A complete inches to millimeters (mm) conversion table and two-way calculator for the fastener industry, engineering drawings, and everyday metric conversions. Find the exact metric equivalent of fractional inch sizes from 1/64" to 15/16", decimal sizes from 0.01" to 100", or convert any value in either direction with our instant calculator.
Inches ⇄ Millimeters Converter
Inches to Millimeters Conversion Tables
Fractional Inches to mm (1/64" to 15/16")
Every fractional inch size from 1/64" to 15/16" with its exact millimeter value and a practical rounded equivalent. Use this table for imperial fasteners, wrench sizes, and fractional drill bits.
| Fraction (in) | Decimal (in) | mm (Exact) | mm (Approx) |
|---|
Decimal & Whole Inches to mm (0.01" to 100")
Decimal inch values from 0.01" up to 100" for wide-range measurements — ideal for machine travel, plate thickness, and long-stroke dimensions.
| Inches (") | Millimeters (mm) |
|---|
Millimeters & Centimeters to Inches (Reverse Lookup)
Convert metric sizes back to inches for metric fasteners, tubing, and 3D printing filament diameters.
| Metric | Inches (") |
|---|
How to Convert Inches to Millimeters
One inch is defined as exactly 25.4 millimeters. To convert inches to millimeters, multiply the inch value by 25.4. To convert back, divide the millimeter value by 25.4. For centimeter and micron conversions use the quick formulas below:
| Conversion | Formula |
|---|---|
| Inches to millimeters | Value (inches) × 25.4 |
| Inches to centimeters | Value (inches) × 2.54 |
| Inches to microns | Value (inches) × 25.4 × 1,000 |
Where Inch-to-Millimeter Conversion Matters
Fasteners & Hardware
Imperial bolts, screws, and nuts are specified in fractional inches — 1/4"-20, 3/8"-16, 5/16"-18 — while metric counterparts use millimeter diameters. This table converts between UNC/UNF sizes and their metric equivalents.
Engineering Drawings & CAD
Blueprints and CAD models often mix imperial and metric dimensions. Use exact millimeter values from the fractional table to avoid tolerance and fit errors in machined parts.
3D Printing
Filament diameters (1.75 mm, 2.85 mm), nozzle sizes, and layer heights are metric; slicing software and STL models from US sources may quote inches. Reverse lookup helps you set the correct values.
Woodworking & DIY
Lumber thickness, drill bits, and hardware from different regions mix units. A quick inches-to-mm conversion keeps shelf pins, dowels, and fasteners compatible.
Best Practices for Inch to mm Conversion
Quick Reference Tips
- Always multiply inches by 25.4 — never approximate with 25 unless precision does not matter.
- For fractional fastener sizes, use the fractional table above; the exact mm value avoids rounding drift in tapped holes.
- Round to 2 decimals for general work, 3–4 decimals for machining tolerances.
- When in doubt, verify with the reverse direction — mm ÷ 25.4 should return your original inch value.
- In CAD and 3D printing, always confirm the document unit is mm before importing STL files.
Frequently Asked Questions
There are exactly 25.4 millimeters in 1 inch. To convert inches to millimeters, multiply the inch value by 25.4 — for example, 2 inches = 2 × 25.4 = 50.8 mm.
3/8 inch equals 9.525 mm. The exact calculation is 3/8 × 25.4 = 9.525 mm, commonly rounded to 9.53 mm for most practical work. 3/8" is a very common fastener and wrench size.
5/16 inch equals 7.9375 mm, usually rounded to 7.94 mm. The exact conversion is 5/16 × 25.4 = 7.9375 mm. 5/16" is a standard UNC bolt and wrench size.
1 inch is exactly 25.4 mm. This value is the international definition of the inch adopted in 1959 and used for all scientific and industrial conversions.
Multiply the inch value by 25.4: d(mm) = d(in) × 25.4. For example, 5 inches = 5 × 25.4 = 127 mm, and 20 inches = 20 × 25.4 = 508 mm.
One millimeter equals approximately 0.03937 inches. To convert mm to inches, divide the millimeter value by 25.4 — for example, 10 mm ÷ 25.4 = 0.394 inches.
