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

A developer built a unit converter in pure vanilla JavaScript that supports seven categories and over 70 units, with 165 tests and zero dependencies. The tool runs entirely in the browser with no tracking or server round-trips, handling both linear and non-linear conversions like temperature. It is part of the devnestio collection of free, zero-dependency developer tools.

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 https://unit-converter-dev.pages.dev 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: 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: js 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: js temperature: { units: { C: { toBase: v = v + 273.15, // °C → K fromBase: v = v - 273.15, // K → °C }, F: { toBase: v = v - 32 5/9 + 273.15, // °F → K fromBase: v = v - 273.15 9/5 + 32, // K → °F }, K: { toBase: v = v, fromBase: v = v }, R: { toBase: v = v 5/9, fromBase: v = v 9/5 }, } } The data category includes both SI prefixes 1 KB = 1000 bytes and binary prefixes 1 KiB = 1024 bytes . Both are represented with the same factor-based approach, just stored as separate units: data: { units: { kB: { factor: 8e3 }, // Kilobyte = 8000 bits kiB: { factor: 8192 }, // Kibibyte = 8192 bits = 8 × 1024 mB: { factor: 8e6 }, // Megabyte = 8,000,000 bits miB: { factor: 8388608 }, // Mebibyte = 8,388,608 bits = 8 × 1024² // ... } } The tricky part is displaying results cleanly. 1.6093440000000001 is ugly; 1.609344 is what you want. toPrecision 10 then parseFloat removes trailing zeros while keeping enough precision: function formatNum n { if isFinite n return ''; if n === 0 return '0'; const abs = Math.abs n ; if abs = 1e-3 && abs < 1e13 { return parseFloat n.toPrecision 10 .toString ; } return n.toExponential 6 ; // very large or very small → scientific notation } 165 tests, all passing, using only Node.js assert : §1 Length conversions 17 tests §2 Weight conversions 12 tests §3 Temperature conversions 13 tests §4 Volume conversions 11 tests §5 Area conversions 10 tests §6 Speed conversions 9 tests §7 Data conversions 12 tests §8 formatNum 15 tests §9 convert edge cases 8 tests §10 Roundtrip conversions 13 tests §11 Category structure 12 tests §12 Known reference values 13 tests §13 Additional coverage 20 tests Highlights from the test suite: -40°C = -40°F the only point where Celsius and Fahrenheit are equal 0 K = -459.67°F absolute zero 1 mi = 5280 ft = 1760 yd = 1.609344 km 1 GiB = 1024 MiB = 8589934592 bits 👉 https://unit-converter-dev.pages.dev https://unit-converter-dev.pages.dev Part of devnestio https://devnestio.pages.dev — a collection of free, zero-dependency developer tools that run entirely in your browser.