Surface finish isn’t just a number on a drawing — it directly affects how parts slide, seal, wear, or even look. The three parameters people talk about most are Ra, Rz, and RMS/Rq. They measure the same thing in different ways, and people constantly ask:
- “What’s the difference between Ra and Rz?”
- “How do I convert RMS to Ra?”
- “Is 32 Ra the same as 125 RMS?”
Let’s break it down clearly, with real-world examples, simple comparisons, and a handy conversion table you can actually use.
1. Ra — The Most Popular One (and Why Everyone Uses It)
Ra = Arithmetic Average Roughness Think of it as: “On average, how bumpy is the surface if I ignore the tallest peaks and deepest valleys?”
- It takes the absolute height difference from the mean line and averages them.
- Super easy to measure, very repeatable → that’s why almost every drawing uses Ra.
- Common values you see every day:
- 0.8 µm (~32 µin) — nice smooth turned or milled finish
- 1.6 µm (~63 µin) — standard for many precision parts
- 3.2 µm (~125 µin) — typical as-machined or ground finish
When to choose Ra → General machining, cosmetic surfaces, most CNC jobs → When the drawing just says “surface finish” without extra symbols, it’s almost always Ra.
2. Rz — The One That Catches the “Dangerous” Peaks & Valleys
Rz = Mean Peak-to-Valley Height (or Ten-Point Height) Imagine you’re worried about leak paths around an O-ring or stress risers that cause cracks. Rz looks at the 5 tallest peaks + 5 deepest valleys and averages them.
- Rz is usually 4–10 times larger than Ra (most commonly ~7–8× on machined surfaces)
- Example: If Ra = 1.6 µm → Rz is often around 10–13 µm
When people care about Rz
- Hydraulic fittings, valve seats, gasket faces
- Medical implants, food/pharma equipment (sealing is critical)
- Any surface where a single deep scratch or high peak can cause failure
3. RMS / Rq — The “Statistically Smarter” Cousin of Ra
RMS (older American term) = Root Mean Square Rq (modern ISO name) = same thing
Instead of just averaging the absolute values, it squares the deviations first → makes big bumps count much more.
- Rq is always a little higher than Ra
- Typical ratio: Rq ≈ Ra × 1.11 to 1.25 (most people use 1.11 as a quick rule)
When to use RMS/Rq
- Optical parts, mirrors, lenses
- Vibration analysis, bearings under dynamic load
- Legacy US drawings (especially aerospace from 1990s–2000s)
Quick Side-by-Side Comparison (The Cheat Sheet)
| You Care About | Best Parameter | Typical Value Compared to Ra | Real-World Example |
|---|---|---|---|
| General machining | Ra | 1× (baseline) | CNC milled aluminum bracket — 1.6 µm |
| Sealing / leak prevention | Rz | 4–10× | O-ring groove — Rz ≤ 8 µm |
| Optical quality / stats | Rq / RMS | 1.1–1.25× | Lens mount — Rq ≤ 0.4 µm |
| Fatigue / crack risk | Rz or Rt | much higher | Aerospace landing gear journal |
Easy Conversion Rules of Thumb (Most Engineers Use These)
| From → To | Quick Formula | Example (Ra = 1.6 µm) |
|---|---|---|
| Ra → RMS/Rq | RMS ≈ Ra × 1.11 | ≈ 1.78 µm |
| Ra → Rz | Rz ≈ Ra × 7.2 (most common) | ≈ 11.5 µm |
| Ra → Rt (max height) | Rt ≈ Ra × 8–9 | ≈ 13–14 µm |
| RMS → Ra | Ra ≈ RMS / 1.11 | — |
| Rz → Ra | Ra ≈ Rz / 7.2 | — |
Important note: These are approximations! The exact ratio depends on how “spiky” or “wavy” the surface is.
- Turned surfaces → closer to 7–8×
- Ground surfaces → sometimes 5–6×
- Shot-blasted or EDM → can be 10× or more
Always measure if the tolerance is tight.
Handy Conversion Table (µin & µm — Print This Out!)
| Ra (µin) | Ra (µm) | RMS / Rq (µin) | Rz approx (µm) | Typical Process |
|---|---|---|---|---|
| 8 | 0.2 | ~9 | 1.0–1.8 | Super-fine lapped / polished |
| 16 | 0.4 | ~18 | 2–3.5 | Precision ground |
| 32 | 0.8 | ~35 | 4–7 | Fine turned / milled |
| 63 | 1.6 | ~70 | 8–13 | Standard CNC finish |
| 125 | 3.2 | ~138 | 16–25 | Rough turned / as-machined |
| 250 | 6.3 | ~275 | 30–50 | Coarse milled / sanded |
Bottom Line — Which One Should You Specify?
- Most drawings today → just use Ra (easiest, most shops understand it)
- Need perfect sealing? → Add Rz requirement
- Working with old US prints or optics? → Check for RMS
- Not sure? → Specify Ra, and add “or equivalent” if someone wants to quote with Rz.
Got a specific part number or drawing in mind? Drop the Ra / Rz value you’re seeing, and I can help you interpret what it really means in practice.
Ryan Wang
Ryan Wang is the CNC Machining Expert at Cncpioneer, with over 15 years of hands-on experience as a CNC programmer, process engineer, senior machinist, and precision manufacturing specialist. He has helped companies in aerospace, automotive, medical, and electronics sectors achieve micron-level tolerances and scale from prototypes to high-volume production. Ryan is also an experienced instructor in advanced CNC techniques, particularly five-axis machining and challenging materials.
