This question has been coming up a lot recently on some of the glass fusing communities I'm on, so thought it would be good to do a post about it with my way of doing it and also links to some other people who have other ideas.
THREE WAYS TO WEIGH YOUR GLASS, WITHOUT MATHS!
These are my 3 top ways of making sure you have the right amount of glass for your pot melt, screen melt or other melt. You can also use the "fill up with water" method for glass casting.
1) Cardboard Box Method: This is my favourite method because it's so simple! Use a piece of cardboard box, cut out the shape of your melt and then cover it in layers of glass pieces to make the depth you want. The good thing about this is you have a permanent "template" to go with your mould. Just add a bit for the pot!
For a "normal thickness" melt you probably want 2 layers of 3mm glass (6mm), or 3 layers of 3mm glass, (9mm). If you don't have enough glass then your ring won't fill up enough, so if you want a perfect circle it's best to slightly over-cater with the glass.
(You could use paper for your template, but I find it tears when I try and tip the pieces into the pot.)
(Note: If you're just dripping on to your shelf, your melt is likely to come out as a circle and will spread so it is approx 6mm thick, so it's always a good idea to put a barrier or ring around the glass to make sure it doesn't run off your shelf!)
2) Weigh It Method: Weigh a 3mm piece of glass the right size for your melt. Write down the weight. If you want to end up with a 6mm piece (using the 3mm circle as a base to drip on to) then put the same amount of glass in the pot, plus a bit (as some always stays in the pot). If you want a 9mm piece put twice as much plus a bit in the pot. If you're not using a circle of glass on the bottom then put more in the pot to compensate. Note: This is the most accurate non-maths version!
2) Fill It With Water Method: Another way, which is really helpful for weird shaped melts like fish and stuff, or for glass casting, is to put plasticine all round your metal melt circle, in a deep tray, fill it up with water as far as you want to have your glass, pour into measuring jug and then use the same volume of glass (not weight as glass is heavier than water) plus a bit for your melt. (This assumes you are melting into a stainless steel casting ring. If you are then you don't need to add extra for the pot as the circle will be a bit smaller once you have lined it with fiber paper so you'll have a bit spare to leave in the pot).
THE "MATHS VERSION"!
Square melt: length x width x depth.
Circular melt: 3.142 (pi) x half width squared (r squared) x depth.
Then multiply by 2.5 for weight of glass, and add a bit for the pot.
(e.g. 10cm square melt x 6mm deep would be 10 x 10 x 0.6, which would be 60, x2.5 for weight of glass = 150g plus a bit for the pot)
(e.g. 10cm circle melt x 6mm deep would be 3.142 x 5x5 x 0.6, which would be 47g, x2.5 for weight of glass = 118g plus a bit for the pot.)
LINKS WHICH CALCULATE IT FOR YOU
There are also several pot melt online calculators and tutorials out there, which can help you with this:
(You could use paper for this, but I find it tears when I try and tip the pieces into the pot.)
Experienced silk painter, glass fuser, teacher, enthusiastic and inspirational.