This calculator is setup for 1 gallon of beer brewed using extract.

Formula:

var gravity = [OG] - 1;
var volume = 1.45;
var alpha = new Array(numberOfHops);
var mass = new Array(numberOfHops);
var time = new Array(numberOfHops);
var util = new Array(numberOfHops);
var mgperl = new Array(numberOfHops);
var bu = 0;

for (i = 0; i < numberOfHops; i++)
{
    alpha[i] = [alpha" + i + "] / 100;
    mass[i] = [weight" + i + "] / 28.3495231;
    time[i] = [time" + i + "];
    mgperl[i] = alpha[i] * mass[i] * 7490 / volume;
    util[i] = 1.65 * Math.pow(0.000125, gravity) * (1 - Math.exp(-0.04 * time[i])) / 4.15;
    bu += (mgperl[i] * util[i]);
}