Understanding your result
The headline is the practical coverage per coat, the area one gallon (or litre) will cover at the film thickness you entered after the application loss. The sub-line gives the theoretical coverage and the dry film. The three figures show the theoretical coverage, the wet film to apply and the paint for the area if you entered one.
The table runs from the volume solids through the dry and wet film per coat to the theoretical coverage, the transfer efficiency for the method, the practical coverage, and with an area, the paint and the cans. The wet film row is the practical takeaway: it is the number to read on a wet-film gauge to know you are applying what the data sheet specifies.
This calculator explains the coverage that every other calculator takes as an input. The paint coverage calculator adds the surface factor for texture and porosity; the sprayer usage calculator uses transfer losses by spray setting; the spray versus roll calculator puts the two methods side by side; and the roof coating calculator works with coatings specified at thick films.
How we calculate this
The constant 1,604 is exact arithmetic: a US gallon is 231 cubic inches, 1 mil is a thousandth of an inch, and 231 ÷ 0.001 ÷ 144 = 1,604.17 sq ft. The metric constant is simpler because a litre spread to 1 µm covers 1,000 m², so 10 × solids % ÷ µm gives m² per litre directly. One mil is 25.4 µm. The film thickness guide derives both and shows how to read the relevant lines on a technical data sheet.
Transfer efficiency by method is the share of paint that reaches the surface: 95% by brush, 90% by roller, 70% by airless and 65% by HVLP or conventional spray. They are mid-range figures from coatings manufacturers’ and equipment makers’ guidance, and the overspray side of the same figures is on the coverage rates page. Measuring wet film follows ASTM D4414, the standard practice for notched wet-film gauges.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Constant | 1,604.17 sq ft per gallon per mil at 100% solids | 231 cu in per US gallon, exact |
| Metric constant | 10 m²/L per (solids % ÷ µm) | 1 L at 1 µm covers 1,000 m², exact |
| Volume solids | 40% | Typical of interior acrylic wall paints, whose data sheets give 30–45% |
| DFT per coat | 1.5 mils (38 µm) | Within the 1.2–2 mils dry most wall paint data sheets specify |
| Transfer efficiency | brush 95%, roller 90%, airless 70%, HVLP 65% | Mid-range figures from coating and spray-equipment makers’ guidance |
| Wet-film measurement | notched gauge | ASTM D4414 |
Assumptions last reviewed October 8, 2026.
The calculator does not model surface roughness, absorption into porous substrates, or the extra film that builds at edges and corners; those reduce real coverage below the practical figure. It assumes volume solids, not weight solids, which data sheets also print and which give a wrong answer in this formula.
Two worked examples
A wall paint to its data sheet
Volume solids 40%, 1.5 mils dry per coat, rolled, 1,000 sq ft, two coats.
- Theoretical: 1,604 × 0.40 ÷ 1.5 = 428 sq ft per gallon
- Practical: 428 × 90% = 385 sq ft per gallon
- Wet film to apply: 1.5 ÷ 0.40 = 3.8 mils
- Paint: 1,000 ÷ 385 × 2 = 5.19 gallons, a pail and a quart
That is why a can says “350 to 400 sq ft per gallon”: the practical figure for a typical film, rounded down for real walls. In metric the same paint at 38 µm is 10.5 m² per litre theoretical and 9.5 practical.
Checking a sprayer with a wet-film gauge
The painter reads 4 mils wet on the gauge, spraying the same 40% solids paint with an airless rig.
- Dry film: 4 × 0.40 = 1.6 mils
- Theoretical: 1,604 × 0.40 ÷ 1.6 = 401 sq ft per gallon
- Practical: 401 × 70% = 281 sq ft per gallon
- Paint for 1,000 sq ft at two coats: 7.12 gallons
The same job by roller would need about 5.2 gallons. The 30% the sprayer loses is the whole difference, which the spray versus roll calculator puts in dollars.
Where to find your inputs
Volume solids. On the product’s technical data sheet, usually under “Physical properties” as “Volume solids” or “Solids by volume”. Use the volume figure, not weight.
Film thickness. The data sheet’s “Recommended spread rate” section gives wet and dry mils (or µm) per coat. On a job, read the wet film with a notched gauge pressed into fresh paint.
Method. How the paint is applied. Choose the spray setting closest to yours.
Area and coats. From the square footage calculator and the coats calculator.
Common mistakes
- Using weight solids. The formula needs volume solids; weight solids are higher and overstate coverage.
- Confusing wet and dry mils. A spec of 4 mils wet is 1.6 dry at 40% solids, not 4.
- Taking theoretical as real. It assumes no loss and a smooth surface.
- Spreading thinner to hit a coverage number. Below the specified film the paint hides and wears less.
- Ignoring transfer efficiency when spraying. A third of the paint can miss.
- Mixing mils and millimetres. A mil is a thousandth of an inch, 25.4 µm.
Questions people ask
- How do you calculate paint coverage from volume solids?
- Theoretical coverage in sq ft per gallon is 1,604 times the volume solids (as a fraction) divided by the dry film thickness in mils. A paint with 40% volume solids applied at 1.5 mils dry covers 1,604 × 0.40 ÷ 1.5 = 428 sq ft per gallon in theory. In metric, m² per litre is 10 × volume solids % ÷ DFT in micrometres.
- Where does 1,604 come from?
- A US gallon is 231 cubic inches. Spread to a film 1 mil (0.001 in) thick it covers 231,000 square inches, which is 1,604 sq ft. Multiplying by the volume solids gives the dry film volume, which is what stays on the wall once the water or solvent evaporates.
- What is the difference between wet and dry film thickness?
- The wet film is what you put on; the dry film is what remains when the liquid has evaporated. Dry film = wet film × volume solids. A 40% solids paint applied at 4 mils wet dries to 1.6 mils. Painters measure wet film with a notched gauge as they go, because the dry film cannot be checked until later.
- What is transfer efficiency?
- The share of the paint that ends up on the surface. About 95% by brush, 90% by roller, 65 to 75% by airless spray and less by conventional or HVLP spray. Practical coverage is theoretical coverage times transfer efficiency.
- What film thickness does wall paint need?
- Most interior latex wall paints specify about 1.2 to 2 mils dry per coat, which at 35 to 45% volume solids is 3 to 5 mils wet. High-build coatings, block fillers and elastomerics specify 5 to 20 mils dry, which is why their coverage is so much lower.
- Why is my real coverage lower than the practical figure?
- Because the formula assumes a smooth surface. Texture increases the surface area and porosity absorbs the first coat. The paint coverage calculator applies a surface factor on top of the figure from this one.