Film thickness is a rate problem, not a paint problem
Material arrives at the panel at some volume per second. The panel goes past at some area per second. Divide one by the other and you have the thickness of the wet film, and there is nothing else in it. The gun sets the numerator, the hand sets the denominator, and the paint has no opinion at all.
That is why the same product can be quoted at wildly different coverage figures by two people who are both telling the truth. Run the defaults on this page: eight fluid ounces a minute, twelve inches a second, a nine inch fan at fifty percent overlap, forty five percent solids and forty five percent transfer efficiency. Two mils wet, nine tenths of a mil dry, three hundred and sixty square feet a gallon. Now slow the hand to eight inches a second and nothing else changes: the film goes up by half, the coverage falls to two hundred and forty square feet a gallon, and the same job needs half as much material again.
The constant behind every coverage figure
A gallon is 231 cubic inches. Spread one mil thick, a thousandth of an inch, it covers 231,000 square inches, which is 1,604 square feet. That number is the whole of coverage arithmetic. Multiply it by the volume solids to account for what evaporates, multiply again by the transfer efficiency to account for what never lands, divide by the dry mils you want, and out comes square feet per gallon.
The calculator computes coverage twice, once from the rate arithmetic and once from that constant, and prints both. They agree because they are the same statement written two ways, and seeing them line up is the fastest way to convince yourself the gun numbers you entered are internally consistent.
Transfer efficiency is the expensive variable
| What moves it | Direction | Why |
|---|---|---|
| Air pressure at the cap | Higher pressure, lower efficiency | More atomising air means more material carried past the panel |
| Gun distance | Further away, lower efficiency | More time in the air, more evaporation and more drift |
| Part size | Small parts, much lower efficiency | A large share of the fan is spraying past the edges |
| Fan width relative to the part | Oversized fan, lower efficiency | Same reason, under the operator control rather than the part shape |
The comparison at the bottom of the results is the point of the page. Holding the film on the metal fixed, going from twenty five percent to sixty five percent efficiency changes what you buy by a factor of more than two and a half. Nothing about the finished paint changes. The material that made the difference went into the air, the filters and the floor.
This also explains a discrepancy that confuses people who use both this page and the auto paint material calculator. That page starts from a coverage figure printed on a data sheet, and a published coverage figure is already a sprayed figure with typical losses inside it, so applying transfer efficiency on top of it would count the loss twice. This page starts from what leaves the gun and works forwards, so the efficiency belongs here. Two correct methods, and the mistake is mixing them.
Measuring the two inputs that matter
Fluid delivery is the easy one and almost nobody does it. Set the gun as you intend to spray it, pull the trigger into a graduated cup for a full minute, read the volume. That single measurement pins down the numerator for good, and it changes when you change the tip, the pressure or the reduction, which is useful information in itself.
Travel speed is harder because it feels fast and is not. Twelve inches a second is a four foot panel crossed in four seconds, which is slower than most people spray when they are concentrating on the edge of the fan rather than the clock. Have somebody time a pass. The number is usually a surprise, and it is the reason the first panel of the day and the last one come out differently.
Everything downstream of those two is arithmetic. What it cannot tell you is whether the film is any good: atomisation, flow, the wet edge, whether the material had time to level before it started to set. Those are properties of the material, the temperature and the reducer grade, and they are why flash times exist at all.
Questions people ask
How do I actually measure fluid delivery?
Fill the cup, set the air pressure and the fluid adjustment exactly as you plan to spray, hold the gun over a graduated container and pull the trigger for sixty seconds by a clock. Read what came out in fluid ounces. Doing it with the fan wide open into the container is fine because you are measuring fluid, not pattern. Repeat it after any change to the tip, the pressure or the reduction ratio, because all three move the number and the arithmetic on this page is only as good as that measurement.
What overlap should I use?
Fifty percent is the common working figure and it is what the default assumes: each pass covers half of the one before, so a nine inch fan advances four and a half inches. Seventy five percent overlap doubles the film for the same hand speed, which is a deliberate technique for some materials and an accident for others. The number matters because it sets how much area a pass actually claims, and a change in overlap has exactly the same effect on film thickness as the same change in travel speed.
Why does the calculator show wet film as well as dry?
Because wet film is what you could measure on the panel with a gauge during the pass, and dry film is what remains after the solvent leaves. The ratio between them is the volume solids. It matters when you are checking work: a wet film gauge reading taken immediately after a pass tells you whether the dry build will land where the data sheet wants it, and it tells you while there is still time to change the hand speed rather than after the coat has flashed.
Is high transfer efficiency the same as a high volume low pressure gun?
Not automatically. Gun design sets the range you can work in, and how you use it decides where in that range you land. The same gun sprayed at higher cap pressure, held further back, with an oversized fan on a small part, will throw a great deal of material past the panel regardless of what the box said. The efficiency figure on this page is deliberately an input rather than a fixed value per gun type, because it is a property of the whole setup and the part, not of the tool alone.
Can I use this for primer and clear as well as base?
Yes, one run per material. Each one has its own volume solids, its own fluid tip, its own delivery rate and usually its own target film, so the answers will differ substantially between them. Clear typically wants a much heavier film than base, which is why it dominates the material cost of a job. What the calculator cannot do is tell you what film any specific product wants, because that is a data sheet figure that changes by product and by temperature.