Quick Answer
Ra (Roughness Average) is the arithmetic average of the absolute values of a surface's microscopic peaks and valleys, measured from the mean line over a defined evaluation length. It is the most widely used parameter for specifying surface finish.
- Defined formally in ASME B46.1 and ISO 4287:1997.
- Interchangeable with the CLA (Center Line Average) parameter.
- Measured in micrometers (µm) or microinches (µin).
- One micrometer equals roughly 40 microinches.
- A lower Ra means a smoother surface.
- The ASME BPE standard sets Ra ≤ 15 µin (0.38 µm) for bio-pharma applications (SF4).
On this page: Ra definition and formula, conversion calculator, full Ra/RMS/Rz chart, how Ra is measured, common Ra values explained, and industry-specific finish requirements.
What Is Surface Roughness, and Why Does It Matter?
Surface roughness is an often-overlooked dimensional aspect of the mechanical design process. While more focus is generally given to the material selection, mechanical properties, or the measured dimensions and tolerances of a part, a surface that is too rough can result in increased friction and premature failure.
Going beyond mechanical operation, high purity manufacturing requires smooth surfaces within the processing equipment to avoid contamination or build-up within it. Simply put, the smoother a surface is, the less likely it is that material will stick to it.
And of course, the smoother the surface, the easier it is to clean.
In our work passivating and electropolishing process equipment for the pharmaceutical and aerospace industries, we routinely see vessels that meet every dimensional spec on the drawing but fail purity audits because the interior Ra was never properly specified or verified.
Ra Definition and Formula
Formally, ASME B46.1 defines Ra as:
"The arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the evaluation length."
In plain terms: a stylus traces the surface, records every peak and valley as a deviation from a calculated mean line, takes the absolute value of each deviation so peaks and valleys do not cancel out, and averages them.
They do not take into consideration other components of the surface topography such as flaws, errors of form, or waviness that would be measured in a 3D evaluation. Two dimensional roughness measurements should be taken across any grain or directionality that may exist on the metal surface.
For those who are fond of math, the formula to calculate the Ra over a measured length is:
AVERAGE ROUGHNESS (Ra) FORMULA
Where L is the evaluation length and Z(x) is the height of the profile at position x.
Figure 1-4.1.1-3 from ASME B46.1 is provided below to help visualize the formula for Ra.
To assist with practical applications, use the conversion calculator below to convert Ra values between micrometers (µm) and microinches (µin):
SURFACE ROUGHNESS (Ra) CONVERSION CALCULATOR
Use the calculator below to convert Ra values between micrometers (µm) and microinches (µin):
- Enter the Ra value received
- Select the conversion direction (µm to µin or µin to µm)
- Click “Convert” to get the result
(Ra) Conversion Calculator
Ra vs RMS: What's the Difference?
Both Ra and RMS describe surface roughness using the same peak and valley measurements, but they use different formulas.
Because RMS squares the values first, a single large peak or deep valley pulls the RMS value higher than the Ra value. RMS is more sensitive to anomalies. For most machined surfaces, RMS is approximately 11% higher than Ra (RMS ≈ Ra × 1.11).
RMS is largely a legacy parameter in the U.S., still appearing on older drawings and in some sanitary specs. Ra is the global standard today under both ASME B46.1 and ISO 4287. Many U.S. drawings show both values: a metric Ra value with the equivalent microinch value in parentheses, like 0.8 (32).
Other Roughness Parameters You'll See
| Parameter | What It Measures | When It Matters |
|---|---|---|
| Ra | Arithmetic average of profile deviations | General-purpose roughness, most common |
| Rq (RMS) | Root mean square of deviations | Original preferred metric, replaced with Ra |
| Rz | Average peak-to-valley height over 5 samples | Sealing surfaces, O-ring seats |
| Rt | Total distance, highest peak to lowest valley | Detecting single critical defects |
| Rp | Height of the highest peak | Bearing surface integrity |
| Rv | Depth of the deepest valley | Crack/defect detection |
| Sa, Sq, Sz | 3D equivalents over an area, not a line | High-performance optics, advanced sealing |
Ra alone is a useful general indicator, but two surfaces with the same Ra can perform very differently. A surface with a single deep valley and a surface with uniformly small peaks can have identical Ra values and very different sealing or wear behavior. That is why parameters like Rz and Rt matter for critical applications.
How Surface Roughness Is Measured
Ra is measured with a profilometer, also called a surface roughness tester or surface texture gauge. The most common type uses a stylus, a diamond-tipped probe that traces across the surface and records the vertical deviations along its path. The instrument then calculates Ra over the evaluation length.
Two Categories of Measurement
Best Practices for Taking a Measurement
- Trace across the grain or lay direction, not along it.
- Take multiple measurements at different locations and record them.
