Surface Finish Chart: 2026 Conversion Guide for Manufacturers

A surface finish chart is a reference table that converts surface roughness values such as Ra, RMS, Rz and ISO N grade numbers between measurement systems, so engineers, machinists and procurement teams can specify and verify the correct finish for any manufactured part. If you have ever needed to translate a 32 microinch callout into micrometers, or work out what N grade corresponds to a 125 Ra finish, this is the guide you will keep bookmarked.

Below you will find the complete surface finish conversion table, a live conversion calculator, a plain English breakdown of every roughness parameter and symbol, the formulas behind them, measurement methods, cost impacts, and an original decision framework we use internally at Meco to help our clients pick the right finish without overspending. Every value in this guide aligns with ASME Y14.36M and ISO 1302 standards.

What Is a Surface Finish Chart?

A surface finish chart, sometimes called a surface roughness chart, roughness chart or metal finishing chart, is a table that lists standard roughness grades and their equivalent values across different measurement units. It lets you convert between Ra in micrometers (µm), Ra in microinches (µin), RMS, CLA, Rz, Rt and ISO N grade numbers all in one place.

Here is why that matters. North American drawings typically call out surface finish in microinches. European and Asian drawings use micrometers. Older drawings might reference RMS or CLA values instead of Ra. Without a conversion chart you are left guessing, and guessing on a sealing surface or bearing bore is a fast way to scrap parts.

Surface Finish Conversion Chart: Ra, RMS, Rz, CLA and N Values

This surface finish conversion table covers every standard roughness grade from ultra fine (N1) to rough (N12). Use it to convert between Ra in micrometers, Ra in microinches, RMS, CLA, Rz, Rt and the ISO N grade scale.

Rz is shown as a range because Ra to Rz is not a fixed mathematical conversion. The ratio depends on profile shape and typically falls between 4 and 6.3 for machined surfaces. Where a drawing specifies both Ra and Rz, each must be verified independently against its own limit.

How to Read This Chart

Find the row that matches one value you know. For example, if your drawing calls out 125 microinches, a common machine finish callout, scan across to see it equals 3.2 µm Ra, 137.5 RMS and N8 on the ISO scale. That is the standard as machined finish most CNC shops deliver by default.

Two quick conversion shortcuts worth memorising: RMS is approximately Ra × 1.1 (multiply Ra in microinches by 1.1 to get the approximate RMS value), and for a rough Ra to Rz estimate, Rz is approximately Ra × 4 to 6.3 depending on how uniform the surface is. These rules of thumb come from the Mitutoyo Surface Roughness Quick Guide and they are handy for shop floor calculations, but do not rely on them for safety critical specs.

Surface Finish Units of Measure Explained

Surface finish is expressed in four unit systems, and mixing them up is the most common cause of parts rejected against an international drawing. Two units are in active use today: micrometers and microinches. RMS and CLA persist on legacy prints and still cause confusion.

How to Convert Metric Surface Finish to Imperial

To convert Ra in micrometers to microinches, multiply by 39.37. To convert microinches to micrometers, multiply by 0.0254. Worked example: Ra 3.2 µm × 39.37 = 126 µin, which is written on drawings as the standard 125 µin callout.

That rounding is worth understanding, because it explains why chart values are not always exact arithmetic matches. The standard grades were defined as a preferred series in each unit system independently, then correlated. When a drawing calls out 125 µin and your inspection report reads 3.2 µm, those are the same specification, not a 1 percent discrepancy.

Surface Finish Conversion Calculator

Enter a value in any one field and the equivalents are calculated instantly. Ra to Rz is shown as an approximate range for the reasons explained above.

Surface Finish Cheat Sheet by Application

Knowing the numbers is one thing. Knowing which number to use for your part is another. This cheat sheet maps each Ra range to the real world applications and manufacturing processes that typically achieve it.

In our experience at Meco, the most over specified finish is Ra 0.8 µm (32 µin) on surfaces that do not actually need it. We regularly see drawings where every surface on a housing gets a 32 finish callout, even faces that bolt against a gasket or never contact another part. That drives CNC machine costs up by 20 to 30 percent for zero functional benefit. We will get into how to avoid that mistake later in this guide.

