Surface Finish Chart with Ra, Rz, RMS and N Grades

Surface Finish Chart: N Grades, Ra and RMS

The surface finish conversion chart below translates between the four ways roughness is usually written: ISO N grade, Ra in micrometers, Ra in microinches and approximate RMS. Find the value from your drawing in any column and read across the row. The last column names processes that typically reach that band.

Three notes apply to every row of the chart:

• Rounded microinches. The µin column holds the rounded values used on US drawings, not exact conversions. One µm equals 39.37 µin, so N8 (3.2 µm) is exactly 126 µin and N9 (6.3 µm) is 248 µin; charts and callouts use 125 and 250.
• Estimated RMS. The RMS columns use RMS ≈ 1.1 × Ra, an estimate that suits regular, periodic profiles.
• Typical processes. The process column shows where a process typically falls, not what a given machine will deliver.

The chart graphic at the top of this page shows the same values. This surface finish chart is also available as a printable PDF for the shop floor.

How to Read the Surface Finish Chart

Read the chart from top to bottom as smooth to rough: a lower N number and a lower Ra mean a smoother surface. Each row doubles the Ra of the row above, so one grade is a large step, not a fine adjustment.

1. Identify the unit. A metric drawing gives Ra in µm, for example Ra 1.6. A US inch drawing usually gives Ra in µin, often as a bare number such as 63 above the symbol.
2. Identify the parameter. Ra, Rz and RMS are different measures. Use the Ra columns only for Ra values.
3. Read across. A US callout of 125 µin is Ra 3.2 µm, which is N8. A metric drawing with Ra 0.8 is N6, about 32 µin.
4. Treat the value as a limit. A single value on a drawing is normally an upper limit, so a part measuring Ra 1.2 µm is within an Ra 1.6 requirement.

Values between rows are valid. Ra 1.0 or Ra 2.5 are legitimate specifications; they simply sit between grades. The doubling series of 0.4, 0.8, 1.6, 3.2 and 6.3 µm is still the set of values most drawings use.

Ra, Rz and RMS in Brief

Ra, Rz and RMS describe the same roughness profile in three different ways. Ra is the arithmetic average of the profile's distance from its mean line. RMS, written Rq in ISO 21920-2 and ASME B46.1, is the root mean square of the same distances. Rz is a height: the highest peak plus the deepest valley in each sampling length, averaged over the evaluation length.

Ra and Rq are averages, so a single scratch or burr-like peak barely changes them. Rz reacts to those extremes, which is why it appears on sealing faces, under thin coatings and on many German drawings. For definitions, formulas and the older meanings of Rz, see our guide to surface roughness Ra and Rz.

Converting Ra to RMS

Rq is always at least as large as Ra on the same profile. On a sine-shaped profile Rq is 1.11 × Ra, which is where the familiar 1.1 factor comes from; it fits regular turned and milled textures. Ground, blasted and other random textures run closer to 1.25 × Ra. In US shops RMS is still often used loosely to mean Ra, so confirm which parameter the customer actually measures.

Ra to Rz Conversion: Why There Is No Exact Factor

There is no exact conversion from Ra to Rz, because the ratio depends on the shape of the profile, not on its size. Two surfaces with the same Ra can have very different Rz values if one has isolated sharp peaks and the other an even texture.

The table shows the ratios for idealized profiles. They follow from geometry and assume a profile that repeats evenly through each sampling length.

The theoretical profile of turning with a round tool nose, a row of shallow arcs, gives Rz of about 3.9 × Ra. Real surfaces scatter around and beyond these cases. Comparison charts and shop rules mostly put Rz at about 4 to 7 times Ra for machined and ground surfaces. Very regular textures can fall near 3, and surfaces with isolated scratches, pits or burr-like peaks can go well above 7.

For early planning only, the next table applies the 4 to 7 rule to common Ra values. Do not use it to accept or reject parts, and do not change a drawing requirement from one parameter to the other without the customer's agreement.

If a drawing gives Rz, measure Rz. If supplier and customer read the same part differently, compare instrument settings first: parameter, cutoff, evaluation length and direction. Older drawings may use the former ten-point Rz definition, so confirm which Rz is meant.

ISO N Grades N1 to N12

N grades are roughness grade numbers from the older ISO 1302 practice. Each grade stands for one Ra value in micrometers from a doubling series, so N7 means Ra 1.6 µm and N8 means Ra 3.2 µm. The 1992 edition of ISO 1302 still listed them. The 2002 edition dropped them, and ISO 21920-1 does not use them.

The grades remain common on older drawings and shop charts. When you meet one, convert it with the chart and write the Ra value on any new revision. Two points matter for quoting:

• An N grade is a single Ra value. N7 does not mean anywhere between N6 and N8. Under the legacy rules it was an upper limit of Ra 1.6 µm.
• The acceptance rule follows the drawing's standard. Legacy ISO 1302 drawings used the 16 percent rule, which accepted a small share of readings above the limit. ISO 21920-1 drawings default to the max rule, where no reading may exceed it.

