# Surface Finish Ra to N Grade Conversion Chart

> Source: https://netray.co/tools/surface-finish-ra-n-grade-conversion-reference
> Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems.
> Section: Free Tools

## Short answer

Searchable chart converting ISO 1302 roughness N grades to Ra in micrometres and microinches, with the machining processes that usually achieve each grade.

## Category

quality

## Industry

Precision Machining

## Audience

- Quality inspector
- Machinist
- Quality engineer

## Tool

**Kind:** worksheet

**Mode:** reference

**Columns:**

#### N grade

**Key:** grade

**Type:** text

#### Ra (um)

**Key:** ra_um

**Type:** text

#### Ra (uin)

**Key:** ra_uin

**Type:** text

#### Typical processes

**Key:** process

**Type:** text

#### Inspection note

**Key:** note

**Type:** text

**Rows:**

**Grade:** N1

**Ra um:** 0.025

**Ra uin:** 1

**Process:** Lapping, superfinishing

**Note:** Optical or gauge block class surfaces; needs a stylus with low noise floor

**Grade:** N2

**Ra um:** 0.05

**Ra uin:** 2

**Process:** Lapping, polishing

**Note:** Mirror finish; use a lab profilometer, not a portable one

**Grade:** N3

**Ra um:** 0.1

**Ra uin:** 4

**Process:** Lapping, fine honing

**Note:** Typical for precision sealing and bearing faces

**Grade:** N4

**Ra um:** 0.2

**Ra uin:** 8

**Process:** Honing, fine grinding

**Note:** Hydraulic rod and cylinder bore range

**Grade:** N5

**Ra um:** 0.4

**Ra uin:** 16

**Process:** Grinding, honing

**Note:** Common for bearing journals and seal contact surfaces

**Grade:** N6

**Ra um:** 0.8

**Ra uin:** 32

**Process:** Fine grinding, fine turning

**Note:** Common fitted shaft and bore finish

**Grade:** N7

**Ra um:** 1.6

**Ra uin:** 63

**Process:** Finish turning, finish milling, reaming

**Note:** Standard general machined finish on many drawings

**Grade:** N8

**Ra um:** 3.2

**Ra uin:** 125

**Process:** Milling, turning, drilling

**Note:** Visible tool marks; typical when no finish is called out tightly

**Grade:** N9

**Ra um:** 6.3

**Ra uin:** 250

**Process:** Rough milling, rough turning, planing

**Note:** Non-mating surfaces

**Grade:** N10

**Ra um:** 12.5

**Ra uin:** 500

**Process:** Rough machining, flame cutting

**Note:** Compare with a visual and tactile roughness comparator plate

**Grade:** N11

**Ra um:** 25

**Ra uin:** 1000

**Process:** Sand casting, saw cutting

**Note:** As-cast or as-cut surfaces

**Grade:** N12

**Ra um:** 50

**Ra uin:** 2000

**Process:** Rough casting, forging scale

**Note:** Coarsest N grade on the ISO scale

**Notes:** Ra is the arithmetic mean roughness. N grades follow the ISO 1302 series and map to Ra values by definition. Process columns are typical ranges, not guarantees. Confirm the cutoff length and evaluation length on the drawing before measuring.

## Content

**Intro:** Drawings in the US usually call surface finish in microinches Ra while many other regions use N grades or micrometres. This chart lets an inspector convert between them in seconds and see which process normally produces each finish.

**Sections:**

#### How to use the chart

**Body:** Type the value you have into the search box, for example 63 or N7, and the matching row is shown. Read across to the equivalent Ra in the other unit and to the processes that usually achieve it. Export the table to Excel or PDF if you want a controlled copy at the inspection station. The Ra values are rounded as they appear in the standard series, so a measured value of 1.58 um is read as N7 compliant against a 1.6 um limit.

#### How roughness is checked

**Body:** Ra is measured with a stylus profilometer over an evaluation length that normally spans five sampling lengths. The cutoff must suit the expected roughness: shorter cutoffs for fine finishes, longer for coarse ones. Always compare the measured Ra to the maximum on the drawing, and check whether the drawing gives a single limit (maximum) or a range, since a lower limit means a too smooth surface is also rejectable.

**Bullets:**

- Measure across the lay direction, not along it
- Clean the part before measuring
- Record the cutoff and evaluation length with the result
- Use a comparator plate only for rough screening

#### Common mistakes

**Body:** The most frequent error is mixing Ra with Rz or Rmax. They are different parameters and cannot be converted with a fixed factor. Another is measuring one spot on a surface whose finish varies, or judging by eye. For ERP users, store the required finish as a characteristic on the inspection plan with the unit stated, so receiving inspection and the shop floor read the same limit.

**Cta:** If surface finish and other characteristics live in drawings and spreadsheets, Netray can help pull them into your ERP inspection plans so results are recorded against the right limit.

**Faq:**

#### What is the difference between Ra and Rz?

**Answer:** Ra is the arithmetic average of profile deviations over the evaluation length. Rz is based on peak to valley heights over sampling lengths. Rz is more sensitive to isolated scratches and peaks. They are different parameters, so you cannot reliably convert one to the other.

#### Is N7 the same as 63 microinch?

**Answer:** Yes for Ra. N7 corresponds to 1.6 micrometres Ra, which is 63 microinches in the usual US call-out. The microinch figure is a rounded conversion of the metric value.

#### Can I use a roughness comparator plate for final acceptance?

**Answer:** Comparator plates are fine for quick screening on coarser finishes. For finishes tighter than about N8 or where the drawing states a measured limit, use a stylus profilometer and record the numeric result.

#### Does the chart replace the drawing requirement?

**Answer:** No. The drawing governs, including the roughness parameter, cutoff and whether a minimum applies. The chart only helps convert units and understand typical processes.

## Search terms

surface finish conversion chart, Ra to N grade, roughness N grade table, Ra micrometre to microinch, surface roughness chart, machined surface finish by process, N7 surface finish Ra, surface finish inspection reference

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