Fractional Inch Reference
Common Metric & Imperial Conversions
| Metric | Imperial / US | Common Engineering Use |
|---|---|---|
| 1 mm | 0.0393701 in | Small mechanical dimensions |
| 10 mm | 0.393701 in | Component dimensions |
| 25.4 mm | 1 in | Standard inch reference |
| 50.8 mm | 2 in | Mechanical dimensions |
| 0.1 mm | 3.937 mil | Precision dimensions |
| 1 mm | 39.370 mil | PCB / electronic dimensions |
| 1 kg | 2.20462 lb | Mass / weight |
| 1 MPa | 145.038 psi | Pressure specifications |
What Is an Engineering Unit Converter?
An engineering unit converter is used to convert measurements between different unit systems. This is particularly useful when engineering drawings, component specifications, datasheets, and customer requirements use different measurement standards.
Metric units such as millimeters (mm), centimeters (cm), and meters (m) are widely used in international engineering. Imperial and US customary units such as inches (in), feet (ft), mils, pounds (lb), and psi are still widely used in mechanical, aerospace, electronics, and industrial applications.
Metric to Imperial Length Conversion
Length conversion is one of the most common requirements when working with engineering drawings and component specifications.
For example, a mounting dimension of 12.7 mm is exactly equivalent to 0.5 inch, while 25.4 mm is exactly 1 inch.
Small Dimension Conversion: mm, µm, mil and inch
Small dimensional units are frequently used in electronics, precision machining, PCB manufacturing, component packaging, and engineering drawings.
- 1 mm = 1000 µm
- 1 inch = 25.4 mm
- 1 mil = 0.001 inch
- 1 mil = 0.0254 mm
- 1 mm = 39.3701 mil
In electronics and PCB specifications, "mil" normally refers to one thousandth of an inch rather than a metric millimeter.
Fractional Inch Conversion
Imperial engineering dimensions are often specified as fractional inches, such as 1/4", 5/16", 3/8", or 1/2". These values can be converted to decimal inches and millimeters for easier comparison with metric drawings.
| Fractional Inch | Decimal Inch | Millimeter |
|---|---|---|
| 1/16" | 0.0625" | 1.5875 mm |
| 1/8" | 0.125" | 3.175 mm |
| 3/16" | 0.1875" | 4.7625 mm |
| 1/4" | 0.250" | 6.35 mm |
| 5/16" | 0.3125" | 7.9375 mm |
| 3/8" | 0.375" | 9.525 mm |
| 1/2" | 0.500" | 12.7 mm |
| 3/4" | 0.750" | 19.05 mm |
| 1" | 1.000" | 25.4 mm |
Engineering Applications
Metric and imperial unit conversion is particularly useful in the following engineering applications:
- Mechanical engineering drawings
- Thread and fastener dimensions
- EMI and EMC filter dimensions
- Electronic component specifications
- PCB and mounting dimensions
- Machining and manufacturing tolerances
- Industrial equipment specifications
- Customer drawings using mixed unit systems
Engineering Unit Conversion Accuracy
Unit conversion is mathematically exact when the relationship between the units is defined exactly. For example, the international definition of an inch is exactly 25.4 millimeters.
However, converting between units does not change the manufacturing tolerance of a component. When converting engineering drawings or specifications, the original dimensional tolerance and required precision should always be considered separately.
Frequently Asked Questions
How many millimeters are in one inch?
One inch is exactly equal to 25.4 millimeters.
How do I convert mm to inches?
Divide the millimeter value by 25.4. For example, 10 mm ÷ 25.4 = 0.393701 inch.
What is a mil in engineering?
A mil is one thousandth of an inch. Therefore, 1 mil = 0.001 inch = 0.0254 mm.
Is 1/2 inch equal to 12.7 mm?
Yes. One-half inch is exactly 12.7 mm.
Is an inch the same as a fractional inch?
A fractional inch is simply an inch measurement expressed as a fraction, such as 1/4", 3/8", or 5/16". The same dimension can also be expressed as a decimal inch or millimeter.
Can metric and imperial dimensions be directly substituted?
Not necessarily. Although dimensions can be mathematically converted, a metric dimension and its approximate imperial equivalent may have different standard tolerances or manufacturing specifications. For precision applications, always check the original drawing, tolerance, thread standard, and applicable specification.