What is the difference between Ra and Rz?

Ra is the arithmetical mean roughness (average deviation from mean line). Rz is the maximum height of the profile (average of 5 highest peaks to 5 deepest valleys). Rz is typically 4-7 times Ra for machined surfaces.

What filter cutoff should I use?

ISO 21920 specifies default cutoff wavelengths: λc = 0.08 mm for Ra ≤ 0.1 μm, λc = 0.25 mm for Ra 0.1-2 μm, λc = 0.8 mm for Ra 2-10 μm, λc = 2.5 mm for Ra 10-80 μm, λc = 8 mm for Ra > 80 μm.

Rq (root mean square roughness) is the RMS of the profile deviations. Rq ≈ 1.11 × Ra for a random (Gaussian) profile. Rq is more sensitive to peaks and valleys than Ra.

Rt (total height of profile) is the vertical distance between the highest peak and the deepest valley across the entire evaluation length. Rt ≥ Rz always. Rt is highly sensitive to scratches and defects.

How does ISO 21920 differ from the old ISO 4287?

ISO 21920 replaces the older ISO 4287 and ISO 3274 standards. It consolidates profile parameter definitions, measurement procedures, and filter specifications into a single updated standard series.

Free Engineering Tool

Surface Parameters Calculator — ISO 21920

Enter Ra value or measurement data to see estimated Rz, Rq, Rt relationships and recommended filter cutoff per ISO 21920.

ISO 21920Ra / Rz / Rq / RtFilter Cutoff

Estimated Parameters

Ra (Arithmetical Mean)

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Rz (Max Height, est.)

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Rq (RMS, est.)

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Rt (Total Height, est.)

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Filter Cutoff λc

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Profile Parameters

Parameter	Definition	Approx. Relation to Ra
Ra	Arithmetical mean of \|Z(x)\| over evaluation length	1.0 × Ra
Rq	Root mean square of Z(x)	1.11 × Ra (Gaussian)
Rz	Max height: average of 5 highest peaks + 5 deepest valleys	4–7 × Ra (process dependent)
Rt	Total height: max peak to min valley over full length	5–10 × Ra

Default Filter Cutoff (λc)

Ra Range (μm)	λc (mm)	Evaluation Length (mm)
0.006 – 0.02	0.08	0.4
0.02 – 0.1	0.25	1.25
0.1 – 2.0	0.8	4.0
2.0 – 10	2.5	12.5
10 – 80	8.0	40.0

Note: The Ra-to-Rz conversion is approximate and depends heavily on the machining process. Always measure directly when possible.

Surface Roughness Calculator Ra Rz Rq Rt | ISO 21920

Published by Nikolai Shelkovenko on February 15, 2026 March 5, 2026

Estimated Parameters

Profile Parameters

Default Filter Cutoff (λc)