You'll need to measure the glass, convert that into cubic measures and determine the type of glass. There are online metrics that will give you a weight per cubic inch or foot metric. I've had to look them up for things like wood (oak, fir, teak, etc.), sand and gravel.
Keep in mind, (Not to be too technical), but this isn't exactly all there is to it as weight changes depending on location and altitude, but that change will amount to something that is relatively small (a few percent.)