Volume is conserved, area is not
Nothing leaves a kiln during a fuse. The glass that went in is the glass that comes out, so the volume is fixed — but it is free to redistribute, and above a certain temperature it does exactly that until it reaches a thickness it is content with. Thicker than that and it flows outwards. Thinner and it draws inwards, edges pulling up and rounding as it goes.
So the working equation is short. Take the volume of glass in the stack. Divide by the thickness the glass settles to. That is the area it will cover. Three layers of eighth inch sheet on a 6 by 6 square is 13.5 cubic inches; at a quarter inch settled thickness that is 54 square inches, against the 36 you started with. The linear growth is the square root of 54 over 36, which is 1.225, so the piece comes out about 7-11/32 by 7-11/32 — around 11/16 of an inch of spread on each edge.
The settled thickness is a number you supply
This page publishes no figure for it, and you should be wary of anyone who does without qualification. Where a particular glass stops spreading depends on the glass itself, on how hot it got and how long it stayed there, on the shelf and its separator, and on whether the piece was dammed. Different manufacturers, different results, and the same glass fired two different schedules will not land in the same place.
The field defaults to a quarter inch simply so the arithmetic on the page is visible, and the hint asks you to replace it. The way to get a figure that is actually yours is a test tile: fire a known stack, undammed, and measure what comes out. That measurement is worth more than any published number because it includes your kiln and your schedule.
Working backwards from a mold or a dam
The other direction is more common in practice. You have a dam, a mold or an opening that has to be filled to a particular thickness, and you need to know how much glass to cut. That is just the volume of the space: 8 by 8 at a quarter inch is 16 cubic inches, which at 156 pounds per cubic foot is about 1.44 pounds.
Turning that into sheet to cut is a division by the sheet thickness. Sixteen cubic inches of eighth inch sheet is 128 square inches, which is 0.889 square feet — a shade under one 12 inch square sheet before waste, and one sheet with the ten percent allowance. Notice that the answer is an area of glass, not a shape: it does not matter whether that 128 square inches arrives as two 8 by 8 squares or as a hundred small offcuts, as long as the volume is right and it is spread reasonably evenly.
When the target thickness is not a whole number of sheets, the leftover has to come from somewhere — a partial layer, frit, stringers, or a sheet of a different thickness. The page flags it when that happens, because a partial layer that is not deliberately spread will fire into a piece with a thick half and a thin half.
Damming is a size decision, not a decoration
An undammed piece that is thicker than the settled thickness will spread, and it will spread unevenly — corners round off, edges wander, and the outline you cut is not the outline you get. If the finished piece has to be a particular size, that is a problem, and a dam is the answer: it holds the outline and forces the surplus into thickness instead of area.
The corollary is worth knowing too. If you are damming, you can stop thinking about spread and start thinking about fill: the dam sets the area, you set the thickness, and the glass you need is the product of the two. The target mode on this page is exactly that calculation.
What is deliberately absent here
No ramp rates. No hold temperatures. No annealing figures. No process temperatures of any kind. Those depend on the glass, the thickness, the kiln, the shelf and the piece, and they come from the glass manufacturer and your own kiln documentation, which is where they should come from. Getting them wrong does not produce a slightly different result; it produces a piece that cracks, either in the kiln or on a shelf weeks later.
The kiln itself is a hazard worth naming plainly and not explaining: it stays hot long after the controller has finished, glass taken out early breaks from the thermal shock, and grinding fused glass makes the same fine dust that grinding any glass makes. Those are covered by published guidance and by the data sheets for what you are firing, not by a calculator.
Questions people ask
How much does fused glass spread?
Enough to matter, and it is calculable. The volume is fixed, so the area it covers is the volume divided by whatever thickness that glass settles to. Three layers of eighth inch sheet on a 6 by 6 footprint is 13.5 cubic inches; at a quarter inch settled thickness that covers 54 square inches instead of the original 36. The linear growth is the square root of that ratio, 1.225, so a 6 inch square comes out about 7-11/32 inches — around 11/16 of an inch out on every edge.
What thickness does fused glass settle to?
This page will not tell you, and that is deliberate. It depends on the specific glass, on how hot the firing went and for how long, on the shelf and separator, and on whether the piece was dammed, and a confidently stated number that does not apply to your glass is worse than no number. Fire a test tile of a known stack, undammed, and measure what comes out. That figure includes your kiln and your schedule, which no published one does.
Why did my single layer come out smaller than I cut it?
Because it was thinner than the thickness the glass wanted to be, so instead of spreading it pulled inwards. The edges draw up, thicken and round over, and the footprint shrinks. It is the same physics as spreading, running the other way. If the piece has to keep its cut size, either stack it to the settled thickness or dam it, and stacking is usually easier.
How much glass do I need to fill a dam or a mold?
The volume of the space, which is interior area times the thickness you want. An 8 by 8 dam at a quarter inch is 16 cubic inches, about 1.44 pounds at 156 pounds per cubic foot. Cut from eighth inch sheet that is 128 square inches, or 0.889 square feet — one 12 inch sheet with a ten percent allowance. The shape of that glass does not matter as long as the volume is right and it is spread reasonably evenly.
Does this page give a firing schedule?
No, and it will not. Ramp rates, hold temperatures, top temperature and annealing all depend on the glass, its thickness, the kiln, the shelf and the piece, and they belong to the glass manufacturer and your kiln documentation. Getting them wrong does not produce a slightly worse result, it produces cracks — sometimes in the kiln and sometimes weeks later on a shelf. What a schedule costs to run in electricity is on the kiln firing cost calculator.