The one constant worth memorising
A US gallon is 231 cubic inches. Spread it a thousandth of an inch thick and it covers 231,000 square inches, which is 1,604 square feet. That figure is not a property of any coating, it is a unit conversion, and every spread rate on every data sheet is that number divided by the wet film and multiplied by the solids fraction.
So a product at 60 percent volume solids, applied to build 8 mils dry, needs 13.33 mils wet. Divide 1,604 by 13.33 and you get 120 square feet per gallon before any losses. Take 15 percent off for what stays in the pail and on the roller and you are at 102. That is the whole calculation, and it is why two people can quote wildly different spread rates for the same tin and both be telling the truth: they are building different films. Ask for 4 mils dry instead of 8 and the wet film halves to 6.67 mils, the theoretical rate doubles to 241 square feet a gallon, and nothing about the product has changed.
Volume solids is the number people skip
Volume solids is the share of the wet film that is still there when the solvent has gone. A product at 100 percent solids leaves everything behind, so wet film and dry film are the same number. A product at 50 percent solids loses half its thickness on the way to cured, so hitting 8 mils dry means putting 16 mils wet on the floor.
Thinning attacks that directly. Adding 10 percent thinner by volume to a 60 percent solids coating takes it to 54.5 percent, because the solids are now spread through 1.1 gallons instead of 1. Every mil of dry film then costs proportionally more wet film, and the spread rate falls with it. Whether the product can be thinned at all, and by how much, is a question for the data sheet and nowhere else.
Working back from a wet film gauge
A wet film comb is a piece of notched metal that costs almost nothing and is the only instrument that tells you anything while there is still time to act. Push it into the wet film, read the last tooth that came out wetted, and multiply by the volume solids to get the dry film you are heading for. The gauge box on this page does that multiplication.
What it will not do is tell you whether the reading is acceptable. Minimum and maximum film per coat are product properties on the data sheet, and both ends matter: too thin and the film does not do what it was specified to do, too thick and solvent gets trapped under a skinned surface. Neither limit is something a calculator can supply.
Where this sits
If you already have a published coverage figure rather than a solids percentage, the garage floor coating calculator takes that route instead and adds the surface profile allowance and the kit rounding. The area you should be feeding either of them is rarely just length times width once cove base and equipment pads are counted, which is the coating area take-off. How much of that area one mixed kit can reach before the working time runs out is the pot life batch size calculator. For spray application rather than roller and squeegee, the film build comes from fluid delivery and travel speed instead, which is the spray gun film build calculator.
Questions people ask
Why does one gallon cover 1,604 square feet at one mil?
Because a US gallon is 231 cubic inches by definition, and a mil is one thousandth of an inch. Divide 231 cubic inches by 0.001 inches of thickness and you get 231,000 square inches of coverage. Divide that by 144 square inches per square foot and you get 1,604.17 square feet. Nothing about the coating enters that calculation, which is why the figure is the same for every liquid product ever made. Everything that varies between products enters afterwards, through the volume solids and the film thickness being built.
What is the difference between wet film and dry film?
Wet film is what is on the floor immediately after application, including everything that is going to evaporate. Dry film is what remains once it has. The ratio between them is the volume solids: a 60 percent solids coating laid at 10 mils wet cures to 6 mils dry. Specifications are almost always written in dry film, because that is the coating that does the work, but the only thickness you can measure while you can still do something about it is the wet film. That is the point of a wet film comb, and it is why this page converts in both directions.
Does thinning make a gallon go further?
It makes a gallon of mixed liquid cover more floor and it makes a gallon of coating build less film, which are not the same thing. Adding 10 percent thinner to a 60 percent solids product drops it to about 54.5 percent solids, so the same wet film leaves less behind. If you are working to a dry film target the thinned material has to go on thicker to land in the same place, and the spread rate in dry film terms gets slightly worse rather than better, because you have added liquid that leaves. Whether thinning is permitted at all is a data sheet question, and on many two-part floor products the answer is no.
What loss allowance should I use?
The default here is 15 percent and it is a starting point rather than a rule. What it has to cover is material left in the pail and on the roller and in the tray, cut-in at the walls where a roller cannot reach, and the extra the surface profile swallows on the first coat over bare concrete. A large open floor with a smooth prepared surface and a single continuous pour loses less. A small room broken up by cabinets, drains and pads, coated over an aggressively blasted slab, loses considerably more. If you have done the same product on the same kind of floor before, your own measured figure beats any default.
Can I use this for a second coat over the first?
Yes, and the loss allowance should usually come down when you do. The first coat over prepared concrete disappears into the profile, which is real material going somewhere useful but is not building film. By the second and third coats you are rolling onto cured film rather than into concrete, the surface is far closer to the smooth panel the theoretical figure assumes, and the practical rate moves back toward the theoretical one. Running the page twice with different loss allowances is a more honest picture than one figure applied to every coat.