What is the difference between Ra and Rz?

Ra (arithmetical mean roughness) is the average of absolute profile deviations from the mean line over the evaluation length. Rz (maximum height) is the average of the five highest peaks and five deepest valleys within the evaluation length. Ra gives an overall average roughness, while Rz is more sensitive to individual peaks and valleys. Roughly, Rz ≈ 4×Ra for most machined surfaces.

How do I convert between Ra and Rz?

A rough approximation is Rz ≈ 4×Ra, but the exact ratio depends on the surface profile and machining process. For precision work, use the ISO standard conversion tables. For general engineering, the ×4 factor provides a reasonable estimate.

What is N class (roughness grade)?

N classes (N1 through N12) are roughness grade numbers defined in ISO 1302. Each grade corresponds to a specific Ra value: N1=0.025 μm, N2=0.05 μm, N3=0.1 μm, up to N12=50 μm. They provide a simplified way to specify surface roughness on drawings.

What is typical Ra for machining operations?

Typical Ra values: Lapping/superfinishing: 0.025–0.1 μm (N1–N3). Fine grinding: 0.2–0.4 μm (N4–N5). Normal grinding/fine turning: 0.8–1.6 μm (N6–N7). Rough turning/milling: 3.2–12.5 μm (N8–N10). Casting/forging: 25–50 μm (N11–N12).

What Ra is needed for bearing seats?

Bearing seats typically require Ra 0.8–1.6 μm (N6–N7) for the shaft and housing bore. For precision bearings, Ra 0.4 μm (N5) or better may be needed. Always check the bearing manufacturer's recommendations.

Free Engineering Tool

Surface Roughness Converter

Convert between Ra, Rz, roughness class (N1–N12), and CLA/AA microinch values. Includes surface finish descriptions and typical machining processes.

Ra ↔ Rz Class N1–N12 CLA / AA μin

Results

Ra (Arithmetical Mean Roughness)

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Rz (Maximum Height)

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Roughness Class

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CLA / AA

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Surface Description

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Typical Process

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Class	Ra (μm)	Rz (μm)	CLA (μin)	Description	Typical Process
N1	0.025	0.1	1	Mirror finish	Superfinishing, lapping
N2	0.05	0.2	2	Mirror finish	Superfinishing, lapping
N3	0.1	0.4	4	Very smooth	Lapping, honing
N4	0.2	0.8	8	Smooth	Fine grinding, honing
N5	0.4	1.6	16	Smooth	Fine grinding
N6	0.8	3.2	32	Fine	Grinding, fine turning
N7	1.6	6.3	63	Fine	Turning, milling
N8	3.2	12.5	125	Medium	Turning, milling
N9	6.3	25	250	Medium rough	Rough machining
N10	12.5	50	500	Rough	Rough machining
N11	25	100	1000	Very rough	Casting, forging
N12	50	200	2000	Very rough	Casting, forging, flame cut

Ra — Arithmetical Mean Roughness

Ra is the arithmetical average of the absolute values of the profile deviations from the mean line, measured over the evaluation length:

Ra is the most commonly used roughness parameter worldwide. It gives a general indication of surface texture but can mask individual peaks or valleys.

Rz — Maximum Height of Profile

Rz is the average of the single roughness depths of five consecutive sampling lengths. It is more sensitive to outlier peaks and valleys:

Approximate Conversion

ℹ️ Note: The Rz ≈ 4×Ra approximation is a rough guide for typical machined surfaces. The actual ratio depends on the surface profile shape and machining process, and can range from 3 to 8.

CLA / AA Conversion

CLA (Centre Line Average) and AA (Arithmetic Average) are older terms for Ra, expressed in microinches. 1 μm = 39.37 μin.

⚠️ Important: Conversions between Ra and Rz are approximate. For critical applications, measure the specific parameter required by the specification directly.