Surface Roughness Chart: Ra, Rz, Grade and Microinch Conversion

Quick answer: A surface roughness chart lines up the different ways of expressing the same finish so you can convert between them at a glance. The key columns are Ra in micrometers, Ra in microinches, the approximate Rz equivalent, the ISO roughness grade (N1 to N12), and the typical process that produces each value. As a quick anchor: Ra 0.8 micrometers equals about 32 microinches, roughly grade N6, a fine milled or ground finish; Ra 1.6 equals about 63 microinches, grade N7, a standard machined finish; Ra 3.2 equals about 125 microinches, grade N8, a rough finish.

A surface roughness chart is the reference every machinist, designer and inspector reaches for when a drawing speaks one language and the shop floor speaks another. It converts metric to imperial, Ra to grade, and value to process, so nobody machines to the wrong target. This article presents the chart and explains how to use it, including the important caveat that Ra-to-Rz conversion is only approximate.

It supports the definitions in surface roughness Ra explained and Ra vs Rz vs Rt, and the drawing context in surface finish symbols explained.

The Master Surface Roughness Chart

The table below is the core reference. The Rz values are typical approximations, not exact conversions, for the reasons explained in the next section.

Image suggestion 1 — A clean visual version of this chart as a graphic, with a sample surface swatch beside each row.

Alt text: "Surface roughness chart showing Ra micrometers, microinches, Rz, ISO grade and process."

Placement: under the table.

How to Use the Chart

The chart answers three everyday questions:

• Convert units. A US drawing says 32, a metric machine reads micrometers. Find 32 microinches in the chart and read across to Ra 0.8 micrometers. This single lookup prevents the factor-of-25 unit error that scraps parts.
• Translate a grade. A drawing marked N7 means Ra 1.6 micrometers, a standard finish. The ISO N-grade column lets you decode older or European callouts.
• Plan a process. If a drawing demands Ra 0.4, the process column tells you that fine grinding or reaming territory is involved, not a single milling pass, so you can plan operations and cost. See best CNC machine for surface finish for the machining side.

Why Ra-to-Rz Conversion Is Only Approximate

The most common mistake with roughness charts is treating the Ra-to-Rz column as exact. It is not, and here is why. Ra averages all deviations while Rz captures peak-to-valley heights, so the ratio between them depends on the shape of the surface profile, which differs by process.

For a regular, periodic surface like a finely turned part, Rz is often roughly four to seven times Ra. For an irregular surface like a bead-blasted or ground finish, the ratio is different. A widely used rule of thumb is Rz is about four times Ra, but this can vary from roughly three to nearly ten depending on the process. Therefore:

• Never substitute a converted value for a measured one on a critical surface.
• If a drawing specifies Rz, measure Rz, do not measure Ra and convert.
• Use the chart for planning and communication, not for certifying compliance.

The deeper explanation of why the parameters diverge is in Ra vs Rz vs Rt.

Matching Finish to Function

The chart also helps you avoid over-specifying. Map the typical requirement to a sensible Ra band:

Choosing one band tighter than necessary can multiply cost by adding grinding or polishing, as discussed in what is surface finish in CNC machining. For cosmetic aluminum specifically, the right pre-finish Ra also affects how downstream treatments look; see aluminum anodizing guide and sandblasted aluminum finish.

Image suggestion 2 — A matching graphic showing application categories mapped onto the Ra scale.

Alt text: "Chart mapping common applications to target Ra surface roughness bands."

Placement: end of this section.

Achieving Chart Values in Practice

Where a value sits on the chart tells you whether machining alone reaches it. Down to about Ra 0.4 micrometers, a capable high-speed machine with sharp tooling, light finishing feed and good rigidity can hit the target by cutting. Below that, into grinding, lapping and polishing territory, secondary operations are required, covered in mirror finish machining and how to achieve mirror finish aluminum. The cleaner the machined surface you start from, the less polishing it takes to reach a mirror, so a low machined Ra pays off downstream.

Recommended HYR Machines

• HYR VMC850 — reaches the N5 to N6 band by machining on aluminum thanks to its high-speed spindle.
• HYR VMC1060 — stable, precise finishing to tight chart values on larger parts.
• HYR 5 Axis Machining Center — consistent low-Ra finishes on contoured surfaces.

Need to hit a chart value on a real part? Use the HYR Machine Selector to match the machine and spindle to your Ra target and material.

Frequently Asked Questions

What is a surface roughness chart?

It is a reference table that lines up equivalent ways of expressing the same finish: Ra in micrometers and microinches, the approximate Rz, the ISO N grade, and the typical process. It lets you convert and plan at a glance.

What is Ra 0.8 in microinches?

Ra 0.8 micrometers is about 32 microinches, which corresponds to ISO grade N6 and a fine milled or ground finish. Ra 1.6 is about 63 microinches (N7) and Ra 3.2 is about 125 microinches (N8).

Is the Ra to Rz conversion exact?

No. The ratio depends on the shape of the surface profile, which varies by process. A common rule of thumb is Rz is about four times Ra, but it can range from roughly three to nearly ten. If a drawing specifies Rz, measure Rz rather than converting.

What do the N grades mean on a roughness chart?

The ISO N grades run from N1 (smoothest, about Ra 0.025 micrometers) to N12 (roughest, about Ra 50 micrometers). Each grade corresponds to a specific Ra value, so N7 means Ra 1.6 micrometers.

What Ra should I specify for a sealing face?

Sealing and gasket faces typically call for Ra 0.4 to 1.6 micrometers depending on the seal type, with lay direction often specified too. Tighter is not always better, since the seal needs some texture to grip in many designs.

Can I reach any chart value by machining alone?

Down to about Ra 0.4 micrometers, a capable high-speed machine with sharp tooling can hit the target by cutting. Below that, into grinding, lapping and polishing territory, secondary operations are required.

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Internal Links Used in This Article

• What Is Surface Finish In CNC Machining (pillar)
• Surface Roughness Ra Explained
• Ra vs Rz vs Rt
• Surface Finish Symbols Explained
• Mirror Finish Machining · How To Achieve Mirror Finish Aluminum
• Aluminum Anodizing Guide · Sandblasted Aluminum Finish
• Best CNC Machine For Surface Finish
• Products: VMC850 · VMC1060 · HYR 5 Axis
• CTA: HYR Machine Selector