Surface Roughness Chart
Convert between Ra, Rz, RMS, and ISO 1302 N-grades. Filter by milling, turning, grinding, and laser cutting to see which process delivers the finish your drawing requires — from mirror polish to rough mill.
Surface Roughness Conversion
Enter Ra in microinches or micrometers, Rz, RMS, or an N-grade (N1–N12) to see all equivalent surface finish values.
- • RMS ≈ Ra × 1.11 for typical machined surfaces.
- • Rz ≈ Ra × 4 (μin) is a shop approximation — actual Rz depends on process and profilometer settings.
- • N-grades come from ISO 1302:1992; the 2002 edition dropped them, and ISO 1302 has since been replaced by ISO 21920-1:2021. They still appear on older drawings. US ASME Y14.36 drawings call out Ra numerically, not N-grade.
Showing 21 finishes
| Ra (μin) | Ra (μm) | Rz (μin) | RMS (μin) | N Grade | Machining Process | Typical Application |
|---|---|---|---|---|---|---|
| 1 | 0.025 | 4 | 1.1 | N1 | Superfinish | Gauge blocks, metrology surfaces |
| 2 | 0.05 | 8 | 2.2 | N2 | Lapping | Precision seals, valve seats |
| 4 | 0.1 | 16 | 4.4 | N3 | Fine grinding | Hydraulic spools, piston rods |
| 8 | 0.2 | 32 | 8.9 | N4 | Cylindrical grinding | Crankshaft pins, cam profiles |
| 16 | 0.4 | 64 | 17.8 | N5 | Finishing pass (milling) | Precision pockets, datum faces |
| 16 | 0.4 | 64 | 17.8 | N5 | Finishing pass (turning) | Seal diameters, O-ring grooves |
| 16 | 0.4 | 64 | 17.8 | N5 | Honing | Cylinder bores, liner seats |
| 32 | 0.8 | 128 | 35.5 | N6 | Fine CNC milling | Fixture plates, gauge surfaces |
| 32 | 0.8 | 128 | 35.5 | N6 | Fine CNC turning | Bearing journals, spindle fits |
| 32 | 0.8 | 128 | 35.5 | N6 | Surface grinding | Machine ways, slide surfaces |
| 32 | 0.8 | 128 | 35.5 | N6 | Fine laser cut | Thin-sheet enclosures, shims |
| 63 | 1.6 | 252 | 69.9 | N7 | Standard CNC milling | Machined flats, mounting faces |
| 63 | 1.6 | 252 | 69.9 | N7 | Standard CNC turning | Shafts, spacers, pins |
| 63 | 1.6 | 252 | 69.9 | N7 | Standard laser cut | Sheet panels, laser-cut brackets |
| 125 | 3.2 | 500 | 138.8 | N8 | As-milled (default) | Brackets, housings, covers |
| 125 | 3.2 | 500 | 138.8 | N8 | General turning | Bushings, adapters, collars |
| 125 | 3.2 | 500 | 138.8 | N8 | Production laser cut | Structural sheet frames, plates |
| 250 | 6.3 | 1000 | 277.5 | N9 | Rough milling | Pre-machined castings, forgings |
| 250 | 6.3 | 1000 | 277.5 | N9 | Rough turning | Heavy stock, large OD turning |
| 250 | 6.3 | 1000 | 277.5 | N9 | Fast laser cut (thick plate) | Thick-plate edges, heavy-gauge cuts |
| 500 | 12.5 | 2000 | 555.0 | N10 | Hogging / heavy roughing | Bulk stock removal, roughing ops |
Quick answer: what is surface finish (Ra, Rz, and RMS)?
Surface finish measures the small peaks and valleys left on a machined part. Ra (roughness average) is the arithmetic mean of profile heights — the most common callout on engineering drawings. Rz is the maximum peak-to-valley height of the profile, usually reported as the average of five sampling lengths, and is more sensitive to deep scratches. RMS (root mean square) weights large peaks more heavily; for typical machined surfaces, RMS ≈ Ra × 1.11.
