How do you calculate coating thickness variation?

Spread over average: (maximum − minimum) ÷ average × 100. Readings of 8 and 12 mils around a 10-mil average give a 4-mil spread and 40% variation.

What is an acceptable coating thickness variation?

Controlled application commonly holds spread under roughly 20 to 25% of the average. The number grades the process, not the specification.

Can a part pass on average and still fail?

Yes. The wild-film example averages 8 mils against a 6-mil specification while its thin spot reads 5. Corrosion and adhesion failures start at the thinnest point.

Why does my film vary so much?

Usually inconsistent gun distance, Faraday-cage recesses, poor part grounding, a reciprocator mismatch or packed racks shadowing each other. The fix list starts at the gun.

Finishing calculator

Coating Thickness Variation Calculator

Grade the film two ways at once. Enter the minimum, maximum and average readings from a gage session plus the specification's floor.

What this calculator does

Grade film uniformity and judge the thin spot against the specification minimum, because uniformity and compliance are different checks.

Formula used

Spread = maximum reading − minimum reading
Variation = spread ÷ average reading × 100
Thin spot margin = minimum reading − specification minimum
Average margin = average reading − specification minimum

Inputs explained

Minimum Film Reading: Thinnest reading in the set; the true low, not a convenient one.
Maximum Film Reading: Heaviest reading, usually an edge or corner facing the guns.
Average Film Reading: Mean of the whole measurement set, not of the two extremes.
Specification Minimum Thickness: The specification's floor for this coating system.

How to use the result

Best suited to grading a first article before release, separating compliance from uniformity failures, trending variation as an application-drift alarm.
Three numbers cannot say where the extremes sit or how the distribution skews. Spread over average is outlier-sensitive by design, which serves floor checks. Conformance judgment across spots belongs to the sampling standard, not this page.

Current U.S. benchmarks

Industrial electricity averages 9.77 cents per kWh across the U.S. (EIA, Jul 2026), up 4.7% from a year earlier. Energy-intensive steps carry this directly into unit cost.
The producer price index for industrial chemicals stands at 336.006 (BLS, Aug 2026), up 13.3% from a year earlier. Quotes priced off last quarter's material cost miss this move.
The U.S. has 14,543 chemical manufacturing establishments employing about 911,245 workers (Census County Business Patterns, 2023).

Common questions

How do you calculate coating thickness variation? Spread over average: (maximum − minimum) ÷ average × 100. Readings of 8 and 12 mils around a 10-mil average give a 4-mil spread and 40% variation.
What is an acceptable coating thickness variation? Controlled application commonly holds spread under roughly 20 to 25% of the average. The number grades the process, not the specification.
Can a part pass on average and still fail? Yes. The wild-film example averages 8 mils against a 6-mil specification while its thin spot reads 5. Corrosion and adhesion failures start at the thinnest point.
Why does my film vary so much? Usually inconsistent gun distance, Faraday-cage recesses, poor part grounding, a reciprocator mismatch or packed racks shadowing each other. The fix list starts at the gun.

Last reviewed 2026-10-01.