Key takeaways
- Coverage and film thickness are two sides of the same number.
- One gallon spread at a given wet thickness covers about 1,604 ÷ wet mils square feet.
- Spreading paint thinner to cover more doesn't give you the film the data sheet assumes.
A coverage figure isn't a fixed property of the paint. It's the area a gallon covers at a stated thickness. That's why data sheets for professional products often list the wet film thickness next to the square feet.
The math
One US gallon is 231 cubic inches. A mil is one thousandth of an inch. Spread that volume at a given wet thickness, and:
Square feet per gallon = 1,604 ÷ wet mils
We worked this out from the volume, and it's a theoretical figure. It matches Sherwin-Williams' ProMar 200 Zero VOC primer sheet, which lists 400 square feet per gallon at 4 mils wet.
| Wet film | Theoretical coverage per gallon |
|---|---|
| 3 mils | 535 sq ft |
| 4 mils | 401 sq ft |
| 5 mils | 321 sq ft |
| 6 mils | 267 sq ft |
| 7 mils | 229 sq ft |
Why real coverage is lower
The table assumes every drop ends up as an even film on a smooth surface. Real surfaces and application differ:
- Surface: Behr says texture can raise how much paint you need, and that a smooth, primed or sealed surface can lower it.
- Color change: Behr also says a subtle color change can reduce what you need, while a bigger change may take more coats.
- Spraying: Behr's ceiling paint page says its coverage figure excludes material lost to spraying.
How to use this
- Find the coverage on your paint's label or data sheet, along with any stated film thickness.
- Put that number in the paint calculator.
- If you've painted the same surface with the same product before, use your own result instead. What you've measured beats any published figure.
Decide: more paint or more coats?
Don't try to stretch a coat to hit a higher coverage number. If a coat looks thin, the question is whether you need another one, and the product's directions set its recoat time and coat count. Regal Select's listing, for example, gives one or two coats with a recoat time of 1 to 2 hours, while the Sherwin-Williams primer sheet gives 4 hours under its stated conditions.
Frequently asked questions
What is wet film thickness?
It's the thickness of the paint just after it's applied, before it dries. The dry film is thinner: Sherwin-Williams' primer sheet lists 4 mils wet and 1.0 mil dry.
Do I need to measure wet mils at home?
Not usually. Professionals use a gauge, while most homeowners can work from label coverage.
Why doesn't the label match my result?
Your surface, tools and color change all matter. Track how much you used, and you'll have a better figure for the next room.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- Sherwin-Williams: ProMar 200 Zero VOC primer data sheet
- Benjamin Moore: Regal Select matte product data
- Behr: How to determine how much interior paint to buy