Surface Finish Values Explained: What 8, 16, 32, 63, 125 and 250 Mean

Most drawings use one of a small set of standard numbers, almost always written in microinches on North American prints. Here is what each callout means in both units, its ISO grade, and where it belongs on a part.

What Is a 125 Surface Finish?

A 125 surface finish means Ra 125 microinches, equal to Ra 3.2 µm or N8. This is the standard machine finish, the default that CNC milling and turning deliver without any additional finishing pass. Parts at 125 feel smooth but show visible tool marks. If a surface has no sealing, bearing or cosmetic function, 125 is almost always the correct and cheapest callout.

What Is a 63 Surface Finish?

A 63 surface finish means Ra 63 microinches, equal to Ra 1.6 µm or N7. It sits one grade finer than standard machining and is produced by fine CNC milling or turning with sharp tooling and reduced feed rates. Specify 63 for mating surfaces, tight fit assemblies and gear teeth, or for parts carrying cyclic load where tool marks could act as crack initiation sites.

What Is a 32 Surface Finish in Microns?

A 32 surface finish converts to 0.8 µm Ra, which is N6 on the ISO scale and 35.2 RMS microinches. It is a precision finish requiring controlled CNC turning or cylindrical grinding. Use it for O-ring grooves, press fit bores and high pressure fluid seals. This is also the single most over specified value we see on incoming drawings, so confirm the surface genuinely needs it before committing.

What Is a 16 Surface Finish?

A 16 surface finish means Ra 16 microinches, equal to Ra 0.4 µm or N5. Reaching it normally requires a secondary operation such as fine grinding or precision honing rather than machining alone, which is why it carries a meaningful cost premium. It is the right specification for high load bearing seats, hydraulic valve seats and aerospace sealing surfaces.

What Is a 250 Surface Finish?

A 250 surface finish means Ra 250 microinches, equal to Ra 6.3 µm or N9. It is a rough machined finish with clearly visible tool marks, produced by rough milling or coarse turning. It is appropriate for clearance surfaces, internal housing walls and any face that never contacts another component. Specifying 250 rather than 125 on non functional surfaces removes finishing passes and reduces cycle time.

What Do Surface Finish Symbols Mean on Engineering Drawings?

If you have looked at a machining drawing and seen a checkmark shaped symbol with numbers scattered around it, that is a surface finish callout. It is standardised by ASME Y14.36M, the American standard, and ISO 1302, the international standard. Here is what each position on the symbol means.

The basic symbol looks like a checkmark resting on the surface line. A circle at the base means no machining is required, so the surface stays as cast or as forged. A bar across the top means material removal is required and the surface must be machined.

Position (a) is the Ra roughness value, the number engineers care about most.

Position (b) is the production method or note, such as GRIND or HONE.

Position (c) specifies the roughness sampling length in millimetres or inches.

Position (d) shows the lay direction, which is the pattern the tool marks follow on the surface: parallel, perpendicular, circular, crossed and so on.

Position (e) indicates minimum material removal.

Position (f) gives an alternate roughness parameter such as Rz, if the design requires it.

You do not always see all six positions filled in. Most machining surface finish callouts only specify position (a), the Ra value. For critical sealing surfaces in automotive or aerospace applications you might see lay direction and Rz controls added as well.

What Is the Difference Between Ra, Rz, and RMS?

These three parameters all describe surface roughness, but they measure it differently. Picking the wrong one, or converting between them incorrectly, can cause parts to fail inspection even when they look and feel fine.

Ra (Roughness Average) calculates the average height of all peaks and valleys along the measured surface profile. It is the most common parameter worldwide. Because it averages everything, Ra can hide occasional deep scratches or burrs. A surface with one bad scratch and one perfectly smooth area might still pass an Ra check.

Rz (Mean Roughness Depth) takes the five highest peaks and five deepest valleys within the sampling length, then averages those extremes. Rz is more sensitive to occasional defects than Ra. European and Asian drawings tend to use Rz more frequently. As a general rule of thumb, Rz is roughly 4 to 6.3 times the Ra value for the same surface, though this varies with surface uniformity.