A lower Ra number means a smoother surface. This chart converts between Ra in microinches (μin) and micrometers (μm), approximate Rz and RMS values, and ISO 1302 N-grades (N1 = smoothest through N12 = roughest).
Standard CNC machining typically delivers Ra 63–125 μin (1.6–3.2 μm, N7–N8). Fine milling or turning can reach Ra 32 μin (0.8 μm, N6). Grinding and lapping achieve smoother finishes; rough milling or as-cast surfaces are coarser.
Common Ra values: N6 → Ra 0.8 μm (32 μin) · N7 → Ra 1.6 μm (63 μin) · N8 → Ra 3.2 μm (125 μin) · N5 → Ra 0.4 μm (16 μin) · N4 → Ra 0.2 μm (8 μin)
Surface finish chart: Ra, Rz, RMS, N-grade & machining process reference
What is surface finish and why it matters in CNC machining
Engineers search surface finish chart, surface roughness chart, and Ra Rz conversion thousands of times per month — usually to interpret a drawing callout, quote a tighter finish, or verify what a CNC shop can hold without secondary grinding.
Surface texture affects sealing, wear, friction, fatigue life, cosmetic appearance, and assembly fit. Over-specifying Ra on non-critical faces increases cycle time and cost; under-specifying on sealing or bearing surfaces causes field failures.
PREMSA's chart lists Ra in μin and μm, approximate Rz and RMS, ISO 1302 N-grades, and typical finishes per process — each filter shows only that process, sorted from smoothest to roughest with no duplicate Ra values.
Ra, Rz, RMS, and ISO 1302 N-grades explained
- Ra (μm or μin) — Arithmetic average roughness per ISO 4287. Most common US and global callout. Example: Ra 1.6 μm = 63 μin = N7.
- Rz — Maximum height of the profile (highest peak to lowest valley) within a sampling length; by convention it is reported as the average of five sampling lengths. Defined in ISO 4287, now ISO 21920-2. Cannot be converted exactly from Ra — depends on process. Use Rz for seals and fatigue-critical surfaces.
- RMS — Root mean square roughness (ASME B46.1). Older US drawings may use RMS. RMS ≈ Ra × 1.11 for typical machined surfaces.
- N-grade (ISO 1302) — Logarithmic shorthand N1–N12 from ISO 1302:1992 (dropped in the 2002 edition; ISO 1302 is now replaced by ISO 21920-1). N6 = Ra 0.8 μm (32 μin). Still seen on older ISO/JIS drawings — write the Ra value instead. ASME Y14.36 prefers explicit Ra values.
Typical CNC surface finish by process
CNC milling — Ra 16–500 μin (0.4–12.5 μm). Default as-milled is Ra 125 μin (3.2 μm, N8). Fine milling reaches Ra 32 μin; a finishing pass under ideal conditions (sharp tool, rigid setup, free-cutting material) can reach Ra 16 μin. Hogging and heavy roughing can exceed Ra 250 μin.
CNC turning — Ra 16–250 μin (0.4–6.3 μm). Standard turning sits at Ra 63 μin (N7); fine turning reaches Ra 32 μin, and a finishing pass under ideal conditions can reach Ra 16 μin. Rough turning on heavy stock tops out around Ra 250 μin — lathes rarely need the 500 μin band that milling hogging does.
Grinding and honing — Ra 1–32 μin (0.025–0.8 μm, N1–N6). Secondary operations after machining. Specify only when function requires it.
Laser cutting — Cut-edge Ra 32–250 μin (0.8–6.3 μm). Ra 32 μin is a best case on thin sheet; most cut edges fall at Ra 63–250 μin. Depends on sheet thickness, assist gas, and feed rate — verify on a sample coupon when the edge is a sealing or cosmetic surface.
Pair this chart with the ISO 2768 Tolerance Chart for general dimensions and the CNC Design Guide for DFM on holes, threads, and finishes.
How to specify surface finish on a drawing
Use the surface texture symbol (check mark with value). Write the Ra value in microinches or micrometers next to it. Place the symbol on the specific face — not as a blanket note on the entire part.
