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Metric and imperial units rarely convert with clean round numbers, which is exactly where doing it by hand goes wrong - this converts length, weight, temperature, area, volume, speed, data and time, with both sides updating live as you type into either one.
When you'd use this
Anywhere a spec, recipe, or measurement is in a unit your audience (or your code) doesn't use natively - converting a US recipe's cups and ounces to metric, or a data sheet's millimetres to inches.
Common errors
Temperature is the one category here that isn't a simple multiplication - Celsius, Fahrenheit and Kelvin each have different zero points, so converting by scaling a ratio (the way you would for length or weight) gives a wrong answer. This tool applies the correct formula for each pair automatically.
Frequently asked questions
Why don't metric-to-imperial conversions come out as round numbers?
Because the two systems aren't related by a clean ratio - a metre is defined independently of a foot, so almost every cross-system conversion produces a repeating or long decimal. That's a property of the units themselves, not a rounding issue in the conversion.