Ra vs Rz: Which Surface Roughness Callout Do You Need?

Both parameters come from the same raw data: a stylus (or optical sensor) traces a line across the surface, and the recorded profile is filtered to separate roughness from waviness. Ra — roughness average — is the arithmetic mean of the absolute values of every deviation from the mean line over the sampling length. Every point on the trace counts equally, so the result describes the overall texture of the surface. It is defined identically in ISO 4287 and ASME B46.1, so Ra travels between US and international drawings without confusion.

Rz — mean roughness depth — ignores most of the profile and concentrates on the extremes. Under ISO 4287, the evaluation length is divided into five sampling lengths; in each, the vertical distance from the highest peak to the lowest valley is measured, and Rz is the mean of those five heights. One caution: the symbol Rz has carried different formulas across eras. The legacy DIN and JIS "ten-point height" Rz averaged the five highest peaks and five deepest valleys within a single sampling length — a different calculation that can return a different number from the same surface, and ISO 21920 has since tightened the definition again. On old drawings and old gauges, "Rz" is not always the same Rz, so when a dispute arises, the first question is which standard and algorithm the instrument is running.

Every machinist's handbook has an Ra-to-Rz conversion chart, and every one of them is a trap. The reason is arithmetic, not opinion: Ra averages every deviation on the profile, so a single deep scratch barely moves it. Rz is built from the highest peaks and deepest valleys, so the same scratch dominates it. Published Ra:Rz ratios range from roughly 2:1 on uniform ground surfaces to over 20:1 on surfaces with isolated defects. A conversion chart cannot know which surface you have — that is the entire problem.

The widely quoted rule of thumb — Rz ≈ 4–7 × Ra — is an empirical screening estimate for typical machined textures, not a standard and not a contract. It has one legitimate job: sanity-checking. If a drawing calls out Rz 16 and your profilometer reads Ra 3.8, the estimate (15–27) tells you the part is probably in the neighborhood — but that is not inspection. If the drawing specifies Rz, measure Rz with the correct sampling length and cutoff; if it specifies Ra, measure Ra. And if a customer's drawing shows one parameter while your quality plan tracks the other, resolve the callout before production — a finish dispute found at final inspection is the most expensive version of this conversation. The same discipline applies to dimensional specs, covered in our CNC machining tolerances guide.

The workhorse instrument is the contact stylus profilometer: a diamond-tipped stylus drags across the surface and its vertical deflection becomes the profile. Three settings decide whether your number matches the customer's number. Cutoff wavelength (sampling length): ISO 4288 sets defaults by roughness range — 0.8 mm cutoff for the Ra 0.1–2 µm band, 2.5 mm for Ra 2–10 µm. The standard evaluation is five sampling lengths, so a 0.8 mm cutoff means a 4 mm evaluation length. Measure a fine ground surface with a 2.5 mm cutoff and you fold waviness into the roughness number. Stylus tip radius: 5 µm is the robust workshop standard, adequate down to about Ra 0.5 µm; finer finishes need a 2 µm tip. Direction: trace perpendicular to the lay — measuring along the tool marks of a turned shaft reads the valleys, not the surface.

Optical instruments — white-light interferometers and confocal systems — map an area without touching the part, protecting soft materials and enabling areal parameters (Sa, Sz) that describe the whole surface rather than one line. They cost more, so most shops verify Ra and Rz with a stylus and reserve optical for critical or delicate surfaces. Note that some budget gauges report only Ra — confirm yours actually computes Rz before accepting an Rz callout. If a spec is disputed, a reference to the standard on the drawing (ISO 4287/4288 or ASME B46.1) settles the measurement conditions — the same logic as any other symbol on the print, covered in our engineering drawing symbols guide.