Surface Finish Specifications
The following surface comparison chart has been compiled for your convenience from numerous resources. This allows easy conversion between various systems. No guarantees are made of its accuracy and it is intended to be non-authoritative. Note that variations of up to 25% may exist in measurements.
| Description | SPI Designation |
Ra Micro-inches |
Ra Micro-meters |
ISO 1302 Roughness grade |
| #1 Diamond Buff | A-1 | 0.5-1 | 0.012 - 0.025 | |
| 1 | 0.025 | N1 | ||
| #2 Diamond Buff | A-2 | 1-2 | 0.025 - 0.050 | |
| 2 | 0.050 | N2 | ||
| #3 Diamond Buff | A-3 | 2-3 | 0.050 - 0.075 | |
| 600 Grit Paper | B-1 | 2-3 | 0.050 - 0.075 | |
| 4 | 0.100 | N3 | ||
| 400 Grit Paper | B-2 | 4-5 | 0.100 - 0.125 | |
| 8 | 0.200 | N4 | ||
| 320 Grit Paper | B-3 | 9-10 | 0.225 - 0.250 | |
| 600 Stone | C-1 | 10-12 | 0.250 - 0.300 | |
| 16 | 0.400 | N5 | ||
| 400 Stone | C-2 | 25-28 | 0.625 - 0.700 | |
| 32 | 0.800 | N6 | ||
| 320 Stone | C-3 | 38-42 | 0.950 - 1.050 | |
| #11 Glass Bead | D-1 | 10-12 | 0.250 - 0.300 | |
| 240 Aluminum Oxide Blast | D-2 | 26-32 | 0.650 - 0.800 | |
| 63 | 1.600 | N7 | ||
| 125 | 3.200 | N8 | ||
| #24 Aluminum Oxide Blast | D-3 | 190-230 | 4.750 - 5.750 | |
| 250 | 6.300 | N9 | ||
| 500 | 12.500 | N10 | ||
| 1000 | 25.000 | N11 | ||
| 2000 | 50.000 | N12 |
Ra = Roughness average, measured in either Micro-inches or Micro-meters
Rrms = Roughness root mean square, measured in either Micro-inches or Micro-meters.
Rrms = approximately 1.1 times Ra.
Rmax = Roughness maximum, measured in either Micro-inches or Micro-meters.
Rmax = 4 times Ra.
