Surface Roughness Conversion Chart of Stainless Steel Pipe

Ra = Roughness Average in micro-meters & micro-inches

China's old standard (finish)	China New standard (roughness) Ra	American Standard(microns), Ra	American Standard (Micro-inch), Ra
▽ 4	6.3	8.00	320
▽ 4	6.3	6.30	250
▽ 5	3.2	5.00	200
		4.00	160
		3.20	125
▽ 6	1.6	2.50	100
		2.00	80
		1.60	63
▽ 7	0.8	1.25	50
		1.00	40
		0.80	32
▽ 8	0.4	0.63	25
		0.50	20
		0.40	16

China Old Grade	China New Ra	China New Rz	USA micron Ra	USA microinch Ra
▽ 1	50	200
▽ 2	25	100
▽ 3	12.5	50
▽ 4	6.3	25	8.00	320
▽ 4	6.3	25	6.30	250
▽ 5	3.2	12.5	5.00	200
			4.00	160
			3.20	125
▽ 6	1.6	6.3	2.50	100
			2.00	80
			1.60	63
▽ 7	0.8	6.3	1.25	50
			1.00	40
			0.80	32
▽ 8	0.4	3.2	0.63	25
			0.50	20
			0.40	16
▽ 9	0.2	1.6	0.20	12.5
				10
				8
▽ 10	0.1	0.8	0.10

RMS (microinch)	RMS (µm)	Ra (microinch)	Ra (µm)	Grit Size
80 58 47 34 17 14	2.03 1.47 1.2 0.6 0.43 0.36	71 52 42 30 15 12	1.32-1.90 0.46-0.64 0.3-0.4 0.18-0.38 ≤0.15 ≤0.08	80 120 150 180 240 320 400

Machining surface finish chart: Ra vs RMS

USA Ra(µm)	USA Ra (Micro inch)	USA RMS (Micro inch)	Machining Finish Method
50.0	2000	2200	The most coarse machining or good rough casting surfaces
25.0	1000	1100	Machining marks very obvious. Rough turning, boring, planning, drilling
12.5	500	550	Machining marks obvious. Normal turning, boring, planning, drilling, grinding
8.00	320	352	Machining marks visible. Normal turning, boring, planning, drilling , grinding
6.30	250	275	Machining marks visible. Normal turning, boring, planning, drilling , grinding
5.00	200	220	Machining marks not obvious. But still visible. Normal turning, boring, planning, drilling, grinding.
4.00	160	176	Machining marks not obvious. But still visible. Normal turning, boring, planning, drilling, grinding.
3.20	125	137.5	Machining marks not obvious. But still visible. Normal turning, boring, planning, drilling, grinding.
2.50	100	110	Machining marks blur, but direction obvious. Number controlled turning, boring, planning, drilling, grinding.
2.00	80	88	Machining marks blur, but direction obvious. Number controlled turning, boring, planning, drilling, grinding.
1.60	63	69.3	Machining marks blur, but direction obvious. Number controlled turning, boring, planning, drilling, grinding.
1.25	50	55	Machining marks direction blur, but still visible. Number controlled turning, boring, planing, drilling, grinding.
1.00	40	44	Machining marks direction blur, but still visible. Number controlled turning, boring, planing, drilling, grinding.
0.80	32	35.2	Machining marks direction blur, but still visible. Number controlled turning, boring, planing, drilling, grinding.
0.63	25	27.5	Machining marks direction blur. Reaming, grinding, boring, rolling.
0.50	20	22	Machining marks direction blur. Reaming, grinding, boring, rolling.
0.40	16	17.6	Machining marks direction blur. Reaming, grinding, boring, rolling.
0.20	12.5	13.75	Machining marks direction invisible. Grinding, super machining
0.20	10	11	Machining marks direction invisible. Grinding, super machining
0.20	8	8.8	Machining marks direction invisible. Grinding, super machining
0.10	4	4.4	Surface dark gloss. Surper machining

Ra is the arithmetic average of surface heights measured across a surface. Simply average the height across the microscopic peaks and valleys.

Ra and RMS are both representations of surface roughness, but each is calculated differently.

Ra is calculated as the Roughness Average of a surfaces measured microscopic peaks and valleys.

RMS is calculated as the Root Mean Square of a surfaces measured microscopic peaks and valleys. Each value uses the same individual height measurements of the surfaces peaks and valleys, but uses the measurements in a different formula.
Related References:
1. ISO 1302 DIN 4768 Comparison of Surface Roughness Values
2. Electrical Discharge Machining EDM Roughness Comparator
3. Polishing Polished Seamless Stainless Steel Pipe Tube
4. Stainless Steel Tubes Surface Roughness Review
5. Stainless Steel Bright Annealing/Annealed Tubes
6. China USA Surface Roughness Standard Comparison
7. Surface Roughness Conversion Chart of Stainless Steel Tube
8. Stainless Steel Tube Surface Finish Types
9. Cleaning of Stainless Steel Tubes
10. Post weld cleaning and finishing of stainless steels
11. Care and maintenance of stainless steel
12. British and American standards for tolerances, surface finish and testing of stainless steel
13. Surface Finish Degree Comparison Table between Rz Ra RMS
14. Surface Roughness Table