Inches to MM Converter - Convert Inches to Millimeters
Convert inches to millimeters for drawings, CNC setup sheets, 3D printing, hardware specs, product dimensions, and repair notes. The conversion is exact: millimeters = inches x 25.4.
Reverse this with mm to inches, or browse all length conversion calculators.
The Exact Inch to Millimeter Rule
One inch is exactly 25.4 millimeters. That definition is why the conversion is reliable for engineering drawings, CAD models, hardware listings, product specs, and shop-floor measurements. There is no approximate inch-to-mm factor hidden behind the result.
Formula
mm = in x 25.4
Reverse
in = mm / 25.4
Anchor
1 in = 25.4 mm
Half inch
0.5 in = 12.7 mm
Inch Fractions People Actually Need
Many inch measurements arrive as fractions, especially on rulers, hardware labels, woodworking plans, and older drawings. Convert the fraction to a decimal inch first, then multiply by 25.4.
| Inch fraction | Decimal inch | mm | Common context |
|---|---|---|---|
| 1/16 | 0.0625 | 1.5875 | Small clearance |
| 1/8 | 0.125 | 3.175 | Hardware spacing |
| 1/4 | 0.25 | 6.35 | Common stock size |
| 1/2 | 0.5 | 12.7 | General layout |
| 3/4 | 0.75 | 19.05 | Pipe or board notes |
Why Millimeter Is the Safer Working Unit
Inches are common in consumer products, US hardware, woodworking, and older drawings. Millimeters are often easier once the measurement enters a CAD model, CNC setup, 3D slicer, inspection report, or international spec. Small differences are easier to see in millimeters than in inch fractions.
CAD and CNC
Hole spacing, offsets, and clearances are easier to compare when inch dimensions are converted into millimeters.
3D printing
Slicers commonly expect mm-based dimensions for layer height, nozzle size, shell thickness, and part clearance.
Hardware matching
Screws, washers, brackets, and parts may mix inch descriptions with metric catalog values.
Inspection and tolerances
A thousandth of an inch is 0.0254 mm. Writing both units clearly helps prevent fit and inspection mistakes.
Use the Right Neighbor Converter
If your measurement is already metric, use mm to inches, mm to cm, or cm to mm. For inch-based routes, compare inches to cm, inches to meters, inches to feet, inches to yards, and feet to inches.
For larger metric or imperial comparisons, the related meters to inches, mm to feet, and cm to inches tools help keep the rest of the unit chain consistent.
Tolerance note
Do the exact conversion first, then round to the tolerance required by the job. Rounding too early can change a fit, clearance, or inspection decision.
Practical Inches to MM Questions
How many millimeters are in an inch?
There are exactly 25.4 millimeters in 1 inch.
How do you convert inches to mm?
Multiply the inch value by 25.4. For example, 2 inches equals 50.8 mm.
Why do CAD and CNC workflows often use millimeters?
Millimeters make small dimensions, clearances, and tolerances easier to enter and read. A 0.25 inch part width becomes 6.35 mm, which fits naturally in metric CAD and machining setups.
Why is 25.4 exact?
The international inch is defined as exactly 25.4 millimeters, so inches to mm conversion does not rely on approximation.
How do I convert inch fractions to millimeters?
Convert the fraction to a decimal inch first, then multiply by 25.4. For example, 1/4 inch is 0.25 inch, so it equals 6.35 mm.
Why do 3D printers and slicers use mm?
Most 3D printing settings are small measurements: nozzle size, layer height, wall thickness, and clearances. Millimeters keep those values easy to control.
Should I round millimeters for machining?
Only round to the tolerance required by the job. For precision work, keep enough decimals to match the drawing, tool, or inspection requirement.
What is 1/8 inch in mm?
1/8 inch equals 3.175 mm.