If your supplier quotes paint coating in g/m² (GSM) and your engineer specifies microns, you are comparing two ways of describing the same dry paint film: GSM is the mass of dry paint per unit area, and micron is the thickness of that film. You can convert between them if you know the paint’s dry film density, but the conversion is only approximate because density varies by paint chemistry, pigment loading and colour. The practical answer: compare both values using the supplier’s stated density, then write your RFQ with both a g/m² target and a minimum micron value, plus the test method, so neither side can reinterpret the requirement later.
Why two units exist at all
Coil coating lines apply paint by roller. The operator controls wet film pickup, line speed and oven cure, not the final dry thickness directly. Because the paint is delivered and consumed by mass, production planning, cost calculation and quality records are naturally kept in grams per square metre of dry film. That is the supplier’s native unit.
Designers and fabricators, by contrast, think in microns because corrosion protection, abrasion resistance and appearance relate to physical film build. A roll former worried about cracking at a tight bend radius wants to know how thick the film is, not how heavy it is. That is the engineer’s native unit.
Neither unit is wrong. The problem only appears when a purchase order states one and the inspection report states the other, and nobody has written down the conversion basis.
How to convert GSM to micron
The relationship is straightforward:
Dry film thickness (µm) = coating weight (g/m²) ÷ dry film density (g/cm³)
The arithmetic works because 1 g/m² spread over 1 µm of thickness corresponds to a density of 1 g/cm³. So a coating weight divided by density gives thickness directly in microns.
A worked example
Suppose a polyester topcoat is quoted at 20 g/m² and the paint supplier states a dry film density of 1.4 g/cm³. Then:
20 ÷ 1.4 ≈ 14.3 µm
If the same 20 g/m² is achieved with a denser paint at 1.6 g/cm³, the film is only about 12.5 µm. Same mass, thinner film. This is why a GSM figure alone does not tell your engineer everything, and why a micron figure alone does not tell the coater how much paint to buy.
Why the conversion is approximate
- Density differs between primer, topcoat and back coat, and between paint families such as polyester, silicon-modified polyester, PVDF and polyurethane.
- Pigment volume concentration changes density; titanium dioxide and inorganic pigments are denser than many resin systems.
- Measurement methods differ. Coating mass is typically determined by stripping a defined area and weighing, while dry film thickness is measured by magnetic or eddy-current gauges on the coated strip. Each method has its own tolerance.
- Substrate roughness and the metallic coating surface affect how paint sits on the steel, so the same wet pickup can produce slightly different dry results.
Treat any GSM-to-micron conversion as an engineering estimate for comparison, not as a substitute for the agreed inspection method.
Practical conversion reference
The table below is general industry guidance only, using representative dry film densities. It is intended to help you sanity-check a quotation. Your contract-specific requirement should always be based on the paint supplier’s declared density and the agreed test method.
| Paint system (typical) | Assumed dry film density | Coating weight | Approximate dry film thickness |
|---|---|---|---|
| Primer, epoxy or polyester | ~1.4 g/cm³ | 5 g/m² | ~3.6 µm |
| Polyester topcoat | ~1.4 g/cm³ | 20 g/m² | ~14 µm |
| Polyester topcoat | ~1.5 g/cm³ | 25 g/m² | ~17 µm |
| Silicon-modified polyester | ~1.5 g/cm³ | 20 g/m² | ~13 µm |
| PVDF topcoat | ~1.5 g/cm³ | 23 g/m² | ~15 µm |
| Back coat, typical | ~1.4 g/cm³ | 5–8 g/m² | ~3.5–5.7 µm |
Note that a “25 µm total coating” and a “25 g/m² topcoat” are not the same thing. Always confirm whether a stated figure covers primer plus topcoat, or topcoat alone, and whether the back coat is included.
Which unit should your RFQ state?
State both, and define the measurement basis. A specification that says only “coating: 20 microns” leaves the coater to guess the density, the paint system and whether primer is included. A specification that says only “coating: 20 g/m²” leaves your engineer unable to verify film build on the finished coil.
A robust RFQ clause normally contains four elements:
- Paint system — for example, polyester topcoat over a primer, with the resin chemistry named.
- Coating weight — target value in g/m², stated separately for primer, topcoat and back coat.
- Minimum dry film thickness — in microns, with the measurement method and instrument type identified.
- Reference standard and edition — for example, the relevant coating mass or thickness test standard. Confirm the required edition and the exact acceptance values in your contract, because standards are revised and testing frequency clauses differ.
Where the metallic coating fits in
Paint is only part of the corrosion system. The zinc or zinc-aluminium coating underneath does most of the sacrificial protection work, and it is specified separately in g/m² per side or as a coating class. If you are comparing a prepainted product against an unpainted one, the metallic coating mass is often the more important variable. Our PPGI coil and GI coil pages set out how these two product families are normally described, and the two specifications should be read together rather than in isolation.
