Convert between Ra, Rz and RMS roughness parameters, and read off the ISO grade number.
Data verified 2026-09-29 · based on n/a — standard engineering relationships, no single governing revision
Quick Answer
Ra is the arithmetic mean deviation from the mean line; Rz is the maximum height of the profile within a sampling length. For most machined surfaces Rz ≈ 4 to 7 × Ra, with the ratio depending on the process.
Roughness Parameter Converter
The Formulas Used
Rz ≈ 5 × Ra (typical machined surface) RMS ≈ 1.11 × Ra The ISO grade N is assigned in steps of √10 in Ra, from N1 (0.025 µm) to N12 (50 µm).
Ra, Rz and RMS
Three different ways of describing the same profile:
Ra — the arithmetic mean of the absolute deviation from the mean line over the sampling length. The most widely specified parameter, because it is stable and easy to measure.
Rz — the average of the peak-to-valley heights within the sampling length. More sensitive to isolated peaks and scratches than Ra, and a better predictor of sealing and fatigue behaviour.
RMS — the root mean square deviation. Historically common in North American practice and roughly 1.1 times Ra for a sinusoidal profile.
The Rz/Ra ratio is not a constant. It runs around 4 for turned surfaces, closer to 5–7 for ground and milled surfaces, and higher still where the profile is dominated by isolated peaks. Treat the conversion as an estimate and specify the parameter you actually intend to inspect.
Frequently Asked Questions
What is the difference between Ra and Rz?
Ra is the average deviation from the mean line; Rz is the average peak-to-valley height. Ra is more commonly specified because it is stable and easy to measure, while Rz is more sensitive to individual peaks and better at predicting sealing and fatigue performance.
How do I convert Ra to Rz?
Multiply by roughly 4 to 7, depending on the process — use 5 as a general estimate. The ratio is not a constant, so for inspection specify the parameter you actually mean rather than converting.
What is the ISO grade N number?
A roughness class from N1 to N12, assigned in steps of √10 in Ra. N1 is 0.025 µm and N12 is 50 µm. A grade of N7 corresponds to Ra 1.6 µm, one of the most common drawing specifications.
What is RMS roughness?
Root mean square deviation — historically common in North American practice and about 1.11 times Ra for a sinusoidal profile. It is rarely specified on new drawings, where Ra is the standard parameter.
Is a lower Ra always better?
Not necessarily. Smoother costs more to produce, and very smooth surfaces can have higher friction because they adhere over a larger real contact area. Specify the roughness the function requires, not the smoothest achievable.
n/a — standard engineering relationships, no single governing revision
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