Coating coverage and paint estimating
Every coverage estimate starts from the same identity: one US gallon of solid material covers 1604 square feet at 1 mil dry. Everything after that is volume solids, target thickness, and honesty about losses.
Theoretical coverage = 1604 x volume solids fraction / DFT in mils, in square feet per gallon. A 70 percent solids product at 5 mils dry covers about 225 sq ft per gallon - before a single percent of loss. Verify volume solids on the current product data sheet; the project specification governs the target DFT.
From theoretical to practical
Real jobs lose paint to overspray, surface roughness and profile, dead volume left in pumps and lines, container residue, and irregular geometry that forces overlap. Divide theoretical coverage by (1 - loss fraction) or equivalently multiply the estimated gallons up.
Loss factors are job knowledge, not physics: airless spray on flat plate might lose 15 to 20 percent, conventional spray on lattice steel 40 percent or more, brush and roller relatively little. Your own past jobs are the best calibration; when in doubt, estimate ranges rather than a single number.
Estimating a work area
Take the coated area, divide by practical coverage, and round up to purchasable units per batch and per coat. Remember that each coat is its own calculation when thicknesses or products differ, and stripe coats consume material the area arithmetic does not show.
CoatSpec's product planner runs this arithmetic per work area from the coat system's solids, target DFT, area, and your loss allowance, and shows the WFT target alongside so application control and purchasing agree.
What the arithmetic looks like
The table below is the planner's own output across the range of volume solids field products actually come in, at a single target thickness, for one work area. It exists because the sensitivity is the part prose hides: the four numbers move together, and seeing them side by side is the difference between knowing the formula and knowing what it costs you.
Two things to read out of it. Coverage scales linearly with volume solids, so an 80 percent solids product covers exactly 1.6 times the area of a 50 percent one at the same dry thickness - which is most of why high-solids products price the way they do. And coverage scales inversely with DFT: at the same solids, a 10 mil target halves every figure in the second and third columns and doubles the gallons.
The loss allowance is the one column you own. Twenty-five percent is a plausible middle for spray work on open steel, not a value this site can source for your job; swap in your own records and the gallons move proportionally.
| Volume solids | Theoretical (sq ft/gal) | Practical at 25% loss (sq ft/gal) | Gallons for 2,400 sq ft |
|---|---|---|---|
| 50% | 160 | 120 | 20.0 |
| 60% | 192 | 144 | 16.6 |
| 70% | 225 | 168 | 14.3 |
| 80% | 257 | 192 | 12.5 |
| 90% | 289 | 217 | 11.1 |
Where estimates go wrong
Almost every bad estimate is bad for one of four reasons, and none of them is the formula. Volume solids was read off a superseded data sheet, or off the mixed-product line when the coating is thinned. The target DFT used was the specified minimum rather than the thickness the applicator will actually aim for, which under a restriction level is somewhere above the minimum. The loss factor was carried over from a job with different geometry. Or stripe coats were never added at all.
The last one is the easiest to size. Stripe coating is brush or roller work along edges, welds and fasteners, so it is driven by linear feet rather than square feet - count the edges on the drawing rather than folding a guess into the area loss factor, which is what hides the cost.
- Volume solids from the current data sheet, for the product as it will be applied
- Target DFT the applicator will aim for, not the specified minimum
- Loss factor from your own records on comparable geometry
- Stripe coat allowance counted from linear feet of edges and welds
- One calculation per coat whenever the product or the thickness changes
Frequently asked questions
How many square feet does a gallon of paint cover?
Theoretically, 1604 x volume solids fraction / DFT in mils. A 100 percent solids coating at 1 mil covers 1604 sq ft; a 60 percent solids coating at 4 mils covers about 241 sq ft. Practical coverage is always lower - divide by one minus your loss fraction.
What loss factor should an estimate use?
Typical territory runs 15 to 40 percent depending on application method, geometry, wind, and crew practice. Use records from similar past jobs where you have them, and state the assumed loss in the estimate so it can be challenged.
Do stripe coats change the estimate?
Yes. Stripe coating edges, welds, and bolts consumes material that plain area arithmetic does not capture. Estimators commonly add an allowance based on the length of edges and welds rather than trying to fold it into the area loss factor.