RFQ checklist for coating specification
Use this list before sending an enquiry. It reduces the back-and-forth that comes from ambiguous coating language.
- Substrate: steel grade, yield strength where relevant, and thickness with tolerance.
- Metallic coating: type (zinc, zinc-aluminium, other) and mass per unit area per side, or coating class.
- Paint system: primer type, topcoat chemistry, back coat type.
- Coating weight: separate figures for primer, topcoat and back coat, in g/m², with a tolerance band.
- Dry film thickness: minimum and typical values in microns, with the measurement method.
- Colour: reference to a colour standard or an approved sample, plus gloss level and tolerance.
- Surface finish: smooth, matt, wrinkle, embossed, printed, or other, with a reference sample if needed.
- Mechanical requirements: bend test, impact, adhesion, pencil hardness, and the test standards to be used.
- Coil parameters: inner diameter, outer diameter, coil weight, width and edge condition.
- Inspection: whether a mill test certificate, coating mass report or third-party inspection is required.
- Packing, marking and shipping terms.
- Quantity, delivery window and destination port.
Common mistakes when comparing micron and GSM
Assuming a fixed conversion factor
Buyers sometimes use a rule of thumb such as “1 g/m² equals 1 µm” and apply it across all paint systems. That only holds at a density of 1.0 g/cm³, which is not typical for coil coatings. The error can be 30% or more, which is enough to turn a compliant coil into a rejected one.
Mixing topcoat-only and total film figures
A quotation stating “coating 25 µm” may mean topcoat only, or primer plus topcoat. Ask which. The same ambiguity applies to g/m² figures.
Ignoring the back coat
Back coats are usually thinner than topcoats and are sometimes omitted from a coating summary. If the reverse side will be exposed, specify it explicitly.
Specifying thickness without a method
Two inspectors using different instruments on the same coil can report different values. Name the method and, where relevant, the number of readings per sample.
Forgetting forming
Film thickness and paint flexibility interact with bend radius. A coating that passes a flat-panel test may still crack at a tight radius. If your line forms aggressively, say so in the enquiry so the paint system can be matched to the application.
Treating a quotation as a specification
A quotation is an offer, not a technical agreement. Convert the accepted offer into a written specification with tolerances before production begins.
Cost drivers and quoting
Coating weight is one of several cost drivers, alongside substrate thickness, metallic coating mass, paint chemistry, colour, coil width, order quantity, tolerance tightness, testing requirements and packing. A higher coating weight generally increases paint consumption per tonne, but the relationship is not linear across all order sizes.
Because raw material and paint costs move, request a dated quotation that states the validity period and the coating values on which it is based. That way, if you later compare two suppliers, you are comparing the same specification rather than two different products.
Performance expectations
Do not treat a coating weight or micron figure as a service-life guarantee. Actual performance depends on the substrate, the metallic coating mass, the paint system, the installation environment, how the material is formed and fixed, and ongoing maintenance. Coastal, industrial and inland environments place very different demands on the same product. If a supplier offers a performance warranty, read the conditions carefully and confirm what is excluded.
Frequently asked questions
Can I convert g/m² to micron exactly?
Only if you know the dry film density of that specific paint. The formula is thickness (µm) = coating weight (g/m²) ÷ density (g/cm³). Because density varies by paint chemistry and pigment loading, the result is an estimate. Use it for comparison, not for acceptance testing.
Which unit is more reliable for inspection?
Both are used in practice. Coating mass measurement is often preferred for dispute resolution because it is less sensitive to operator technique on textured surfaces. Dry film thickness gauges are faster and are widely used for routine checks. The right answer is whichever method your contract names.
Does a higher GSM always mean better corrosion resistance?
Not on its own. The metallic coating beneath the paint usually dominates long-term corrosion performance. A thicker paint film can improve barrier protection and appearance retention, but it cannot compensate for an inadequate zinc coating.
Should the RFQ specify primer and topcoat separately?
Yes, where the application is demanding. Separate figures make it clear what is being supplied and allow meaningful comparison between quotations. A combined figure is acceptable only if the split is defined elsewhere in the contract.
What tolerance should I allow on coating weight?
Tolerances are contract-specific and depend on the paint system, line capability and test method. Rather than adopting a number from a blog, agree the tolerance with your supplier and record it in the purchase specification, referencing the applicable standard edition.
How do I compare two quotations with different units?
Ask each supplier for the dry film density of the quoted paint system, convert both to microns using the formula above, and compare like with like: same substrate, same metallic coating mass, same paint chemistry, same colour, same test method. Then compare price on that normalised basis.
If you are preparing an enquiry and want the coating clause reviewed against your application, send your draft specification and target values through our contact page. Include the substrate, metallic coating mass, paint system, coating weight or micron target, colour, coil dimensions and quantity, and we will respond with a dated quotation and the technical basis behind it.
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