Unit Converter

Most conversion mistakes are not arithmetic mistakes, they are category mistakes: converting a weight when the form wanted a mass-per-volume, or reading a fluid ounce as an ounce of weight. Fix the category first. Length covers distance in metres, kilometres, miles, feet, inches, and yards. Weight covers grams, kilograms, pounds, ounces, and tonnes. Volume covers litres, millilitres, gallons, pints, and cups. Temperature covers Celsius, Fahrenheit, and Kelvin. Area and speed follow the same pattern. Once the family is right, the number takes care of itself.

Two traps are worth naming. The US gallon is 3.785 litres while the imperial gallon used in the UK is 4.546 litres, a 20 percent gap that quietly ruins recipes and fuel-economy figures. And a US fluid ounce measures volume, not weight, so it converts to millilitres and never to grams unless you also know the density of what you are measuring.

Every other unit here scales: double the metres and you double the feet. Temperature does not, because Celsius and Fahrenheit start counting from different places. Fahrenheit is Celsius multiplied by 9/5 and then offset by 32, so 0 degrees C is 32 degrees F, not 0 degrees F. That offset is why you cannot convert a temperature difference the same way you convert a temperature. A rise of 10 degrees C is a rise of 18 degrees F, not 50 degrees F.

Kelvin has no offset problem in the other direction because it shares Celsius' step size: Kelvin is simply Celsius plus 273.15. That is why scientific work uses it. If a value has to survive further maths, converting to Kelvin first usually saves trouble.

The converter keeps a long decimal tail because it does not know how precise your source number was. That decision is yours. If you measured a room as 12 feet with a tape measure, converting to 3.6576 metres implies a precision you never had; 3.66 metres is the honest answer. The rule of thumb is to keep the same number of significant figures you started with.

Precision does matter in a few places, so do not round early there. Shipping and customs forms are checked against declared weights and a rounded kilogram can trip a threshold. Medicine dosing by body weight needs the real figure. Engineering tolerances and machining dimensions are specified to a decimal place for a reason. Everywhere else, a clean two-decimal answer reads better and communicates more accurately than a string of digits.