Deriving the coverage constant
Every powder coverage figure in the world comes from one number, and it is worth building it from scratch once so you never have to trust a chart again.
Water weighs about 62.4 pounds per cubic foot. A cubic foot is 1,728 cubic inches, so water is 0.03611 pounds per cubic inch. Now take one square foot of surface and put a film one mil thick on it: that is 144 square inches at 0.001 inches, which is 0.144 cubic inches of material. At the density of water that film weighs 0.144 times 0.03611, or 0.0052 pounds. Turn it upside down and one pound would cover 1 divided by 0.0052, which is 192.3 square feet.
So a powder with a specific gravity of 1.0 covers 192.3 square feet per pound per mil, and everything else scales from there. Divide by the actual specific gravity and by the mils you are building, and you have the theoretical coverage. Nothing about powder chemistry enters that calculation at any point, which is why it is the same arithmetic for a liquid coating, for a plated deposit or for anything else measured in thickness and bought in mass.
Why two colours at one price are not one price
Specific gravity is largely set by the pigments and fillers, and those vary far more than the resin does. Run the default job on this page at 240 square feet and 2.5 mils and the arithmetic is direct: a powder at specific gravity 1.3 needs 4.06 pounds on the part and a powder at 1.8 needs 5.62 pounds for exactly the same film. That is 38.5 percent more powder, which is the ratio 1.8 divided by 1.3, and it appears in full on the invoice.
Two suppliers quoting the same dollars per pound for those two powders are quoting prices that differ by 38 percent per part. This is the single most common way powder costs get mis-estimated, and it is invisible unless you go looking for the specific gravity on the data sheet, which is where it always is.
What transfer efficiency and reclaim actually do
Transfer efficiency divides. If 60 percent of the powder lands, you must spray 1 over 0.6, so 1.67 pounds for every pound of film. Reclaim then subtracts from the overspray rather than from the total: at 60 percent transfer and 70 percent reclaim you spray 1.67 pounds, 0.67 of it misses, 0.47 of that comes back, and you have consumed 1.20 pounds. Utilisation goes from 60 percent to 83 percent, which is a large gain and is not the same as the 70 percent recovery figure that produced it.
The formula behind that is worth keeping: consumption per pound of film is 1 divided by the transfer fraction, times one minus the product of the miss fraction and the reclaim fraction. It tells you something useful about where to spend effort. At high transfer efficiency, reclaim has little left to recover. At low transfer efficiency, reclaim is doing most of the work, and a booth that reclaims badly is expensive in a way that never shows up as a defect.
Where this sits
The area you feed this page is the whole coatable surface, which for fabricated steel is rarely obvious: the fabricated steel surface area calculator builds it from a bill of materials. What has to be plugged, capped and taped before any of it gets sprayed is the masking and hanging take-off, which is usually the larger labour figure. For liquid coatings the equivalent arithmetic runs on volume solids rather than specific gravity, and that is the coating spread rate calculator; for spray gun setup rather than material quantity, the spray gun film build calculator works from fluid delivery and travel speed.
Questions people ask
Where does 192.3 square feet per pound come from?
From the density of water and nothing else. Water is about 62.4 pounds per cubic foot, which is 0.03611 pounds per cubic inch. One square foot of film one mil thick is 144 square inches by 0.001 inches, or 0.144 cubic inches, and at that density it weighs 0.0052 pounds. One divided by 0.0052 is 192.3 square feet per pound. Specific gravity is by definition the ratio to water, so dividing by it converts the figure to any material. The constant is a unit conversion, which is why it is identical for every powder ever made and why anything that varies between products has to enter afterwards.
Is transfer efficiency the same as first pass transfer efficiency?
Not necessarily, and the difference matters when reclaim is involved. First pass transfer efficiency is the share that lands the first time powder leaves the gun. What this page wants in the transfer efficiency box is that same first pass figure, because the reclaim box then handles what happens to the rest. If you enter a number that already includes the benefit of reclaim and then also fill in the reclaim box, you will double count the recovery and the answer will be optimistic. When in doubt, weigh what left the hopper for a run and weigh the film you got, and use that ratio with reclaim set to zero.
Can I use this for a liquid coating?
Only if you are working in mass rather than volume, which is unusual for liquids. Liquid coatings are bought and quoted by volume, so their coverage arithmetic uses volume solids and the 1,604 square feet per gallon per mil constant instead. The coating spread rate calculator on this site does that version. The two constants are the same idea expressed in different units: 231 cubic inches in a gallon spread a mil thick gives 1,604 square feet, and one pound of a specific gravity 1.0 material spread a mil thick gives 192.3.
Why is my measured consumption higher than this page says?
Several ordinary reasons, in rough order of size. The film is thicker than the target, which is very common because operators build extra where coverage looks thin and nobody measures the edges. The transfer efficiency figure is optimistic, particularly on open or complex parts where a lot of the spray is aimed at nothing. Powder is lost outside the booth in hopper changes, sieve residue and floor sweepings, which never enters the arithmetic at all. And the coated area is larger than the drawing suggests, especially on tube, angle and mesh. Weighing a run and working backwards through the same formula will tell you which of those it is.
Does the specific gravity change the film thickness I should target?
No, and that is the point of separating them. Film thickness is a performance question decided by your specification or the powder data sheet, and it is expressed in thickness because that is what determines what the coating does. Specific gravity only tells you what that thickness costs in mass. A denser powder at the same mils does not give you a better coating, it gives you the same coating and a larger invoice. That is exactly why the two are separate inputs here rather than combined into a single coverage figure.