Many factors are exact by definition (1 in = 25.4 mm, 1 lb = 0.453 592 37 kg). The = or ≈ beside the answer says whether the box shows every digit; click it to see them all.
°C, °F, K and °R relate by scale plus offset, not one multiplier. “Twice the temperature” only makes sense in kelvin.
kg and lb are mass; N and lbf are force. On Earth 1 kg “weighs” about 9.81 N.
Miles per gallon and litres per 100 km are reciprocals, so equal steps in one are not equal steps in the other.
A graph that is a straight line through the origin (metres and feet) lets percentages and ratios carry across unchanged. A line that misses the origin (every temperature scale) does not: 20 °C is not twice as warm as 10 °C, though differences still carry across, scaled; the one temperature where the two scales agree is −40. A hyperbola (fuel economy) does not even let differences carry across, and the consequences are expensive.
kB, MB and GB use powers of 10³ (marketing, networking); KiB, MiB and GiB use powers of 2¹⁰ (memory, file systems). A “500 GB” drive is about 465 GiB. Both scales are listed so you can match the context.
A US gallon is smaller than a UK gallon, and cups, pints and fluid ounces differ too. Always check which system a recipe or spec means.
Real-world unit choice is messy: cooking uses cups and teaspoons, engineering pascals and psi, shipping different “tons” by country. Categories are kept separate because there is no general conversion between different physical quantities; density is its own category because mass over volume is a defined quantity.
Every other category uses a fixed physical constant. Exchange rates are official European Central Bank reference rates, published once per working day around 16:00 CET, not intraday prices, so do not use them for anything time-sensitive. This is the only place the tool talks to an external service (Frankfurter / ECB); the request goes from your browser and nothing passes through this site’s servers.
1 L of water ≈ 1 kg ≈ 10 N of weight, and a cubic metre is a tonne: water’s 1,000 kg/m³ makes it the universal mental anchor for mass, force and volume checks.
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. Density is its own category because mass/volume is a defined physical quantity with standard units — not an arbitrary cross-category multiply.
No — they're official European Central Bank reference rates, published once per working day around 16:00 CET. They're not intraday trading prices, so don't use this for anything time-sensitive.
Every other category converts using a fixed physical constant. Exchange rates change daily, so there's no fixed factor to bake in; this is the only place this tool talks to an external service (Frankfurter / ECB), and the request goes from your browser — nothing passes through this site's servers.
Releases in which this page changed, newest last. Derived from the archived copy of every release, not from notes written afterwards — so it reflects what actually shipped. Site-wide passes are left out; they are in the full changelog.