Surface Finish Ra Values: 125, 63, 32, 16, 8

Surface Finish Ra Values Explained: 125, 63, 32, 16 and 8

Surface finish Ra values are numbers that tell you how rough or smooth a machined surface is, measured as the average height of its tiny peaks and valleys. A lower number means a smoother surface, so an 8 finish is far smoother than a 125 finish. This guide breaks down each common value, what it is used for, how it is made, and what it costs.

You will find a quick benchmark table first, then a plain-English section for every value engineers call out most: 125, 63, 32, 16, and 8 microinches, plus their metric twins of 3.2, 1.6, and 0.8 micrometers. Every value here matches the ASME B46.1 and ISO 21920 standards.

What Do Surface Finish Ra Values Mean?

An Ra value is the average roughness of a surface, written in either microinches or micrometers. It measures the average distance between the high points and low points across the surface. The smaller the number, the smoother and more polished the part.

Most North American drawings use microinches, so you will see callouts like 125 or 32. Most other countries use micrometers, so you will see 3.2 or 0.8. They describe the same thing in different units.

Is a Lower Ra Number Smoother or Rougher?

A lower Ra number is smoother. The value measures roughness, so less roughness equals a smaller number. An 8 finish is glassy, a 125 finish has visible tool marks, and a 250 finish feels coarse to the touch.

Common Surface Finish Ra Values Explained

Here is what each common Ra value means in practice. We cover what it looks like, where it is used, how it is made, and how it affects cost, so you can match the number to the job.

250 Microinch (6.3 µm) Finish

A 250 finish is rough. It has clear, coarse tool marks you can feel with a fingernail. Use it on surfaces that never touch another part, like the inside of a clearance pocket or a rough housing wall. It is the cheapest machined finish because the tool moves fast with deep cuts.

125 Microinch (3.2 µm) Finish

A 125 machine finish is the standard "as machined" surface, equal to 3.2 micrometers or N8. Most CNC milling and turning produces this by default with no extra steps. The surface looks even with light, regular tool marks. Use 125 ra on general parts, structural brackets, and any face that does not seal, slide, or show. This is the best default for cost, which is why a ra 125 surface finish appears on so many drawings.

63 Microinch (1.6 µm) Finish

A 63 surface finish is noticeably finer than 125, with only faint tool marks. It works well for mating surfaces, tight-fit assemblies, and gear teeth where parts touch but do not seal under pressure. A 63 ra surface finish needs sharper tooling and slower feeds, so it costs a bit more than the standard finish. It is a common step up when 125 is not quite smooth enough.

32 Microinch (0.8 µm) Finish

A 32 machine finish is smooth, with no tool marks you can see at arm's length. It is the go-to for O-ring grooves, press fits, and high-pressure fluid seals, because a smooth surface keeps seals from leaking or wearing fast. A 32 surface finish usually needs precision turning or grinding, so it costs more than a 63 finish. Many engineers treat 32 as the line between "normal machining" and "precision work."

16 Microinch (0.4 µm) Finish

A 16 ra surface finish is very smooth and starts to look reflective. It suits high-load bearings, hydraulic valve seats, and aerospace seal surfaces where friction and wear must stay low. Hitting 16 usually takes fine grinding or honing after machining. The extra steps add real cost, so reserve it for parts that truly need it.

8 Microinch (0.2 µm) Finish

An 8 surface finish is polished and reflects light like a mirror. It is used on high-precision seals, fine bearings, and sliding parts that must move with almost no friction. Reaching an 8 finish takes honing or fine lapping, which is slow and skilled work. This finish carries a high cost, so it shows up only on critical surfaces. For even finer work, a 2 micro inch finish requires superfinishing or lapping and is rare outside optical and gauge work.

What Is the Standard Machined Surface Finish?

The standard machined surface finish is 3.2 micrometers Ra, which equals 125 microinches or N8. This is the finish a CNC mill or lathe produces by default, with no extra finishing passes. It balances good appearance with low cost, which is why it is the most common default on engineering drawings.

If a drawing does not list a finish at all, most shops will produce something close to 125. When you need smoother, you must call it out, because every step finer adds time and cost. You can see how finishes line up across units in our surface finish conversion chart.

Metric Ra Values: 3.2, 1.6 and 0.8 Micrometers

Metric drawings use micrometers instead of microinches, but they describe the exact same surfaces. Knowing the pairs keeps you from misreading a global drawing.

A 3.2 surface finish (or ra 3.2 surface finish) equals 125 microinches, the standard machined finish. A surface roughness 3.2 callout means the same thing. A 1.6 surface finish equals 63 microinches, the fine mating-surface finish. And 0.8 micrometers equals 32 microinches, the smooth sealing finish.

How Ra Value Affects Part Cost

Surface finish is one of the biggest hidden cost drivers in machining. The relationship is not a straight line. Each step toward a smoother finish can add far more cost than the step before it.

Going from 125 to 63 might add a small amount because it only needs sharper tools and slower feeds. But going below 32 often means adding a whole second operation, like grinding or polishing, which can raise the per-part price sharply. The table below shows the general pattern.

The most common mistake we see is over-specifying. Engineers copy a tight finish like 32 onto every surface, even faces that never touch another part. That forces extra finishing passes for no real benefit. Our engineering team flags this in the DFM review on every quote, which typically saves 15 to 30 percent. To see how finish fits into total part price, read our guide on how much it costs to get a metal part made, or explore our surface finishing services.

How to Choose the Right Ra Value: The Three Questions

Picking the right Ra value does not have to be guesswork. Before you write a finish callout, our team runs through three quick questions about what the surface actually does.

• Does it seal? Sealing surfaces like O-ring grooves need 32 or smoother to stop leaks.
• Does it move? Sliding or rotating surfaces like bearings need 32 to 16 to cut friction and wear.
• Does it show? Visible cosmetic faces may need a finer finish for looks, even if function does not require it.

If the answer to all three is no, use 125 (3.2 µm) and save the money. Here is a real example. A customer sent us a bracket drawing with a 32 microinch finish called out on every face, including six that bolt out of sight inside a housing. Only one bore actually sealed an O-ring.

We recommended keeping 32 on the sealing bore, dropping the bolt faces to 125, and leaving hidden faces rough. The change cut machining time per part without affecting how the part worked, and it passed first-article inspection on a calibrated profilometer. That is the kind of finish tuning built into our CNC machining quotes. For a deeper look at every roughness unit and symbol, see our full surface finish chart guide, and for textured finishes, our guide to sandblasted and bead blasted finishes.