Surface roughness parameters explained
Ra, Rz, Rq and Rt describe the same surface in different ways. Choosing the wrong one on a drawing or inspection report can cause rejected parts, leaking seals or premature wear. Below is a plain-English guide to each parameter, how they relate, and when engineers specify one over the other.
What do Ra, Rz, Rq and Rt mean?
Ra — Arithmetic average roughness
Ra is the arithmetic average of the absolute deviations from the mean line over the evaluation length. It is the most common surface finish parameter worldwide because it is stable, easy to measure and directly comparable across machines. However, Ra alone can hide deep scratches or flat plateaus, so sealing and fatigue-critical surfaces usually require Rz as well.
Rz — Mean peak-to-valley height
Rz averages the vertical distance between the five highest peaks and five deepest valleys within each sampling length. It is more sensitive to occasional scratches or burrs than Ra, which is why German and Japanese automotive drawings often specify Rz for sealing surfaces and bearing races.
Rq — Root-mean-square roughness (RMS)
Rq is the square root of the average squared deviation from the mean line. For a Gaussian-like profile, Rq is approximately 1.11 × Ra. RMS is common in optics, electronics and American drawings; always confirm whether the drawing means Ra or RMS before interpreting a value.
Rt — Maximum peak-to-valley height
Rt is the single largest peak-to-valley distance within the entire evaluation length. It is the most extreme parameter and is useful for detecting scratches, dent or burrs, but it is statistically noisy. As a rule of thumb, Rt ≈ 1.2 × Rz for a stable, in-control process.
How to convert Ra to Rz (and why it depends on the process)
There is no universal Ra-to-Rz constant. The ratio depends on the shape of the profile, which is driven by the manufacturing process. A polished surface has a low Rz/Ra ratio because peaks and valleys are shallow; a sand casting or FDM print has a much higher ratio because of deep pores and layer steps.
For quick estimating, use the process-aware ratios in the calculator above. For contract or final inspection, always measure Rz directly or reference a validated process capability study.
| Process | Typical Rz/Ra ratio | So 0.8 Ra ≈ |
|---|---|---|
| Lapping | 3.60 | 2.88 Rz µm |
| Polishing | 4.00 | 3.20 Rz µm |
| Grinding (precision) | 4.50 | 3.60 Rz µm |
| Honing | 4.00 | 3.20 Rz µm |
| Turning (finish) | 4.50 | 3.60 Rz µm |
| Milling (finish) | 5.00 | 4.00 Rz µm |
| EDM (finish) | 5.50 | 4.40 Rz µm |
| Drilling | 5.50 | 4.40 Rz µm |
| Broaching | 4.50 | 3.60 Rz µm |
| Casting (sand) | 7.00 | 5.60 Rz µm |
| FDM 3D print | 7.00 | 5.60 Rz µm |
| SLS 3D print | 6.00 | 4.80 Rz µm |
| SLA / DLP 3D print | 4.50 | 3.60 Rz µm |
Common surface finish conversions used in industry
These examples use the generic Rz/Ra ratio of 4.0 and Rq/Ra of 1.11, which is a safe starting point when the process is unknown. Select the actual process in the calculator above for a more accurate, process-aware result.
| Ra (µm) | Ra (µin) | Rz (µm) generic | Rq / RMS (µm) | ISO 1302 grade | Typical application |
|---|---|---|---|---|---|
| 0.025 | 1.0 | 0.10 | 0.028 | N1 | Optical-quality finish |
| 0.1 | 3.9 | 0.40 | 0.111 | N3 | High-precision shafts |
| 0.4 | 16 | 1.60 | 0.444 | N5 | Hydraulic cylinder bores |
| 0.8 | 31 | 3.20 | 0.888 | N6 | Common machined fit surfaces |
| 1.6 | 63 | 6.40 | 1.776 | N7 | General-purpose parts |
| 3.2 | 126 | 12.80 | 3.552 | N8 | Structural components |
| 6.3 | 248 | 25.20 | 6.993 | N9 | Non-sealing surfaces |
| 12.5 | 492 | 50.00 | 13.875 | N10 | As-cast or as-printed |
When should you specify Ra vs Rz on a drawing?
Use Ra when: you need a stable, repeatable number for general machining control, cost estimation or supplier comparison. Ra is the default on most ISO 1302 callouts and is sufficient for non-sealing, non-fatigue surfaces.
Use Rz when: the surface seals, slides, rolls or is subject to fatigue. Rz catches the deep scratches and peaks that Ra can average away. Automotive sealing faces, bearing races, hydraulic cylinders and gears are common Rz applications.
Use Rq / RMS when: the specification comes from optics, electronics or North American suppliers that quote RMS. Confirm the parameter symbol before accepting parts; an RMS value is about 11% higher than the equivalent Ra value.
Use Rt when: you need to catch the single worst defect, such as a scratch from handling or a burr from machining. Rt is not a stability metric and should be paired with Ra or Rz.
ISO 1302 drawing symbols and callouts
Modern ISO 1302 drawings use a simple check mark (tick) symbol with the roughness parameter and limit value next to it. The parameter symbol tells the inspector what to measure:
- — (no symbol) defaults to Ra unless the drawing states otherwise.
- Rz next to the tick means the Rz value is the requirement.
- Rmax (older drawings) is roughly equivalent to Rt.
- Rq or RMS must be written explicitly if required.
A complete callout usually looks like:Ra 0.8 / Rz 3.2 max, λc 0.8 mm. The filter (Gaussian), evaluation length and sampling length are often added in a note for critical parts.
Frequently asked questions
What is 0.8 Ra in Rz?
For a generic machined surface, 0.8 Ra ≈ 3.2 Rz. For a ground surface the ratio is closer to 4.5, giving ≈ 3.6 Rz; for a sand casting or FDM print the ratio can be 6–7, giving ≈ 4.8–5.6 Rz.
How do I convert Ra to microinches?
1 µm = 39.37 µin (microinches). Multiply the Ra value in µm by 39.37. For example, 0.8 Ra µm = 31.5 µin.
Is RMS the same as Ra?
No. RMS (Rq) weights larger deviations more heavily, so it is always slightly larger than Ra for the same profile. A typical conversion is Rq ≈ 1.11 × Ra.
What ISO grade is 0.8 Ra?
0.8 Ra corresponds to ISO 1302 N6. It is a common machined finish for fit and seal surfaces.
Need help with surface finish requirements?
CADfinity helps engineering teams choose the right roughness parameters, inspect as-printed and as-machined surfaces, and convert scan data into production-ready CAD. Get in touch for reverse engineering, 3D inspection or GD&T support.
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