Only call out finishes tighter than standard (Ra 3.2 μm / 125 μin) when sealing, sliding, or cosmetic requirements demand it. For threaded holes and drill sizes, cross-check the Tap & Drill Size Chart and Drill Bit Size Chart.
Popular surface finish chart searches
Direct answers to the roughness lookups engineers and machinists search most often:
- What is Ra 32 surface finish?
- Ra 32 μin = 0.8 μm = N6. Typical fine CNC turning or milling with sharp tools and light cuts. Cut marks usually not visible to the naked eye.
- What is Ra 63 surface finish?
- Ra 63 μin = 1.6 μm = N7. Standard fine CNC machining finish — common for shafts, bores, and general machined faces.
- What is Ra 125 surface finish?
- Ra 125 μin = 3.2 μm = N8. Default as-machined CNC finish at many shops. Visible tool marks; suitable for non-critical structural surfaces.
- N6 surface finish in micrometers
- N6 = Ra 0.8 μm = 32 μin. Fine CNC milling or fine turning. A laser-cut edge reaches it only in the best case (thin sheet, tuned parameters).
- N7 vs N8 surface finish
- N7 = Ra 1.6 μm (63 μin) — fine machined. N8 = Ra 3.2 μm (125 μin) — standard as-machined. N7 is roughly twice as smooth as N8.
- Ra to Rz conversion
- Ra and Rz measure different profile properties — there is no exact conversion. Shop rule of thumb: Rz ≈ 4–7 × Ra depending on process. This chart uses Rz ≈ Ra × 4 (μin) as a reference approximation.
- Ra to RMS conversion
- RMS ≈ Ra × 1.11 for typical machined surfaces. Example: Ra 63 μin → RMS ≈ 69.9 μin.
- What surface finish can CNC milling achieve?
- Standard CNC milling: Ra 63–125 μin (N7–N8). Fine milling with sharp end mills and light stepover: Ra 32 μin (N6). Ra 16 μin (N5) is possible with a finishing pass under ideal conditions; anything smoother, or N5 held consistently in production, usually calls for grinding or polishing.
- Best surface finish for O-ring groove
- Per the Parker O-Ring Handbook: static sealing surfaces Ra 32 μin (0.8 μm, N6) maximum, and 16 μin (0.4 μm) maximum for gas or vacuum face seals. Dynamic sealing surfaces (rod and bore) Ra 8–16 μin (0.2–0.4 μm). The groove itself is less critical than the surface the O-ring seals against.
- ISO 1302 N-grade chart
- N1 (Ra 0.025 μm) through N12 (Ra 50 μm) — each ISO 1302 step doubles Ra. This chart lists N1–N10 across grinding, milling, turning, and laser cutting rows — use All or filter by process.
How to use this surface finish chart
- PREMSA publishes this surface finish chart with Ra in microinches and micrometers, approximate Rz and RMS, and ISO 1302 N-grades — separate rows per process (milling, turning, grinding, laser cutting), each sorted ascending without repeated Ra steps.
- Ra is the arithmetic average roughness — the default callout on most engineering drawings. RMS ≈ Ra × 1.11. Rz cannot be converted exactly from Ra; this chart uses Rz ≈ Ra × 4 (μin) as a shop approximation for quick cross-reference.
- Standard as-machined CNC at PREMSA is typically Ra 3.2 μm (125 μin, N8) unless your drawing specifies tighter. Ra 1.6 μm (63 μin, N7) is achievable on critical faces, and Ra 0.8 μm (32 μin, N6) on selected faces depending on material and geometry — tell us on the drawing and we confirm it in the quote.
- Use the converter above to translate Ra μin ↔ μm, estimate Rz/RMS, or look up N-grades. Filter the table by process to see what finish milling, turning, grinding, or laser cutting typically delivers.
- Values are nominal reference data based on the ISO 1302 N-grade steps and common shop practice. Verify with a profilometer on production parts. For CNC quotes with specific Ra requirements in Monterrey, request a quote with your STEP file and drawing callouts.
Related Resources
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ISO 2768 Tolerance Chart
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ISO 286 Fit & Tolerances Chart
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CNC Design Guide for Machining
DFM rules for holes, threads, tolerances, materials, and surface finishes.
