Coating thickness is measured using handheld gauges that detect the distance between a probe and the base material beneath the coating. The right method depends entirely on what your substrate is made of. Steel, aluminum, wood, plastic, and concrete each require different gauge technologies, and using the wrong one gives unreliable readings or no reading at all.
Magnetic and Eddy Current Gauges for Metal
Most coating thickness measurement happens on metal substrates, and two technologies handle the vast majority of these jobs. Magnetic gauges measure nonmagnetic coatings (paint, powder coat, galvanizing, chrome) applied to ferrous metals like steel and iron. Eddy current gauges measure nonconductive coatings on nonferrous metals like aluminum, copper, and brass. Many modern handheld gauges combine both technologies in a single probe, automatically detecting the substrate type and switching modes.
Both work on the same basic principle: the gauge sends a signal from its probe tip through the coating and measures the gap between the probe and the base metal. That gap distance is displayed as coating thickness. Magnetic gauges use either a permanent magnet (pull-off type) or an electromagnetic field to sense the distance to a ferrous substrate. Eddy current gauges generate a small alternating current in the probe coil and measure how the substrate’s conductivity affects the signal strength, which changes with distance.
These electronic gauges are fast, accurate, and nondestructive. You press the probe against the coated surface, and a reading appears in under a second. For routine work on steel or aluminum, this is the standard approach across automotive, marine, construction, and manufacturing industries. ASTM D7091 is the governing standard for this type of measurement, covering calibration, verification, adjustment procedures, and proper techniques for obtaining readings on both ferrous and nonferrous metal substrates.
Ultrasonic Gauges for Non-Metal Substrates
Magnetic and eddy current gauges rely on a metal substrate to function. If your coating sits on wood, plastic, fiberglass, or concrete, you need an ultrasonic gauge instead. These send an ultrasonic vibration through the coating via a probe. A drop of coupling gel (similar to ultrasound gel) is applied to the surface to help the pulse enter the coating cleanly.
As the ultrasonic pulse travels downward through the coating, it encounters changes in material density at each interface: between coating layers and between the bottom coating layer and the substrate. Each density change creates a signal “peak” that the gauge detects. The time between peaks translates to thickness. On rough surfaces, the gauge typically reads from the top of the coating peaks down to the substrate, while the couplant fills voids between the probe and the coating surface.
Ultrasonic measurement is now a well-established technique in wood finishing and plastics manufacturing. ASTM D6132 covers the standard test method for ultrasonic coating thickness measurement. One advantage is that ultrasonic gauges can also measure individual layers in a multi-coat system if the layers have sufficiently different densities, something magnetic and eddy current gauges cannot do on their own since they only read total coating thickness from the surface down to the metal.
Destructive Testing With a Paint Inspection Gauge
When you need to verify individual layer thicknesses in a multi-coat system or when no electronic gauge suits the substrate, a destructive method using a Paint Inspection Gauge (PIG) is an option. This physically cuts into the coating, so it leaves a small mark that needs touch-up afterward.
The process works like this: you draw a broad line on the surface with a contrasting marker pen. Using the PIG, you make a precise angled cut at a right angle to the marker line, slicing all the way through the coating down to the substrate. Then you look at the cut through a supplied microscope and count the graticule divisions across each visible coating layer. The thickness calculation uses basic trigonometry based on the blade angle. A 45-degree blade gives a 1:1 ratio between the width you see under the microscope and the actual thickness. A shallower 26.6-degree blade cuts at a ratio where the true thickness is half the measured width, effectively magnifying thin coatings so they’re easier to read.
Different blade angles suit different thickness ranges. Some PIG models hold a single cutter, while others feature a rotating carousel with multiple blade angles so you can switch between them without swapping parts. Despite being destructive, this method is particularly valuable when you need to distinguish and measure each individual layer in a complex coating system.
How to Calibrate Your Gauge
An uncalibrated gauge can be off by enough to cause a failed inspection or a premature coating failure. Calibration is one of three operational steps ASTM D7091 identifies as necessary for accurate measurement, alongside verification and adjustment.
Most electronic gauges use certified reference foils (thin plastic shims of known thickness) placed on an uncoated sample of the same substrate material. A one-point calibration zeros the gauge on bare substrate, then takes a reading on a single reference foil. This corrects the gauge at one thickness value. A two-point calibration adds a second foil at a different thickness, bracketing the range you expect to measure. After completing the first one-point procedure (a “CAL” indicator appears on screen), you repeat the process with the second foil until a “2CAL” indicator confirms both reference points are set.
For best accuracy, choose reference foils that bracket your expected coating thickness. If you’re measuring a coating you expect to be around 75 microns, calibrate with one foil below and one above that value. Verify on a known standard periodically throughout your measurement session, especially if you’re covering a large area. SSPC-PA 2, a companion standard, even prescribes how frequently to take thickness readings based on the size of the area being coated.
Taking Accurate Readings
Where and how you place the probe matters as much as which gauge you use. Surface roughness, curvature, edge proximity, and temperature all affect readings. On rough surfaces like blast-cleaned steel, the coating fills the valleys of the profile before it builds thickness above the peaks. Gauges read from the top of the coating down to the substrate, so readings on rough surfaces include some of that valley-filling material.
Take multiple readings across the surface rather than relying on a single spot. Coating thickness varies naturally, even on well-sprayed surfaces. Most specifications call for an average of several readings within a defined area, along with minimum and maximum acceptable values. The governing specification for your project is responsible for providing minimum and maximum thickness requirements for each layer and for the total system.
Hold the probe perpendicular to the surface. Tilting it introduces error. On curved surfaces like pipes or tanks, use a probe with a small contact area and keep it centered on the curve’s high point. Near edges and corners, coatings tend to thin out or build up depending on the application method, so readings within about 25 mm of an edge are often excluded unless the specification says otherwise.
Units and Conversions
Coating thickness is expressed in microns (µm) in most of the world and in mils (thousandths of an inch) in the United States. One mil equals 25.4 microns. For quick conversion, multiply microns by 0.03937 to get mils, or divide mils by 0.03937 to get microns.
A typical industrial paint coat runs 50 to 125 microns (roughly 2 to 5 mils). Galvanized coatings on structural steel often fall in the 75 to 150 micron range. To put a practical number on the conversion: a specification calling for 75 microns equals about 2.95 mils, or 530 grams per square meter, or 1.74 ounces per square foot. Those weight-based units show up primarily in galvanizing specifications where coating weight is sometimes measured instead of thickness.
The full set of conversion factors:
- Microns to mils: µm × 0.03937
- Microns to g/m²: µm × 7.067
- Microns to oz/ft²: µm × 0.02316
- Mils to microns: mils ÷ 0.03937
Choosing the Right Method
Your substrate determines your starting point. For steel substrates, a magnetic gauge is the default. For aluminum or other nonferrous metals, you need eddy current capability. For wood, plastic, or concrete, an ultrasonic gauge is the only nondestructive option. Combination gauges that include magnetic and eddy current modes cover the most common metal scenarios in a single tool.
If you need individual layer measurements in a multi-coat system, ultrasonic gauges can sometimes distinguish layers nondestructively, provided the layers differ enough in density. When they don’t, or when definitive layer-by-layer data is required, a destructive PIG cut and microscope examination is the most reliable path. For simple total-thickness checks on metal, a basic electronic gauge gets the job done in seconds with accuracy within a few microns.