US customary versus metric on this site

The Unit Converter opens on US customary units because that is the everyday language of most readers here: inches on a tape, pounds on a bag, Fahrenheit on a porch thermometer, and gallons at a pump. The metric switch is a starting-unit control. It does not hide the US units, and it does not replace the US gallon with the imperial gallon.

Length uses 1 inch = 2.54 centimeters exactly. A foot is 12 inches, so it is 30.48 centimeters. If you need millimeters or kilometers, choose them in the menus after you pick Length. Zero inches is zero centimeters; an empty box is an error, because there is nothing to convert.

Weight uses the avoirdupois pound of 0.45359237 kilograms. That is the grocery-store pound, not a troy pound. Ounces on this page are 1/16 of that pound.

Temperature is the place people skip the offset. Fahrenheit = Celsius × 9/5 + 32. 32 °F is 0 °C. If you only multiply by 9/5, or only subtract 32, the result is wrong by a wide margin. The converter uses the full formula in both directions.

Volume uses the US gallon of 231 cubic inches, which is 3.785411784 liters. The imperial gallon used in some other countries is larger. If a recipe or a fuel note came from abroad, confirm which gallon the writer meant before you scale a tank or a stockpot. On this site the word gallon means the US gallon.