone working across industries needs to cross-convert regardless of which country they are in.

Three quick checks cover most everyday cases. For weight: 1 kg is about 2.2 lb, so 10 kg should give roughly 22 lb; if the result is closer to 4.5 or 45, you have applied the factor in the wrong direction. For temperature: 20°C is a comfortable room temperature and should convert to about 68°F. For length: 1 inch is exactly 2.54 cm, so 10 inches should be 25.4 cm. The most common error is dividing when you should multiply, which typically produces a result that is either implausibly small or unexpectedly large for the context.