Sheet Metal Surface Finish Standards Explained: Ra, Rz, Lay & How to Specify Correctly

Specifying “smooth” or “polished” on a sheet metal drawing is a guarantee of disagreement. One supplier reads “smooth” as Ra 1.6 µm; another reads it as Ra 3.2 µm; the inspector measures Ra 2.5 µm and the part is rejected. Surface finish has to be quantified to be meaningful — and the quantification system uses parameters most engineers learned briefly in school and never revisited.

This guide breaks down what Ra, Rz, and the other surface roughness parameters actually measure, how to specify finish correctly per ISO 1302, common Ra values for sheet metal finishes, and the specification mistakes that cause inspection disputes.

What These Parameters Actually Measure

A real surface, viewed under a profilometer, looks like a mountain range — irregular peaks and valleys at the microscopic scale. The roughness parameters describe different aspects of that profile.

Ra — Arithmetic average roughness

Ra = average distance from the mean line, regardless of peak/valley direction

Ra = (1/L) × ∫|y(x)| dx

This is the most common parameter because it’s easy to measure and stable. Two surfaces with similar Ra usually look and feel similar.

Ra’s blind spot: Ra is an average. A surface with one deep scratch but otherwise smooth has roughly the same Ra as a uniformly textured surface. For sealing, food contact, and aesthetic applications, this matters.

Rz — Maximum height of profile

Rz describes a profile height characteristic; its evaluation and aggregation follow the selected standard and measurement settings. Identify the standard edition rather than assuming every instrument’s Rz output uses the same definition.

Rz provides different information from the arithmetic averaging used for Ra. Select it where the design calls for that profile information; it is not a substitute for every defect or functional inspection.

There is no universal fixed conversion between Ra and Rz. Measure and report the parameter required by the drawing instead of multiplying one result to infer the other.

Rmax — Maximum peak-to-valley

Rmax = single largest peak-to-valley measurement in the evaluation length

Rmax catches one-off defects: a scratch, a tool mark, a contamination indent. Used in critical sealing and optical applications.

Other parameters (less common)

Parameter	What It Measures	When It Matters
Rsm	Mean spacing between profile peaks	Adhesion, friction
Rsk	Skewness (peaks vs valleys dominant)	Bearing surfaces
Rku	Kurtosis (sharp peaks vs gradual)	Wear, sealing
Rt	Total peak-to-valley over full length	Critical defect detection

The design authority selects the required parameters; neither one parameter nor a fixed pair is automatically sufficient for every sheet-metal application.

Common Sheet Metal Finishes Mapped to Ra

These are typical achievable Ra values for sheet metal finishes:

Finish	Typical Ra (µm)	Typical Ra (µin)	Visual Description
Mill finish (cold-rolled)	0.2 - 1.0	8-40	Smooth, slight texture
Mill finish (hot-rolled)	3.2 - 12.5	125-500	Visible scale, rough
2B (cold-rolled stainless)	0.1 - 0.5	4-20	Smooth, slight reflectivity
#3 (coarse brushed)	0.4 - 1.0	16-40	Visible brushed grain
#4 (architectural brushed)	0.4 - 0.8	16-32	Fine satin grain
#6 (fine satin)	0.2 - 0.4	8-16	Soft satin
#7 (high reflective)	0.05 - 0.1	2-4	Near-mirror
#8 (mirror)	0.025 - 0.05	1-2	Mirror finish
Bead blasted (fine)	1.6 - 3.2	63-125	Matte, uniform
Bead blasted (coarse)	3.2 - 6.3	125-250	Rough matte
Electropolished	0.05 - 0.4	2-16	Bright, smooth
As-laser-cut edge	1.6 - 6.3	63-250	Striated edge
As-machined (typical)	0.8 - 3.2	32-125	Tool marks visible

Lay — The Direction of the Texture

Two surfaces with identical Ra can perform completely differently based on lay direction — the dominant direction of the texture pattern.

ISO 1302 lay symbols

Symbol	Meaning	Common Use
=	Parallel to projection line	Linear brushed (#4)
⊥	Perpendicular to projection line	Cross-grain machining
X	Angular in two directions	Honing, cross-hatching
M	Multi-directional	Bead blasting, lapping
C	Circular	Lathe-turned features
R	Radial	Disc finishing
P	Pitted, non-directional	Tumbled, etched

Why lay matters:

• Sealing: Lay perpendicular to gasket compression direction can break the seal
• Friction: Linear lay creates anisotropic friction
• Appearance: Brushed finishes look different at different viewing angles
• Stress corrosion: Lay direction can create preferential crack propagation paths

For applications like food contact, ISO requires lay perpendicular to flow direction to prevent contamination buildup.