RMS (Root Mean Square) squares all the height deviations, averages those squares, then takes the square root. For most machined surfaces RMS is approximately 1.1 times Ra. RMS appears mainly on older drawings and has been largely replaced by Ra in modern standards, but you will still see it on legacy prints and in some stainless steel and pharmaceutical specifications.

Surface Finish Formulas and Calculations

Ra Formula

Ra is the arithmetic mean of the absolute vertical deviations from the mean line, across the sampling length:

Ra = (1 / n) × Σ | y_i |

Where n is the number of measured points and y_i is the height deviation of each point from the mean line. Because deviations are averaged as absolute values, Ra is insensitive to whether the profile is dominated by peaks or valleys, which is precisely why it can mask isolated defects.

RMS (Rq) Formula

Rq = √ [ (1 / n) × Σ y_i² ]

Squaring the deviations before averaging weights large excursions more heavily than Ra does. For typical machined profiles this produces a value about 1.1 times Ra, which is where the common conversion factor comes from.

Theoretical Turning Surface Finish Formula

For single point turning, the theoretical roughness produced by the feed marks can be predicted from feed rate and tool nose radius:

Rz (µm) = ( f² / (8 × r) ) × 1000

Where f is feed in millimetres per revolution and r is the tool nose radius in millimetres. Worked example: at f = 0.2 mm/rev with a 0.8 mm nose radius, Rz = (0.04 / 6.4) × 1000 = 6.25 µm, giving an estimated Ra of roughly 1.6 µm once divided by four.

Treat this as a floor, not a prediction. It accounts only for the geometric feed marks and ignores tool wear, built up edge, vibration, workpiece rigidity and material behaviour. Real measured values are typically worse than theoretical, and the gap widens as tooling dulls. Use the formula to choose a starting feed rate, then verify on the part with a profilometer.

How Is Surface Roughness Measured?

There are four main methods for measuring surface finish. The right choice depends on your accuracy requirements, budget, and whether you can touch the surface.

Contact profilometer (stylus method): This is the industry standard. A diamond tipped stylus drags across the surface and records every peak and valley as a profile trace. The instrument calculates Ra, Rz, RMS and other parameters automatically. Brands such as Mitutoyo and ZEISS make profilometers accurate to fractions of a micrometer. The downside is that the stylus can scratch delicate surfaces, and it only measures along a single line rather than the whole surface area.

Non contact optical methods: Laser triangulation, confocal microscopy and white light interferometry scan the surface with light instead of a physical probe. These methods are faster, will not damage the surface, and can map an entire area in 3D. They are standard for inspecting soft metals, coated surfaces and very fine finishes below Ra 0.1 µm.

Comparator plates: A set of metal reference plates with known Ra values. The machinist holds the part up against each plate and compares by look and touch. It is fast and cheap, but not precise enough for anything beyond rough shop floor checks.

Portable testers: Battery powered handheld profilometers that can be carried to the machine. Good for in process checks during a production run. At Meco, our quality team uses both benchtop CMM integrated profilometers and portable testers across our Thailand and China production facilities to verify finishes at every stage, from custom CNC machining through final inspection.

What Surface Finishes Can Different Manufacturing Processes Achieve?

Not every manufacturing process can hit every finish. Knowing the achievable Ra range for each process helps you avoid specifying a finish that is impossible, or unnecessarily expensive, for your chosen method.

Surface Finish by Secondary Finishing Process

Machining sets the baseline roughness. Secondary finishing then modifies it, sometimes smoothing the surface and sometimes deliberately roughening it for appearance or adhesion. These are typical ranges rather than guarantees, since the result depends heavily on media, pressure, duration and the incoming machined finish.

Two points that catch buyers out. Anodising does not smooth a part, it faithfully reproduces whatever roughness is underneath and adds a little, so a cosmetic anodised finish has to be achieved before the tank, not in it. And bead blasting changes appearance far more than it changes measured Ra, which is why a blasted part can look uniform while still failing a roughness check. If your drawing specifies both a measured Ra and a visual finish, those are two separate requirements and both need to be called out. Our surface finishing services page covers process selection in more detail.

How Does Surface Finish Affect Part Cost?

Here is the part most guides skip. A smoother surface finish costs more, sometimes dramatically more. The relationship is not linear. Going from Ra 3.2 µm to Ra 1.6 µm might add 2 to 5 percent to your machining cost. But going from Ra 1.6 µm to Ra 0.4 µm can add 10 to 15 percent or more, because it typically requires secondary operations such as grinding or polishing on top of the initial machining.

