Convert surface roughness measurements between common Ra, Rq and Rz reference values and switch between micrometres and microinches.
| Reference | Value | Unit |
|---|
Surface roughness describes the fine-scale irregularities present on a manufactured surface. Machining, grinding, polishing, casting, forming and finishing processes can produce different surface profiles.
Surface texture specifications are commonly used in manufacturing drawings, machining processes, quality inspection and component specifications.
Ra represents the arithmetic average of the absolute profile deviations from the mean line over the evaluation length.
Ra is widely used because it provides a simple numerical description of average surface texture. However, two surfaces with the same Ra can still have substantially different profile shapes.
Rq is the root mean square value of the profile deviations from the mean line.
Because deviations are squared before averaging, larger peaks and valleys have greater influence on Rq than on Ra.
For some idealized profile distributions, Rq may have a predictable relationship with Ra. A universal exact conversion should not be assumed for real measured surfaces.
Rz is a height-based surface roughness parameter rather than an average-deviation parameter like Ra. The precise calculation and definition can depend on the applicable surface-texture standard.
For this reason, an Ra value cannot simply be multiplied by one universal factor to obtain an exact Rz measurement.
Surface roughness is commonly expressed in micrometres (µm) in metric specifications and microinches (µin) in inch-based specifications.
The conversion is a unit conversion only. It does not change the underlying roughness parameter.
| Ra | Approx. µin | General Reference |
|---|---|---|
| 0.025 µm | 1 µin | Very fine finish |
| 0.05 µm | 2 µin | Very fine finish |
| 0.1 µm | 4 µin | Fine finish |
| 0.2 µm | 8 µin | Fine finish |
| 0.4 µm | 16 µin | Fine machined finish |
| 0.8 µm | 32 µin | Machined finish |
| 1.6 µm | 63 µin | Common machined finish |
| 3.2 µm | 126 µin | General machining range |
| 6.3 µm | 248 µin | Rougher machined finish |
| 12.5 µm | 492 µin | Coarse surface |
These are general Ra references for comparison and are not substitutes for a drawing specification or process requirement.
A numerical Ra value summarizes average profile deviation but does not fully describe peak shape, valley depth, spacing, waviness or directional texture.
Two components can therefore have the same Ra while having visibly or functionally different surfaces.
When a technical drawing specifies a surface roughness value, use the stated parameter and measurement standard. Do not automatically substitute an Ra value for an Rz requirement or vice versa.
Measurement conditions, cutoff length, evaluation length, filtering and instrument settings can influence reported surface-texture values.
This tool provides unit conversion and reference information. It does not measure a physical surface and does not establish whether a component meets a drawing or specification.
Actual surface roughness should be measured using appropriate calibrated equipment and the measurement procedure required by the applicable standard.
What does Ra mean? Ra is the arithmetic average roughness based on the absolute deviations of the assessed profile from its mean line.
What does Rq mean? Rq is the root mean square roughness and gives greater mathematical weighting to larger profile deviations.
Can I convert Ra directly to Rz? Not exactly. Rz and Ra are different surface-profile parameters, so there is no universal exact conversion factor.
Can I convert Ra from µm to µin? Yes. Multiply the micrometre value by approximately 39.37 to obtain microinches.
Can I convert microinches to micrometres? Yes. Multiply the microinch value by 0.0254 to obtain micrometres.
Does the tool measure actual surface roughness? No. It converts entered values and provides reference information. A surface roughness instrument is required for physical measurement.