Ra to Rz Conversion (and Rz to Ra)

Converting Between Ra and Rz

You got a drawing with a callout for Rz. However, you are used to using Ra. Can you convert the Rz callout to Ra? The answer is not straightforward. Learn why, or head directly to the Ra-Rz converter calculator.

What is the difference between Ra & Rz ?

Difference betwwen Ra and Rz
  • Ra The arithmetic average of the absolute values of the profile height deviations from the centerline.
  • Rz the vertical distance from the highest peak to the lowest valley within the scanned profile.

You can learn more in our Surface Finish Units Dictionary.

How to Convert between Ra & Rz ?

A common question among machinists and engineers is, “How to convert from Ra to Rz?”. A precise conversion like one can make between Kw and HP is impossible since Ra and Rz are two different properties.

It is like asking, “how do I convert height to weight?”. You cannot convert. However, you can make a rough estimation based on statistics. A person with a height of 6 feet will weigh between 140 and 290 pounds (At a probability of 99%). We know he will not weigh 100 pounds, nor 400.

The same logic is applied when converting Ra to Rz (or Rz to Ra). Based on statistics, we can forecast the Rz range for a specific Ra with reasonable probability.

Ra / Rz Conversion formulas

Rz to Ra

\begin{array} \Large R_{a\,Min} = 0.03\,\times\,{R_z}^{1.3} &\\&\\ R_{a\,Max} = 0.24\,\times\,{R_z}^{1.06} &\\ \end{array}

Ra to Rz

\begin{array} \Large R_{z\,Min} = 3.8\,\times\,{R_a}^{0.95} &\\&\\ R_{z\,Max} = 14.5\,\times\,{R_a}^{0.75} &\\ \end{array}

Converting between Ra and Rz is not a good engineering practice. It is always recommended to Measure according to the method in which surface roughness is indicated on the drawing.

  • As you can see in the below graphs, the inaccuracy grows in higher Ra /Rz values.
  • The explanation is more clear when presented on the Log-Log graph.

Ra/ Rz Conversion Graph

Ra-Rz Conversion Graph Linear scale

Ra/ Rz Log-Log Conversion Graph

Ra-Rz Convertion Graph Logarithmic scale

Ra / Rz Convention Calculator

Ra / Rz Conversion Charts

Ra to Rz Metric

Ra [µm] Rz Range [µm]
0.1 0.4 - 2.5
0.2 0.8 - 4
0.5 2 - 9
1 4 - 14
1.5 6 - 20
2 7 - 25
3 11 - 33
5 17 - 49
10 34 - 84
15 50 - 115
20 65 - 140

Ra to Rz Inch

Ra [µ Inch] Rz Range [µ Inch]
4 16 - 100
8 32 - 160
20 80 - 360
40 160 - 560
60 240 - 800
80 280 - 1000
120 440 - 1320
200 680 - 1960
400 1360 - 3360
600 2000 - 4600
800 2600 - 5600

Rz to Ra Metric

Rz [µm] Ra Range [µm]
1 0.03 - 0.2
5 0.3 - 1.3
10 0.6 - 3
20 1.5 - 6
50 5 - 15
100 13 - 32
150 21 - 49

Rz to Ra Inch

Rz [µ Inch] Ra Range [µ Inch]
40 1 - 8
200 12 - 52
400 24 - 120
800 60 - 240
2000 200 - 600
4000 520 - 1280
6000 840 - 1960
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