Unit Converters

Why a general converter is not enough

Search for a unit converter and you get a page that knows metres, feet, kilograms and miles. That answers a school physics question and almost nothing you will actually be asked. The conversions that cost marks are the ones nobody lists: kilonewtons to kilogram-force, horsepower to kilowatts, gauss to tesla, poise to pascal-second.

This one is built around the units an engineering course puts in front of you. There are 95 categories and 1,012 units, grouped by the discipline that uses them rather than dropped into one alphabetical list.

The awkward conversions are the point

Most units are one multiplication away from their base unit, and any converter manages those. Temperature is not one of them: Celsius, Fahrenheit and Kelvin differ by a zero point as well as by a scale, so the conversion is a multiply and a shift rather than a multiply.

That zero-point offset is stored against each unit instead of being special-cased in code, which is why the thermal category behaves exactly like every other category, and why adding a new scale is a change to data rather than a change to code.

Numbers, not approximations of numbers

Conversion factors are written as the relations they are, not as pre-rounded decimals. A thousandth is written as one over a thousand. The arithmetic comes out the same either way, but it means the line in the data file is something you can check by eye, rather than a decimal that somebody rounded once and nobody has looked at since.

Where the numbers come from

Every factor is defined against SI. Within a category the units run from smallest to largest, so the list reads the way the quantity does, and the categories are ordered by discipline — a mechanical student and an electrical student open different lists first.

Currency is deliberately absent. Exchange rates move daily, and a converter that ships a rate inside the app is a converter that is quietly wrong by the end of the week. When you are checking a number before submitting it, a missing conversion is better than a wrong one.

Where it fits

Conversion is a scratchpad, not a subject. It sits alongside the formula library, which covers the equations these units appear in, and the GATE virtual calculator, which is the calculator you will actually be holding in the exam hall.