Pick a category, choose a "from" and "to" unit, and type a value — the conversion happens instantly, at full precision internally, rounded only for display. Every category shares one design: type in either box, or hit swap to flip direction without re-typing.
Converting 100°F to Celsius isn't a simple multiply — temperature scales have an offset as well as a ratio. This tool applies the exact affine formula (°C = (°F − 32) × 5/9) rather than a rule-of-thumb approximation, so the result — 37.8°C — matches a lab thermometer, not just a mental-math shortcut.
Where a factor is defined by international standard — the inch, the pound, the international foot — it's used to full precision, not rounded early. A few less common units use the best publicly available conversion factor, clearly no less precise than typical reference tables.
Because real-world unit choice is messy: cooking uses cups and teaspoons, engineering uses pascals and psi, shipping uses different "tons" depending on the country. The category list reflects units people actually encounter, not just the metric/imperial pair.
No — categories are kept separate because there's no general conversion between fundamentally different physical quantities. If you need a derived quantity (like density from mass and volume), that's a calculation, not a unit conversion, and belongs in a dedicated tool.
Swap moves the current output into the input box so you can convert the other way immediately. If there's no valid output yet (empty or invalid input), there's nothing to carry over, so the input box stays as you left it.