When comparing powder coatings, it’s tempting to shop by price per kilogram. But that number alone can be misleading, and it can quietly cost you more in the long run. One key powder characteristic that has a meaningful impact on job cost is specific gravity.
What Is Specific Gravity, and Why Does It Matter?
Specific gravity (SG) is the density of the powder relative to water. In powder coating, it determines how much volume you get from a given weight of material. Since coating performance and coverage are governed by film thickness, a volume-based property, SG is the bridge between what you buy (kilograms) and what you actually use (coated surface area).
Two powders can cost the same per kilogram and still perform very differently once sprayed, simply because one is denser than the other.
The Direct Link to Powder Mileage
Powder mileage, the surface area a given weight of powder will cover at a specified film thickness, is calculated using specific gravity. Since volume equals area multiplied by thickness, a lower-SG powder occupies more volume for the same weight, and at a fixed thickness, that extra volume covers more surface area. As SG decreases, mileage increases.
The relationship is captured in a simple formula:
Coverage (m² per kg) = 1000 / (Specific Gravity × Film Thickness in microns)
(For reference, 1 mil = 25.4 microns.)
In practical terms, a lower-SG powder will coat more parts per kilogram than a higher-SG powder, assuming the same target film build. This is why two products with identical price tags per kilogram can have meaningfully different real-world costs per part.
Suppose you’re comparing two powders for a job requiring a 60-micron film thickness:
| Powder A | Powder B | |
|---|---|---|
| Specific gravity | 1.5 | 1.8 |
| Price per kilogram | $8.00 | $7.50 |
| Coverage per kilogram | 1000 / (1.5 × 60) = 11.11 m²/kg | 1000 / (1.8 × 60) = 9.26 m²/kg |
| Cost per m² coated | $8.00 / 11.11 = $0.72 | $7.50 / 9.26 = $0.81 |
Powder B looks like the better deal at a glance, since it’s $0.50 cheaper per kilogram. But because it’s denser, it covers less surface area per kilogram, and it actually costs more per square meter coated.
Now scale that up to a job requiring 10,000 m² of coverage:
- Powder A: 10,000 ÷ 11.11 = 900 kg needed, at $8.00/kg = $7,200 total
- Powder B: 10,000 ÷ 9.26 = 1,080 kg needed, at $7.50/kg = $8,100 total
Choosing Powder B based on its lower price per kilogram ends up costing $900 more to complete the exact same job.
The only way to make an apples-to-apples comparison is to calculate cost per unit of coverage, factoring in specific gravity and target film thickness, rather than relying on the sticker price per kilogram.
The Takeaway
Specific gravity isn’t a technical footnote; it’s a core driver of your true material cost. When evaluating powder coating suppliers, ask for the specific gravity alongside the price per kilogram, and calculate coverage-based cost comparisons before making a purchasing decision. The powder that looks cheaper on paper may end up being the more expensive choice once it’s actually sprayed.