Convert Microinches (Ra) to Millimeters
Instantly convert Microinches (Ra) (μin) to Millimeters (mm) with our free online calculator.
Formula: μin to mm — multiply by 2.5400e-5
Reference Table
| Microinches (Ra) (μin) | Millimeters (mm) |
|---|---|
| 1 | 0.0000254 |
| 5 | 0.000127 |
| 10 | 0.000254 |
| 25 | 0.000635 |
| 50 | 0.00127 |
| 100 | 0.00254 |
How to Convert Microinches (Ra) to Millimeters
Formula
To convert Microinches (Ra) (μin) to Millimeters (mm): multiply by 2.5400e-5
Step-by-Step
- Start with your value in Microinches (Ra) (μin).
- Multiply by 2.5400e-5 to perform the conversion.
- The result is your value expressed in Millimeters (mm).
Conversion Factor
1 μin = 0.0000254 mm
Reverse Factor
1 mm = 39370.1 μin
Worked Example
Convert 25 Microinches (Ra) to Millimeters: 25 μin = 0.000635 mm
About Microinch (Ra) (μin)
An imperial surface-roughness unit equal to exactly 0.0254 μm (1 μin = 25.4 nm). Microinches (μin) Ra is the traditional US unit on machining drawings per ASME B46.1 (Surface Texture — Surface Roughness, Waviness and Lay) and US-edition Machinery's Handbook. Typical achievable values per ASME B46.1 surface-finish charts and US machining-shop practice: sand-cast iron 500-1,000 μin Ra; rough turned/milled 125-500 μin Ra; fine turned/finish milled 32-125 μin Ra; ground 8-63 μin Ra; honed 4-16 μin Ra; lapped 1-8 μin Ra; mirror-polished/superfinished 1-4 μin Ra or less; precision optical mirror surfaces <1 μin Ra; silicon-wafer surfaces (after CMP polish) <0.1 μin Ra. US production-machining drawings still routinely call out '63' or '125' (μin Ra) for general-purpose finish; '32' for sealing surfaces; '16' for bearing fits per ANSI/ASME B46.1 + the unified-screw-thread standard ANSI/ASME B1.1. Converting μin Ra to μm Ra (multiply by 0.0254, or divide by 39.37) is a routine step when US-designed parts are manufactured in metric-default shops overseas (Mexico maquiladoras, China contract manufacturers).
About Millimeter (mm)
A roughness unit equal to exactly 1,000 μm — used for very coarse surfaces where μm-Ra would produce unwieldy 4-5 digit numbers: sand-cast metal surfaces before machining cleanup (cast-iron sand-cast Ra typically 0.5-3 mm = 500-3,000 μm), raw cut-off saw and oxyacetylene-cut steel surfaces in steel fabrication, weld-bead surface finish before grinding, and most importantly the absolute roughness ε of pipe inner walls in fluid-mechanics Darcy-Weisbach pipe-flow friction calculations per the Moody diagram / Colebrook-White equation. Standard pipe-roughness reference values per Crane Technical Paper 410 / ASHRAE Handbook Fundamentals chapter 22: drawn copper tubing ε = 0.0015 mm (1.5 μm); commercial steel pipe ε = 0.045 mm; galvanized iron ε = 0.15 mm; cast iron ε = 0.26 mm; concrete pipe ε = 0.3-3.0 mm; riveted steel pipe ε = 0.9-9.0 mm. These mm ε values are used directly in the Colebrook equation 1/√f = -2·log₁₀(ε/(3.7D) + 2.51/(Re·√f)) to compute friction factor f for any pipe Reynolds number Re and diameter D. HVAC duct designers per ASHRAE 90.1 and water-distribution engineers per AWWA M11 standards use mm pipe roughness daily.
