Most of a frit layer is air
Pour frit with a bulk density of 1.4 grams per cubic centimetre and the glass it is ground from is 2.5. The packing fraction is 56 percent, so 44 percent of what you laid down is space between grains. An eighth of an inch of loose frit fuses to about 0.07 in — it loses 0.055 in of height, and none of that is glass leaving.
That is the number people are missing when a layer that looked right on the shelf comes out of the kiln thin. It did not shrink; it was never that thick to begin with.
What a square foot takes
At 1.4 bulk density an eighth of an inch over a square foot is 413 grams — a pound of frit for one 12 by 12 tile at one depth, before any allowance for what blows off the sifter. Halve the depth and you halve the weight; the relationship is a straight line because the packing fraction does not change with how deep you pour. On the defaults, with 10 percent written off to waste, a 12 inch square wants 459 grams weighed out, which is 16.2 ounces, or two of the 8.5 ounce jars.
Two jars for one tile is the moment most people discover why frit work is priced the way it is.
Grain size changes the answer
The bulk density is the whole calculation and it is different for every grade. Fine powder settles tighter than coarse frit because the small grains fill the gaps the big ones leave, so the same visual depth of powder weighs more and fuses thicker. There is no table of it here because it also depends on how you pour — tapped down reads higher than sifted loose, and a jar that has been shipped across the country has settled in transit.
Weighing a level measuring cup of each grade takes about a minute per jar and it is the only way to get the number that belongs to your material. Write it on the lid in marker.
Going the other way
The thickness mode is the one to use when the design fixes the outcome. Ask for a 0.07 in fused layer at the same densities and it tells you to lay 0.125 in and weigh out 459 grams for the square foot. It is the same arithmetic run backwards, and it is more useful than it sounds, because a fused thickness is something you can specify and a loose depth is something you can only rake and hope.
Neither mode has an opinion on whether the frit and the base sheet belong together. Compatibility is settled by the supplier and by firing your own test tiles, and no amount of weighing changes it.
Questions people ask
How many grams of frit per square foot?
At a bulk density of 1.4 g per cubic centimetre, an eighth of an inch deep is 413 grams a square foot, a sixteenth is 206, and a quarter inch is 826. The figure is proportional to depth and proportional to bulk density, so the grade of frit matters as much as the depth. Measure your own bulk density rather than taking the 1.4 in the box, which is only there to make the arithmetic visible.
How thick does frit fuse down to?
It collapses by the packing fraction — the bulk density divided by the solid glass density. At 1.4 over 2.5 that is 56 percent, so an eighth of an inch of loose frit ends up near 0.07 in. Nothing is lost; the air between the grains goes. Fire a measured depth on a test tile and measure the result to get the figure for your own material and schedule.
How do I measure the bulk density of frit?
Fill a container of known volume level with the frit, poured the way you would actually pour it, weigh it, subtract the container and divide the grams by the cubic centimetres. Do not tap it down unless you tap it down at the bench, because packed frit reads a good deal higher than sifted frit and the answer has to match how you work.
Does powder cover differently from coarse frit?
Yes, and it is the main reason a single coverage figure does not exist. Fine powder packs tighter because the small grains fill the voids the coarse grains leave, so it weighs more for the same depth and fuses to a thicker layer. Each grade needs its own bulk density number, which is why the page asks for one rather than publishing a table.
Is the frit I bought compatible with my base glass?
That is not a question this page can answer and it does not try. Compatibility is a property of the two glasses and is settled by the supplier who tested them and by firing your own samples. The calculator only converts a depth and an area into a weight of material, and it would produce the same weight for glasses that do not belong anywhere near each other.