Machining Surface Finish Chart: Typical Ra by Process

Each manufacturing process produces roughness within a typical band. The values below are the average application ranges given in general engineering references such as Machinery's Handbook. They are typical ranges, not guarantees: tooling, feeds, material and machine condition move results in both directions, and the drawing governs. The table gives each range in micrometers and microinches with the matching N grades.

Use the table to see which processes can reach a requirement without a second operation. An Ra 3.2 µm (125 µin) face is inside the normal band of turning and milling. An Ra 0.4 µm face usually needs grinding, honing or polishing. A requirement tighter than the function needs adds cost, so check it before quoting.

Surface Finish of Cut, Ground and Satin Sheet Metal

The classic process chart covers sheet metal surfaces only in part. Thermal cut faces have their own standard, and stainless finishes are defined by mill standards rather than by process.

Laser, plasma and oxy-fuel cut faces

ISO 9013 classifies thermal cuts made by oxy-fuel, plasma and laser cutting. It does not grade the cut face by Ra. It uses the perpendicularity or angularity tolerance and the mean height of the profile, Rz5, each sorted into quality ranges whose limits grow with material thickness.

If a cut quality matters, the drawing should name the ISO 9013 range. Dross and slag on the bottom edge are a separate issue from face roughness; see slag removal and finishing plasma and oxy-fuel cut parts.

Belt-ground, brushed and satin surfaces

Belt grinding has no standard Ra range, because the result depends on grit, belt condition, pressure and the marks left by earlier steps. Stainless finish standards give the most useful reference points. The table places them, and the cut faces, on the chart.

ASTM A480 notes that readings vary between instruments, laboratories and operators, and that No. 3 and No. 4 can overlap. A brushed or satin sheet also measures higher across the grain than along it, so a value without a direction is incomplete. For how these finishes are made and named, see the guide to types of metal finishes.

Surface Finish Symbols on Drawings

Surface finish is called out with a check-mark symbol that carries the parameter and its value. ISO 21920-1, published in 2021, replaced ISO 1302 as the ISO standard for indicating surface texture on drawings. Drawings made to ISO 1302, including older ones with N grades, remain in circulation and are read under their original rules.

The basic symbol has three forms. The open check mark allows any process, a bar closing the V requires material removal, and a circle in the V forbids it. The requirement is written as parameter plus value, such as Ra 1.6 or Rz 6.3, with optional entries for production method, lay direction and measurement settings.

Legacy and US callouts

• N grade at the symbol: old ISO 1302 practice allowed an N number in place of the Ra value.
• Bare number on an older metric drawing: read as Ra in µm.
• Bare number on a US drawing: under ASME Y14.36 practice, a number such as 63 or 125 without a parameter means Ra in µin.
• RMS callout: confirm whether Rq or Ra is meant before quoting.

Symbol variants, lay symbols and the measurement settings that can sit on the symbol are covered in the Ra and Rz roughness guide. The lay symbol matters on brushed and satin sheet, where roughness changes with direction.

Specifying a Surface Finish for Sheet Metal Parts

A sheet metal finish callout should state the parameter, the value, the measuring direction and the faces it applies to. A grit number or a finish name alone does not define the result, because the same name means different surfaces at different suppliers.

1. Parameter and value: for example Ra 0.8 µm max, or Ra 32 µin.
2. Direction: across or along the grain, with the grain tied to a named edge.
3. Faces: usually the visible face only.
4. Edges: a burr limit or edge radius, stated separately. Ra on a flat face says nothing about the edge; edges are handled by sheet metal deburring and edge rounding.
5. Reference sample: a signed sample held by both sides.

On flat laser, plasma, oxy-fuel and punched parts, these requirements are usually met on a through-feed machine. TFON Surfacer machines deburr, round edges, remove oxide and slag and apply a satin finish. The TF-RF-3013 surface finishing machine has a 1,300 mm working width, an abrasive belt plus 8 flap wheels (Ø350 mm) and a conveyor speed of 0.6-4.0 m/min. The TF-RBF-3013 deburring and finishing machine deburrs, rounds edges and applies a satin finish in one pass on the same width.

This chart contains no TFON-measured roughness values. The finish a machine reaches depends on material, incoming surface and abrasive sequence, so qualify it with a trial on your own parts and measure before and after. The sheet metal surface finishing page explains the stations, and the deburring and edge rounding machines page lists every model.

How to Measure Surface Finish

Surface finish is measured with a stylus profilometer that traces the surface and calculates Ra, Rz and other parameters from the profile. The result depends on the settings, so the settings belong on the report next to the value.

• Cutoff (λc): the filter that separates roughness from waviness. Under ISO 4288, now replaced by ISO 21920-3, non-periodic profiles with Ra over 0.1 up to 2 µm used a 0.8 mm cutoff. The other defaults were 0.25 mm up to Ra 0.1 µm, 2.5 mm up to 10 µm and 8 mm up to 80 µm.
• Evaluation length: normally five sampling lengths, so 4 mm with a 0.8 mm cutoff.
• Direction: across the lay unless the drawing says otherwise. On brushed sheet, a trace along the grain reads lower.
• Location: a clean, flat area away from edges, holes and handling marks.

Roughness comparison specimens let an inspector judge a surface by sight and touch against samples of known Ra. They suit quick screening, not acceptance, and only work when the specimen was made by the same process as the part.