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Tap & Drill Size Chart
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Drill Bit Size Chart
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Frequently asked questions about surface finish
- What is Ra in surface finish?
- Ra is the arithmetic average roughness — the mean height of peaks and valleys on a surface profile. Lower Ra = smoother finish. It is the most common surface texture callout on engineering drawings worldwide.
- What is the difference between Ra and Rz?
- Ra averages all profile heights. Rz measures peak-to-valley height over a sampling length and is more sensitive to isolated deep scratches. They cannot be converted exactly — Rz is typically 4–7× Ra depending on the machining process.
- What is the difference between Ra and RMS?
- Ra is arithmetic average roughness. RMS (root mean square) gives more weight to large peaks. For typical machined surfaces, RMS ≈ Ra × 1.11. Older US drawings may specify RMS instead of Ra.
- What surface finish can CNC machining achieve?
- Standard CNC milling and turning: Ra 63–125 μin (1.6–3.2 μm). Fine machining with sharp tools: Ra 32 μin (0.8 μm), and down to about Ra 16 μin (0.4 μm) under ideal conditions. Smoother finishes require grinding, honing, or lapping as secondary operations.
- What are N-grade surface finish numbers?
- N-grades are the roughness grade numbers of ISO 1302:1992 (no longer in the current standards) from N1 (smoothest, Ra 0.025 μm) to N12 (roughest, Ra 50 μm). Example: N6 = Ra 0.8 μm (32 μin), N7 = Ra 1.6 μm (63 μin), N8 = Ra 3.2 μm (125 μin).
- How do I specify surface finish on a drawing?
- Use the surface texture symbol and write the Ra value in μin or μm next to it. Place it on the specific face. ASME Y14.36 prefers explicit Ra — not N-grade alone. Only tighten beyond Ra 3.2 μm when function requires it.
- What is a good surface finish for bearing fits?
- Press-fit and bearing journal surfaces typically need Ra 16–63 μin (0.4–1.6 μm, N5–N7) depending on load and lubrication. Verify with your bearing supplier's specification.
- Does tighter surface finish cost more?
- Yes. Finishes below Ra 1.6 μm (63 μin) often require slower feeds, lighter finishing passes, sharper tools, or a secondary operation — which can add significant cycle time. Specify the loosest Ra that meets function.
Ra is the arithmetic average roughness — the mean height of peaks and valleys on a surface profile. Lower Ra = smoother finish. It is the most common surface texture callout on engineering drawings worldwide.
Ra averages all profile heights. Rz measures peak-to-valley height over a sampling length and is more sensitive to isolated deep scratches. They cannot be converted exactly — Rz is typically 4–7× Ra depending on the machining process.
Ra is arithmetic average roughness. RMS (root mean square) gives more weight to large peaks. For typical machined surfaces, RMS ≈ Ra × 1.11. Older US drawings may specify RMS instead of Ra.
Standard CNC milling and turning: Ra 63–125 μin (1.6–3.2 μm). Fine machining with sharp tools: Ra 32 μin (0.8 μm), and down to about Ra 16 μin (0.4 μm) under ideal conditions. Smoother finishes require grinding, honing, or lapping as secondary operations.
N-grades are the roughness grade numbers of ISO 1302:1992 (no longer in the current standards) from N1 (smoothest, Ra 0.025 μm) to N12 (roughest, Ra 50 μm). Example: N6 = Ra 0.8 μm (32 μin), N7 = Ra 1.6 μm (63 μin), N8 = Ra 3.2 μm (125 μin).
Use the surface texture symbol and write the Ra value in μin or μm next to it. Place it on the specific face. ASME Y14.36 prefers explicit Ra — not N-grade alone. Only tighten beyond Ra 3.2 μm when function requires it.
Press-fit and bearing journal surfaces typically need Ra 16–63 μin (0.4–1.6 μm, N5–N7) depending on load and lubrication. Verify with your bearing supplier's specification.
Yes. Finishes below Ra 1.6 μm (63 μin) often require slower feeds, lighter finishing passes, sharper tools, or a secondary operation — which can add significant cycle time. Specify the loosest Ra that meets function.
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