- Use a cutoff length appropriate to the surface (commonly 0.8 mm for general Ra work; ASME B46.1 specifies the standard cutoffs).
Why Ra Matters for Cleanability
As mentioned earlier, a smooth surface makes it harder for residual product within the system to stick to the sides of a vessel or piping. Similarly, should free iron or other unwanted material be introduced into the system, there is less likelihood that it will become embedded into the metal and become a source of contamination. With high-purity processes, any contamination can spoil an entire batch of products.
While the cost to clean and purge a system can quickly add up, there is also the cost of the lost production time to consider as well. Overall, the lower the Ra, the higher-purity production application of the vessel. Not only is it easier to clean, but a smoother finish means that it is easier to empty product.
The savings in time between batches can increase the throughput of product, which benefits the bottom line.
For our pharmaceutical and biotech clients, the cost difference between specifying Ra ≤ 20 µin (0.5 µm) and Ra ≤ 15 µin (0.38 µm) is small compared to the cost of a single contaminated batch or a failed FDA audit.
Surface Finish Conversion Chart: Ra, RMS, Rz, and Typical Processes
Use this surface finish conversion table to convert between Ra (µin), Ra (µm), RMS, Rz (peak-to-valley), and the typical manufacturing process or abrasive grit that produces each finish.
| Ra (µin) | Ra (µm) | RMS (µin) | Rz (µm) | Grit # | Typical Process / Application |
|---|---|---|---|---|---|
| 2000 | 50 | 2200 | 200 | — | Hot-rolled, sand-cast surfaces |
| 1000 | 25 | 1100 | 100 | — | Rough machining, flame cutting |
| 500 | 12.5 | 550 | 50 | — | Heavy turning, milling |
| 250 | 6.30 | 275 | 25 | 60 | Standard machining, #1 finish (HRAP) |
| 125 | 3.20 | 137.5 | 13 | — | "As-machined" CNC default |
| 71 | 1.80 | 78 | 7 | 80 | — |
| 63 | 1.60 | 64.3 | 6.3 | — | Fine machining |
| 52 | 1.32 | 58 | 5 | 120 | — |
| 42 | 1.06 | 46 | 4 | 150 | — |
| 19 – 32 | 0.48 – 0.80 | 21 – 33 | 1.9 – 3.2 | 180 – 220 | #4 brushed, dairy/sanitary range |
| 16 | 0.40 | 17.6 | 1.6 | — | Honing, lapping, fine polishing |
| 15 | 0.38 | 17 | 1.5 | 240 | ASME BPE SF4 (bio-pharma EP) |
| 12 | 0.30 | 14 | 1.2 | 320 | — |
| 9 | 0.23 | 10 | 0.9 | 400 | Fine lap, mirror prep |
| 8 | 0.20 | 8.8 | 0.8 | — | — |
| 4 | 0.10 | 4.4 | 0.4 | 500 | — |
| 2 | 0.05 | 2.2 | 0.2 | — | Optical / mirror finish |
| 1 | 0.025 | 1.1 | 0.1 | — | #8 buffed mirror, lapped |
- Ra: Arithmetic average roughness, the most common surface parameter
- RMS (Rq): Root mean square roughness, more sensitive to large peaks
- Rz: Average peak-to-valley height over multiple sampling lengths, critical for sealing surfaces
- Grit #: Abrasive particle size, higher equals finer
Surface Roughness vs Surface Finish
It should be noted that Surface Roughness differs from Surface Finish. The term “Finish” is used to describe the visual appearance of a stainless plate or sheet and can be highly subjective. Surface Roughness is objectively measured with calibrated equipment.
Stainless steel with a 2B mill finish is a bright, relatively defect-free finish produced by a final cold roll pass using polished rollers. It does not show any grain and has been compared to a “cloudy mirror” in terms of appearance. Any defects pressed into the surface will be revealed if the surface is electropolished.
Since thinner metal is passed through the rollers more times than thicker sheets, the thinner metal usually has a lower Ra and a more uniform finish. Therefore, the same surface finish callout on different product forms can result in different values of surface roughness.