According to data shared by Komacut, a CNC service provider, the cost uplift for standard aluminium alloy parts roughly follows this pattern: Ra 3.2 µm as the baseline with no extra cost, Ra 1.6 µm at around 2.5 percent, Ra 0.8 µm at around 5 percent, and Ra 0.4 µm at 11 to 15 percent. Complex geometries make those percentages even steeper.

Which Surface Finish Should You Choose? The FAST Framework

We developed the FAST Framework internally at Meco to help clients select the right finish the first time. FAST stands for Function, Application, Speed, and Tolerance budget. Walk through these four questions before writing a single surface finish callout on your drawing.

F. Function: What Does the Surface Do?

Is it a sealing surface, a bearing bore, a cosmetic face, or a structural bracket buried inside a housing? The answer determines your starting Ra target. Seals and bearings need Ra 0.8 µm or finer. Mating surfaces need Ra 1.6 µm. Structural non contact surfaces are fine at Ra 3.2 to 6.3 µm.

A. Application: What Is the Operating Environment?

A part exposed to corrosive fluids, extreme temperatures or high humidity may need a smoother finish to resist pitting and fatigue cracking. An interior bracket in a climate controlled electronics enclosure does not. Environment drives the spec, not habit.

S. Speed and Motion: Is the Part Moving?

Static parts are forgiving. Parts that rotate, slide or vibrate against other surfaces need tighter finish control. High RPM shafts and bushings typically require Ra 0.4 to 0.8 µm. Slow moving or stationary mating faces can often work fine at Ra 1.6 µm.

T. Tolerance Budget: What Can Your Program Afford?

Every micrometer of surface improvement costs money. Balance the functional need against your production budget and volume. For a 10 piece prototype, the per part impact of specifying Ra 0.4 is small. For a 500,000 unit annual run, that same spec adds significant cost. Our engineering team provides DFM feedback with every quote to flag exactly these tradeoffs, and turnkey manufacturing works best when finish specs are dialled in early.

Real World Example: How One Surface Finish Change Saved 22 Percent on a Production Run

An automotive OEM sent us drawings for an aluminium sensor housing with Ra 0.8 µm called out on all exterior surfaces, six faces in total. The housing bolts into a subframe behind the dashboard. Nobody sees it. Nobody touches it. Only one bore, the sensor pocket, actually needed a tight finish for O-ring sealing.

Our engineering team in the U.S. flagged this during DFM review. We recommended Ra 0.8 µm on the sensor bore only, Ra 3.2 µm on the bolt face interfaces, and Ra 6.3 µm on all other exterior surfaces. The client approved the change.

The result was that machining cycle time dropped from 14.2 minutes to 11.1 minutes per part. Across their 80,000 unit annual volume, that translated to a 22 percent reduction in per part CNC machining cost, with no change in part performance. The housing passed every functional test and PPAP validation on the first submission.

That is the kind of result a turnkey partner can deliver that a parts broker cannot. When the same team that machines the part also reviews the drawing and manages quality, finish optimisation happens naturally rather than as an afterthought. Need help specifying the right finish for your program? Reach out to our engineering team and we will review your drawings.

Surface Finish Standards You Should Know

Two standards govern how surface finish is specified and measured on engineering drawings in 2026.

ASME Y14.36M is the North American standard for surface texture symbols. It defines the checkmark symbol, the position of each parameter around the symbol, and how to call out roughness, waviness and lay direction on drawings. If your parts ship to or from the U.S. or Canada, this is the standard your drawings should follow.

ASME B46.1 is the companion standard that defines the actual measurement methods and calculations for Ra, Rz, RMS, waviness and other roughness parameters. It tells your quality lab how to set up the profilometer, what sampling length to use, and how to compute each value.

ISO 1302 is the international equivalent, widely used in Europe and Asia. It uses the same roughness parameters but may specify different default sampling lengths and symbol conventions. When Meco runs production across our facilities in Thailand and China for North American clients, we bridge both standards, and all our inspection reports reference the customer's specified standard with full traceability documentation.