ISO 1302 Surface Finish Symbol

The standard symbol has multiple positions:

        Rz3.2
   √    Ra0.8 / M
        Process: ground

Where:

• Symbol shape: Indicates material removal (open V), no removal allowed (circle on V), or material removal required (line under V)
• Top value: Upper limit of roughness (e.g., Ra 0.8)
• Bottom value (if shown): Lower limit
• Process designation: Method to achieve (ground, milled, brushed)
• Lay symbol: After ”/”
• Sampling length: e.g., “0.8” for 0.8 mm

A complete callout looks like:

Ra 0.8 / 0.8 mm / =
(0.8 µm Ra, 0.8 mm sampling length, parallel lay)

When Ra Alone Is Dangerous

Specifying “Ra ≤ 0.8 µm” by itself works for many applications. But for these specific cases, Ra alone will get you a part that fails inspection:

Sealing surfaces

A surface can hit Ra 0.8 µm with a single deep scratch (high Rz) that creates a leak path. Specify Ra + Rmax for sealing.

Food contact

Bacteria harbor in surface peaks and valleys that Ra averages away. 3-A requires Ra ≤ 0.8 µm but auditors also evaluate visible defects and Rz. See our food processing stainless guide for the full requirement.

Bearing surfaces

The shape of the asperities matters as much as the average height. Specify Ra + Rsk (negative skewness means more valleys than peaks, better for oil retention).

Adhesive bonding

Adhesives need a specific roughness profile for mechanical interlock. Specify Ra + Rsm (peak spacing) to ensure the profile suits the adhesive.

Optical / cosmetic

Visible scratches matter more than average roughness. Specify Rmax and visual inspection criteria.

Surface Finish by Application

Application	Typical Spec
Architectural exterior	Ra ≤ 0.8 µm, brushed #4, lay vertical
Food contact (3-A)	Ra ≤ 0.8 µm + visual + passivated per ASTM A380
Pharma (USP)	Ra ≤ 0.4 µm electropolished + 100% borescope on tubing
Sealing surface (gasketed)	Ra ≤ 1.6 µm + Rmax ≤ 6.3 µm
Powder coat substrate	Ra 1.6 - 3.2 µm (good mechanical adhesion)
Anodizing substrate	Ra ≤ 0.8 µm if cosmetic
General industrial cosmetic	Ra ≤ 1.6 µm
Hidden / structural	Ra mill finish acceptable

For finish selection in specific contexts, see our powder coating color guide and anodizing vs powder coating comparison.

Specification Mistakes to Avoid

Mistake	Consequence
”Smooth finish” (no number)	Subjective — supplier picks easiest
”32 µin finish” (no parameter)	Ra? Rz? Rmax? Disputes guaranteed
Ra only, sealing application	Pass Ra, fail sealing test
No lay specification	Anisotropic surfaces, sealing issues
Specifying tighter than process can deliver	Re-work cycles, cost increases
No sampling length	Different equipment can give different readings
No process designation when one matters	Brushed vs blasted can both hit Ra 0.8 but look totally different

Inspection Considerations

For Ra/Rz to be measurable and meaningful:

1. Sampling length: Must be specified — different lengths produce different values
2. Cutoff filter: Typically 0.8 mm for most applications
3. Measurement direction: Across or with the lay direction (specify)
4. Number of measurements: 5+ measurements averaged for representative value
5. Calibration: Profilometer calibrated against standards
6. Operator technique: Light surface contact, perpendicular to surface

For our quality control capabilities including profilometer inspection, see the capability page.

How DRAmetal Handles Surface Finish

Our surface finish capabilities:

• Inspection: Contact profilometer (Mahr / Mitutoyo), 3D optical for critical surfaces
• Standards: ISO 1302, ASME B46.1, full Ra/Rz/Rmax reporting
• Processes: Mechanical polish, electropolish, brushed, bead-blasted, vibratory
• Reporting: Surface finish report with Ra, Rz, Rmax, lay, sampling length — per part for critical applications
• Industries: Aerospace, medical, food, pharma, optical, architectural

For production RFQs and finish workflow, see our surface finishing service.

To request a surface-finish review for a specific application, submit your design. The initial response target is within 24 working hours; parameters, values, lay and process recommendations require drawing and service-condition review.

Bottom Line

Surface finish on a drawing is not the same as surface finish in service. A complete specification identifies the required parameter or parameters, limits, surface regions, measurement conditions and applicable standard. Ra and Rz are not interchangeable, and both are not automatically required for every part.

The drawings that produce parts matching the engineer’s intent are the ones where finish is specified as rigorously as critical dimensions. The drawings that produce surprise rejections at first article are the ones with vague terms like “smooth” or single Ra values where Ra alone isn’t enough.

Pick the parameter that matters for the application and agree how it will be verified. The Ra and Rz acceptance-clause guide covers supplier exceptions and surface-zone requirements for formed medical sheet metal.