Unit of Measure Conversion Library

Here's a little script I threw together just for the heck of it. It handles all the SI conversions for grams, bytes, meters and liters, and also I've added ounces and pounds as an example of non-SI units. To add more, you'll need to:

1. Add the base type to the "units" list for items that follow SI or
2. Add the conversion ratios for items that don't follow SI

Usage:

$u(1, 'g').as('kg').val(); // converts one gram to kg

You can get the value out with .val(), a string representation using .toString() or the full details via .debug()

(function () {
    var table = {};

    window.unitConverter = function (value, unit) {
        this.value = value;
        if (unit) {
            this.currentUnit = unit;
        }
    };
    unitConverter.prototype.as = function (targetUnit) {
        this.targetUnit = targetUnit;
        return this;
    };
    unitConverter.prototype.is = function (currentUnit) {
        this.currentUnit = currentUnit;
        return this;
    };

    unitConverter.prototype.val = function () {
        // first, convert from the current value to the base unit
        var target = table[this.targetUnit];
        var current = table[this.currentUnit];
        if (target.base != current.base) {
            throw new Error('Incompatible units; cannot convert from "' + this.currentUnit + '" to "' + this.targetUnit + '"');
        }

        return this.value * (current.multiplier / target.multiplier);
    };
    unitConverter.prototype.toString = function () {
        return this.val() + ' ' + this.targetUnit;
    };
    unitConverter.prototype.debug = function () {
        return this.value + ' ' + this.currentUnit + ' is ' + this.val() + ' ' + this.targetUnit;
    };
    unitConverter.addUnit = function (baseUnit, actualUnit, multiplier) {
        table[actualUnit] = { base: baseUnit, actual: actualUnit, multiplier: multiplier };
    };

    var prefixes = ['Y', 'Z', 'E', 'P', 'T', 'G', 'M', 'k', 'h', 'da', '', 'd', 'c', 'm', 'u', 'n', 'p', 'f', 'a', 'z', 'y'];
    var factors = [24, 21, 18, 15, 12, 9, 6, 3, 2, 1, 0, -1, -2, -3, -6, -9, -12, -15, -18, -21, -24];
    // SI units only, that follow the mg/kg/dg/cg type of format
    var units = ['g', 'b', 'l', 'm'];

    for (var j = 0; j < units.length; j++) {
        var base = units[j];
        for (var i = 0; i < prefixes.length; i++) {
            unitConverter.addUnit(base, prefixes[i] + base, Math.pow(10, factors[i]));
        }
    }

    // we use the SI gram unit as the base; this allows
    // us to convert between SI and English units
    unitConverter.addUnit('g', 'ounce', 28.3495231);
    unitConverter.addUnit('g', 'oz', 28.3495231);
    unitConverter.addUnit('g', 'pound', 453.59237);
    unitConverter.addUnit('g', 'lb', 453.59237);


    window.$u = function (value, unit) {
        var u = new window.unitConverter(value, unit);
        return u;
    };
})();

console.log($u(1, 'g').as('kg').debug());  
console.log($u(1, 'kg').as('g').debug());
console.log($u(1, 'g').as('mg').debug());
console.log($u(1, 'mg').as('g').debug());
console.log($u(1, 'mg').as('kg').debug());

console.log($u(1, 'g').as('oz').debug());
console.log($u(1, 'g').as('lb').debug());

console.log($u(1, 'oz').as('lb').debug());

console.log($u(1, 'lb').as('g').debug());

// this last one throws an exception since you can't convert liters to mg
console.log($u(1, 'l').as('mg').debug());

I've moved this to a small repo on Github so if anyone wants to improve/enhance they can do so: https://github.com/jerodvenemafm/jsunitconverter

You might check out this port of Ruby Units to Javascript:

https://github.com/gentooboontoo/js-quantities