I Built a Unit Converter in Pure Vanilla JS — 7 Categories, 70+ Units, 165 Tests, Zero Dependencies

Unit converters are everywhere online, but they all seem to either require an account, run ads that cover half the screen, or send your input to a server for no reason. I built one that runs entirely in your browser, with no dependencies, no tracking, and no round-trips. 👉 https://unit-converter-dev.pages.dev What It Does Seven conversion categories, 70+ units, real-time bidirectional conversion: Category Example units Length mm, cm, m, km, in, ft, yd, mi, nmi, light-year Weight mg, g, kg, t, oz, lb, st, short ton Temperature °C, °F, K, °R Volume ml, l, m³, fl oz, cup, pint, quart, gallon, tbsp, tsp Area mm², cm², m², km², ha, acre, ft², in², mi², yd² Speed m/s, km/h, mph, ft/s, knot, Mach Data bit, byte, KB/KiB, MB/MiB, GB/GiB, TB — both SI and binary Features: Bidirectional — type in either field, the other updates instantly Swap button — flip from/to with one click All-units panel — see your input converted to every unit in the category simultaneously Formula display — shows the conversion factor (e.g. "1 Mile = 1.609344 Kilometer") Zero dependencies — single HTML file, no build step, no npm Implementation Notes Linear vs. non-linear conversions Most unit conversions are linear: multiply by a factor to get to the base unit, divide by another factor to get to the target. The approach: function convert ( catKey , fromUnit , toUnit , value ) { const base = toBase ( catKey , fromUnit , value ); // → base unit return fromBase ( catKey , toUnit , base ); // base unit → target } function toBase ( catKey , unit , value ) { const u = CATEGORIES [ catKey ]. units [ unit ]; if ( u . toBase ) return u . toBase ( value ); // non-linear (temperature) return value * u . factor ; } Temperature is the classic non-linear case. You can't just multiply to convert between Celsius, Fahrenheit, and Kelvin — you need offset arithmetic: temperature : { units : { C : { toBase : v => v + 273.15 , // °C → K fromBase : v => v - 273.15 , // K → °C }, F : { toBase : v => ( v - 32 ) * 5 / 9 + 2