Ra vs AARH: Surface Roughness Conversion

Quick Answer

Ra and AARH measure the same surface roughness parameter; the arithmetic average of surface deviations from the mean line. Ra is expressed in micrometers (um), while AARH is expressed in microinches (uin). The conversion is: 1 um Ra = 40 AARH (microinches). The standard flange finish of 125-250 AARH equals 3.2-6.3 um Ra.

Ra vs AARH confusion is common because ASME and ISO standards use different units for the same measurement. ASME B16.5 specifies flange surface finish in AARH (microinches), while European standards (EN 1092-1) and ISO standards use Ra (micrometers). Both are arithmetically identical measurements expressed in different unit systems.

Ra and AARH Definitions

Parameter	Ra	AARH
Full name	Roughness Average	Arithmetic Average Roughness Height
Unit	Micrometers (um)	Microinches (uin)
Standard	ISO 4287, ISO 1302	ASME B46.1
Measurement	Arithmetic average of absolute deviations	Arithmetic average of absolute deviations
Identical to	CLA (Centre Line Average)	AA (Arithmetic Average)

Ra and AARH are mathematically the same parameter. The only difference is the unit of measurement. CLA (Centre Line Average), used in older British standards, is also the same measurement.

Conversion Table

AARH (microinches)	Ra (micrometers)	Typical Application
16	0.4	Precision instrument fittings
32	0.8	Lens ring seats, high-precision seals
63	1.6	RTJ flange grooves, tongue & groove
125	3.2	RF/FF flange face (lower limit)
250	6.3	RF/FF flange face (upper limit)
500	12.5	MSS SP-6 maximum for standard flanges
1000	25.0	Rough machined surfaces

Conversion formula: Ra (um) = AARH (uin) / 40, or equivalently, Ra (um) = AARH (uin) x 0.025.

Ra vs AARH vs RMS

RMS (Root Mean Square) is a different roughness parameter sometimes confused with Ra/AARH. RMS gives a slightly higher value than Ra for the same surface because it squares the deviations before averaging:

Surface	AARH (uin)	Ra (um)	RMS (uin)	RMS (um)
RTJ groove	63	1.6	70	1.8
RF flange (min)	125	3.2	139	3.5
RF flange (max)	250	6.3	278	7.0

The approximate relationship is: RMS = 1.11 x Ra (AARH). When a specification calls for AARH or Ra, do not substitute an RMS reading without applying the correction factor.

Practical Application for Flanges

For ASME B16.5 flanges, always specify roughness in AARH (microinches) since this is the native unit of the standard. For EN 1092-1 flanges, use Ra (micrometers). When purchasing flanges from international suppliers, confirm which unit the inspection report uses and convert if necessary.

Standard	Native Unit	RF/FF Requirement
ASME B16.5	AARH (uin)	125-250 AARH
ASME B16.47	AARH (uin)	125-250 AARH
EN 1092-1	Ra (um)	3.2-6.3 Ra
API 6A	AARH (uin)	63 max (RTJ)

Pro Tip

When reviewing mill test certificates from non-US manufacturers, the surface roughness is often reported in Ra (um). To verify ASME compliance, multiply the Ra value by 40 to convert to AARH. A certificate showing Ra 5.0 um equals 200 AARH, which is within the 125-250 AARH specification for RF flanges.

Read the full guide to flange face finishes