Quick Facts
- 1 Microinch (Ra) equals 0.0000254 Millimeters
- 1 Millimeter equals 39370.1 Microinches (Ra)
- Microinch (Ra) is a unit of surface roughness
- Millimeter is a unit of surface roughness
- This conversion is commonly used in machining, quality control, and tribology
- The Microinch (Ra) belongs to the imperial system
- The Millimeter belongs to the metric system
Common Microinch (Ra) to Millimeter Conversions
| Microinches (Ra) (μin) | Millimeters (mm) |
|---|---|
| 0.01 | 2.540000e-7 |
| 0.1 | 0.00000254 |
| 0.25 | 0.00000635 |
| 0.5 | 0.0000127 |
| 1 | 0.0000254 |
| 2 | 0.0000508 |
| 3 | 0.0000762 |
| 5 | 0.000127 |
| 10 | 0.000254 |
| 15 | 0.000381 |
| 20 | 0.000508 |
| 25 | 0.000635 |
| 50 | 0.00127 |
| 75 | 0.001905 |
| 100 | 0.00254 |
| 250 | 0.00635 |
| 500 | 0.0127 |
| 1000 | 0.0254 |
| 5000 | 0.127 |
| 10000 | 0.254 |
Understanding Microinches (Ra)
The Microinch (Ra) (symbol: μin) is a unit of surface roughness. An imperial surface-roughness unit equal to exactly 0.0254 μm (1 μin = 25.4 nm). Microinches (μin) Ra is the traditional US unit on machining drawings per ASME B46.1 (Surface Texture — Surface Roughness, Waviness and Lay) and US-edition Machinery's Handbook. Typical achievable values per ASME B46.1 surface-finish charts and US machining-shop practice: sand-cast iron 500-1,000 μin Ra; rough turned/milled 125-500 μin Ra; fine turned/finish milled 32-125 μin Ra; ground 8-63 μin Ra; honed 4-16 μin Ra; lapped 1-8 μin Ra; mirror-polished/superfinished 1-4 μin Ra or less; precision optical mirror surfaces <1 μin Ra; silicon-wafer surfaces (after CMP polish) <0.1 μin Ra. US production-machining drawings still routinely call out '63' or '125' (μin Ra) for general-purpose finish; '32' for sealing surfaces; '16' for bearing fits per ANSI/ASME B46.1 + the unified-screw-thread standard ANSI/ASME B1.1. Converting μin Ra to μm Ra (multiply by 0.0254, or divide by 39.37) is a routine step when US-designed parts are manufactured in metric-default shops overseas (Mexico maquiladoras, China contract manufacturers).
It belongs to the imperial measurement system.
Microinches (Ra) are commonly used in machining, quality control, and tribology.
Understanding Millimeters
The Millimeter (symbol: mm) is a unit of surface roughness. A roughness unit equal to exactly 1,000 μm — used for very coarse surfaces where μm-Ra would produce unwieldy 4-5 digit numbers: sand-cast metal surfaces before machining cleanup (cast-iron sand-cast Ra typically 0.5-3 mm = 500-3,000 μm), raw cut-off saw and oxyacetylene-cut steel surfaces in steel fabrication, weld-bead surface finish before grinding, and most importantly the absolute roughness ε of pipe inner walls in fluid-mechanics Darcy-Weisbach pipe-flow friction calculations per the Moody diagram / Colebrook-White equation. Standard pipe-roughness reference values per Crane Technical Paper 410 / ASHRAE Handbook Fundamentals chapter 22: drawn copper tubing ε = 0.0015 mm (1.5 μm); commercial steel pipe ε = 0.045 mm; galvanized iron ε = 0.15 mm; cast iron ε = 0.26 mm; concrete pipe ε = 0.3-3.0 mm; riveted steel pipe ε = 0.9-9.0 mm. These mm ε values are used directly in the Colebrook equation 1/√f = -2·log₁₀(ε/(3.7D) + 2.51/(Re·√f)) to compute friction factor f for any pipe Reynolds number Re and diameter D. HVAC duct designers per ASHRAE 90.1 and water-distribution engineers per AWWA M11 standards use mm pipe roughness daily.
It belongs to the metric measurement system.
Millimeters are commonly used in machining, quality control, and tribology.
Why Convert Microinches (Ra) to Millimeters?
Converting between Microinches (Ra) and Millimeters is a frequent requirement for engineers, scientists, and students working with surface roughness values. Different industries and regions favour different unit systems, so having a dependable conversion tool saves time and prevents errors in technical calculations. Whether you are verifying a specification sheet, cross-checking simulation results, or preparing a report for an international audience, accurate surface roughness conversion is essential.
Frequently Asked Questions
How do I convert Microinches (Ra) to Millimeters?
An imperial surface-roughness unit equal to exactly 0. To convert Microinches (Ra) to Millimeters, multiply by 2.5400e-5. For example, 25 μin equals 0.000635 mm.
How many Millimeters are in 1 Microinch (Ra)?
There are 0.0000254 Millimeters in 1 Microinch (Ra).
How many Microinches (Ra) are in 1 Millimeter?
There are 39370.1 Microinches (Ra) in 1 Millimeter.
What is the formula for Microinch (Ra) to Millimeter conversion?
The formula is: multiply by 2.5400e-5. This means 1 μin = 0.0000254 mm.
Is a Microinch (Ra) bigger than a Millimeter?
Yes. One Microinch (Ra) is larger than one Millimeter because 1 μin equals 0.0000254 mm, which is less than 1.
When do you need to convert between Microinches (Ra) and Millimeters?
A roughness unit equal to exactly 1,000 μm — used for very coarse surfaces where μm-Ra would produce unwieldy 4-5 digit numbers: sand-cast metal surfaces before machining cleanup (cast-iron sand-cast Ra typically 0. Microinch (Ra) and Millimeter are both surface roughness units, so conversion comes up whenever one source of information uses one unit and another uses the other — a classic cross-reference challenge in engineering, trade, travel, and everyday life.