Unit	= Meters	Notes
1 nm	1×10⁻⁹ m	Nanometer
1 μm	1×10⁻⁶ m	Micrometer (micron)
1 mil	2.54×10⁻⁵ m	Thousandth of an inch
1 mm	0.001 m	Millimeter
1 cm	0.01 m	Centimeter
1 inch	0.0254 m	International inch
1 foot	0.3048 m	12 inches
1 yard	0.9144 m	3 feet
1 m	1 m	SI base unit
1 km	1000 m	Kilometer
1 mile	1609.344 m	Statute mile

Common Engineering Conversions

From → To	Multiply by	Example
mm → inch	0.03937	25.4 mm = 1 inch
inch → mm	25.4	1 inch = 25.4 mm
m → feet	3.28084	1 m = 3.281 ft
feet → m	0.3048	1 ft = 0.3048 m
km → miles	0.62137	1 km = 0.621 mi
miles → km	1.60934	1 mi = 1.609 km
μm → mil	0.03937	25.4 μm = 1 mil
mil → μm	25.4	1 mil = 25.4 μm

Handy Memorization Tips

• 1 inch = 25.4 mm (exact, by international agreement since 1959)
• 1 foot = 12 inches = 304.8 mm
• 1 meter ≈ 39.37 inches ≈ 3 ft 3⅜ in
• 1 mile ≈ 1.6 km (for quick estimation: divide km by 8, multiply by 5)
• 1 mil (thou) = 0.001 inch = 25.4 μm (used for paint/coating thickness and shim sizes)
• 1 micron (μm) = 0.001 mm (surface roughness, bearing clearances)

Application-Specific Units

• Surface roughness: Usually in μm (Ra) or μin (microinch)
• Bearing clearance: μm or mil (thou)
• Shaft alignment: mm or mil
• Pipe sizes: DN (mm) in metric, NPS (inches) in imperial
• Sheet metal: mm or gauge number
• Wire diameter: mm or AWG (American Wire Gauge)
• Vibration displacement: μm peak-to-peak or mil peak-to-peak
• Machining tolerances: μm or 0.001 mm, often expressed as IT grades (ISO 286)

Historical Note

The meter was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole. Since 1983, it is defined as the distance light travels in vacuum in 1/299,792,458 of a second, making it the most precisely defined SI base unit.