Most Conversions Are Just Multiplication
Converting kilometers to miles, or grams to ounces, is fundamentally a one-step operation: multiply by a fixed ratio. A kilometer is always exactly 0.621371 miles, no matter the input value, because both units measure the same physical quantity along a scale that starts at the same zero point — zero kilometers is zero miles. That shared zero point is what makes the math a simple multiplication, and it's true of nearly every conversion category: length, weight, volume, area, speed, time, and data storage all share this property.
Why Temperature Breaks the Pattern
Temperature is the outlier, and the reason is that Celsius, Fahrenheit, and Kelvin don't share a common zero. Zero degrees Celsius is the freezing point of water — but zero degrees Fahrenheit is an arbitrary colder point Daniel Fahrenheit picked based on a brine mixture, and zero Kelvin is absolute zero, roughly -273.15°C. Because the zero points don't line up, a straight multiplication doesn't work; converting between them requires first shifting the scale, then multiplying, which is why the formula for Celsius to Fahrenheit is (C × 9/5) + 32 rather than just C × 1.8.
This is a common source of bugs in hand-rolled conversion code — it's easy to build a converter that treats every unit as "multiply by a factor" and get everything right except temperature, where that assumption silently produces wrong answers instead of an error.
The Right Way to Structure a Converter
The cleanest approach for multi-category converters is to define one "base unit" per category — meters for length, kilograms for weight, seconds for time — and store every other unit as a ratio to that base. Converting from any unit to any other becomes two multiplications: source value into the base unit, then base unit into the target. Temperature needs its own small set of functions instead of a ratio table, since it's the one category with an offset instead of a pure scale factor — but keeping it isolated that way means the other seven categories stay simple and bug-resistant.
Why "Works Offline" Matters More Than It Sounds
Every conversion described here needs nothing but arithmetic — no lookup table that changes over time, no external data source, unlike currency conversion which depends on live exchange rates. That makes unit conversion a genuinely good fit for a tool that works with zero network dependency: once the page has loaded once, it keeps working exactly the same on a flight, in a basement with no signal, or on a job site where the nearest random web-based converter simply won't load.