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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.