The average peak-to-valley height of the profile over the sampling length.
Rz is far more sensitive than Ra to isolated peaks and valleys, which makes it the better parameter where a single deep scratch matters — sealing surfaces, fatigue-critical parts, and anything that will be coated.
The ratio between Rz and Ra is not a constant: it runs around 4 for turned surfaces and 5 to 7 for ground ones. Converting between them is an estimate, not a conversion.
Treating Rz as exactly 4 × Ra and specifying one while inspecting the other. The ratio depends on the process, and the two parameters are independent measurements.
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