A blown piece is a skin round a lot of air
Take the shape sitting in the form — 3.2 in at the rim, 4.4 at the belly two and a half inches up, 2.4 at the base, six inches tall, with a 0.14 in wall and a base three times that. The skin holds 11.03 cubic inches of glass. At 2.5 g per cubic centimetre that is 452 grams in the finished piece, and 626 grams on the pipe once 55 grams of moil, 25 grams of punty pad and a 15 percent loss rate are carried. Only 72 percent of the gather reaches the shelf; 174 grams a piece does not.
That ratio surprises people who have never weighed a moil. Moil and punty together are 80 grams of it, and they are invisible because they go straight back into the cullet bin, where they look like thrift rather than like a fixed 80 gram tax on every piece regardless of size.
Why the gather goes up faster than the piece
The block near the bottom computes the same form at 0.75, 1, 1.25, 1.5 and 2 times every dimension, twice over: once with the wall scaled with everything else, once with the wall held. Scale the wall too and the glass follows volume, so it goes as the cube — the two-times piece takes eight times the gather. Hold the wall and the glass follows surface area, so it goes roughly as the square, about four times.
Neither is the doubling people assume. On the default form the two-times piece is 3,616 grams against 452, and holding the wall it is still 1,873. A vase scaled up by a quarter for a bigger version of a good seller is 95 percent more glass at proportional wall, not 25 percent, and it is more glory hole time and more annealer hours to go with it.
Reading it the other way
The gather mode is the useful one after the fact. Weigh a finished piece cold, add back what you know the moil and the punty took, put the measured outside dimensions in, and the page returns the wall you actually blew. Almost nobody guesses that within a third. It gets there by halving an interval until the wall lands within a millionth of an inch, and it says so in the output rather than presenting the answer as if it had been solved in closed form, because for this shape it cannot be.
Run three pieces from a set through it and the spread between them tells you more about consistency at the bench than any amount of looking at them does.
What it does not know
It does not know your glass. The density field is 2.5 because something has to be in the box; the figure that belongs there is the one on the data sheet for what is in your furnace, and a borosilicate or a lead glass is a different number entirely. It has no opinion on annealing, on compatibility or on whether the wall you get is a wall the piece can live with, and it never will — those are the supplier and your own testing.
It also assumes an even wall, which no blown piece has. Use it to size a gather before the first one of a run, then correct it from the piece you break.
Questions people ask
How much glass does a blown vase take?
For the shape in the form — 3.2 in rim, 4.4 in belly, 2.4 in base, six inches tall with a 0.14 in wall and a base three times that — the skin holds 452 grams, and the gather comes out at 626 grams once the moil, the punty pad and a 15 percent loss rate are in it. That is 1.38 lb on the pipe for a 1 lb piece. Your own moil and loss figures move it more than the dimensions do.
How do I work out the wall from a piece I already made?
Switch the mode to gather, weigh the piece cold, add back what the moil and the punty took, and put in the outside dimensions you measure with callipers. The page solves the wall by halving an interval until it closes, and it reports how many halvings it took. It is the honest way to find out whether you are blowing as thin as you think you are.
What density should I use for glass?
Whatever the data sheet for your glass says. Soda-lime studio glass tends to sit near 2.5 g per cubic centimetre and borosilicate lower, but the field is an input rather than a constant precisely because that varies by maker and by colour. If you want your own figure, weigh a cut-off and read its volume by displacement.
Why is the moil worth counting?
Because it is a fixed weight on every piece regardless of how big the piece is, so it costs proportionally more on small work. At 55 grams against a 452 gram vase it is 12 percent of the glass again, and on a set of small tumblers it can be more glass than the tumblers. Weighing a dozen moils once is an afternoon that pays for itself.
Does the calculator tell me if a piece came out annealed?
No, and it will not. Annealing depends on the glass, the thickness, the schedule and the kiln, and every one of those figures belongs to your supplier and your own firing log. This page only counts glass. The schedule side is a separate question and the numbers for it come from the data sheet for the glass in front of you.