Pick a category, type a number, and read it in every other unit at once.
Most converters make you set a "from" unit and a "to" unit before you see anything, which is two menus of work for one number. This page skips the second menu: type the amount, pick what it is, and the whole category is on screen. If you came to turn inches into centimeters you also get millimeters, feet and yards for free.
The Clinical tab is the short shelf: pounds and kilograms, Celsius and Fahrenheit, micrograms through grams, milliliters and liters, centimeters and inches. Picking a unit there shows the other units it can actually become, so temperatures never appear in a list of weights.
The pound is exact, not approximate. One pound is defined as 0.45359237 kilograms exactly, by international agreement since 1959, and that is the number this page multiplies by. The inch is exactly 2.54 centimeters for the same reason. Nothing here is a rounded lookup table.
Cups, tablespoons and teaspoons use the kitchen conventions. This page uses the US nutrition-labelling values: a cup is 240 mL, a tablespoon is 15 mL and a teaspoon is 5 mL. The older US customary cup is 236.588 mL and a UK cup is 250 mL, so a recipe from another country can be a few percent off. For baking, weigh it.
Temperature uses formulas, not a factor. Celsius and Fahrenheit have different zero points, so the page converts through Celsius: Fahrenheit is C times 9/5 plus 32, and Kelvin is C plus 273.15. That is why a temperature of zero does not stay zero.
Results are rounded to five significant digits. That is enough to be exact for everyday work and short enough to read, but it means a long chain of conversions can drift in the last place. Trailing zeros are trimmed, so 2.54 shows as 2.54 and not 2.5400.
There is no currency here on purpose. Money is not a fixed factor: converting it needs live exchange rates from somewhere else, which would mean this page calling out to an outside service every time you typed. Every unit on this page is a fixed, defined relationship that never changes, so the page never has to ask anyone anything.
You don't have to take our word for it. Every browser has a built-in panel that shows everything a page sends or receives over the internet. On a computer (this doesn't work on a phone):