Surface Finish Designations on Stainless Steel
The term "finish" describes the visual and tactile character of a surface and can be subjective. Surface roughness is the objective, measurable component of finish.
| Designation | Description | Typical Ra |
|---|---|---|
| #1 (HRAP) | Hot Rolled, Annealed, Pickled, as-from-mill plate | 125 to 500 µin (3.2 to 12.5 µm) |
| 2B Mill | Cold-rolled bright finish, "cloudy mirror" | 12 to 40 µin (0.3 to 1.0 µm) |
| 2D | Cold-rolled, annealed, pickled, matte | 16 to 40 µin (0.4 to 1.0 µm) |
| #3 | Coarse brushed | 40 to 60 µin (1.0 to 1.5 µm) |
| #4 (Standard) | Brushed, most common architectural finish | 40 to 50 µin (1.0 to 1.3 µm) |
| #4 Dairy/Sanitary | Brushed, finer abrasive | 32 µin (~0.8 µm) |
| #6 | Soft satin, fine-grit brushed | 20 to 40 µin (0.5 to 1.0 µm) |
| #7 | Buffed, near-mirror | 4 to 12 µin (0.1 to 0.3 µm) |
| #8 | Buffed mirror finish | 1 µin (~0.025 µm) |
For bio-pharmaceutical applications including injectables and otic solutions, the typical specification is Ra ≤ 15 µin (0.38 µm) with an electropolished surface, defined in the ASME BPE standard as Surface Designation SF4.
Powder and tablet manufacturing often uses a slightly rougher surface around 20 µin (0.5 µm), specified under BPE’s SF1 Surface Designation, which does not require electropolishing.
Common Ra Values, Explained
These are the Ra callouts engineers and machinists encounter most often.
How to Reduce Surface Roughness
Once a vessel or part is fabricated, Ra can be improved through finishing processes. The two main approaches are mechanical and electrochemical.
The surface finish of a vessel, as well as its Ra, determines what product can be reliably produced within it, and as stated above, increasing levels of purity require increasingly finer surface finishes with lower Ra numbers. Each industry has specific finish standards that must be met. Sanitary food grade finishes generally fall in the 20 µin (0.5 µm) to 32 µin (0.8 µm) range. This range reduces places where bacteria or other contaminants can gain a foothold.
Mechanical polishing uses progressively finer abrasives (belts, pads, compounds) to physically remove the high peaks of the surface profile. It is the right choice for surfaces with visible damage, weld scale, or descaling requirements, heavy impact damage or chloride micropitting, and reducing Ra from a rough starting point before electropolishing.
Mechanical polishing alone has limits. The abrasive process can embed dust and fine metal particles into the surface.
Electropolishing uses a controlled electrochemical reaction to dissolve the high points of the surface profile. Typically, only 5 to 10 µm of material is removed, mostly from the peaks, making the valleys appear shallower by comparison.
Electropolishing can reduce starting Ra by up to 50%, depending on the base material and starting condition. It also removes embedded abrasive debris and metal fines left by mechanical polishing, which is why mechanical polishing plus electropolishing is the standard one-two punch for bio-pharmaceutical and semiconductor finishes.
Electropolishing is not a fix for heavy damage. A scratched or pitted surface will be a shinier scratched or pitted surface after EP. Always address mechanical damage first with mechanical polishing.
Need Help Specifying or Achieving a Surface Finish?
Astro Pak’s engineering team works with manufacturers across aerospace, pharmaceutical, semiconductor, and high-purity industries to spec, achieve, and verify surface finishes that meet the most demanding industry standards. From precision cleaning to electropolishing to passivation, we deliver the surfaces high-purity production requires.
Frequently Asked Questions
Ra stands for Roughness Average (sometimes called Arithmetic Average Roughness). It is the average of the absolute values of the surface profile’s height deviations from the mean line, measured over a defined evaluation length.
Ra is measured in micrometers (µm) internationally and microinches (µin) primarily in the United States. One micrometer equals about 40 microinches. The unit is typically written next to the Ra callout on a drawing.
There is no universal “good” Ra value, only an appropriate value for the application. Common benchmarks: 125 µin (3.2 µm) is the standard as-machined finish, 32 µin (0.8 µm) is the sanitary minimum, and 15 µin (0.38 µm) is the finest surface finish designation defined in the ASME BPE standard.
Surface roughness (Ra) is an objective, measured parameter. Surface finish is a broader, often subjective term that includes appearance, lay direction, waviness, and roughness together. Two surfaces can have the same Ra and look completely different. Conversely, two surfaces can have the same surface finish, but different Ra values
Multiply Ra by approximately 1.11. For a typical 32 µin Ra surface, RMS is roughly 35 µin. This is an approximation; the exact ratio depends on the surface profile.
Rz (average peak-to-valley height) is the average of the individual Rt values from each sampling length within the full evaluation length. Rz is typically 4 to 7 times Ra, depending on how uniform the surface is. Rz is more sensitive than Ra to occasional deep valleys, which is why it is preferred for sealing applications.
Electropolishing typically reduces the starting Ra by 10 to 50%. For high-purity applications, electropolishing is used to reach ASME BPE SF4 (Ra ≤ 15 µin / 0.38 µm).
ASME B46.1 is the American Society of Mechanical Engineers standard that defines surface texture parameters, measurement methods, and reporting conventions in the United States. It is the formal source for the Ra definition used throughout this article.
