Inches to Millimeters Converter (in to mm)
Convert inches (in) to millimeters (mm) instantly with real-time two-way calculation, exact formulas, and comprehensive reference tables.
Conversion Formula & Exact Calculation
How to convert Inches to Millimeters: Multiply by 25.4.
Fractional Inches to Millimeters (1/64" to 1" Engineering Chart)
Exact decimal inches and metric millimeter equivalents for all standard imperial fractions. Critical for CNC machining, 3D printing tolerance, carpentry, and drill bit selection:
| Fractional Inch | Decimal Inch | Millimeters (mm) | Typical Application |
|---|---|---|---|
| 1/64" | 0.0156" | 0.3969 mm | Micro drill bits / sheet metal shim |
| 1/32" | 0.0313" | 0.7938 mm | PCB trace spacing, fine gaskets |
| 1/16" | 0.0625" | 1.5875 mm | Standard cotter pins, 1.6mm PCB board |
| 1/8" | 0.1250" | 3.1750 mm | 1/8" audio jacks (3.5mm nominal), 3mm screws |
| 3/16" | 0.1875" | 4.7625 mm | Automotive brake lines, 5mm bolts |
| 1/4" | 0.2500" | 6.3500 mm | 1/4" hex driver bits, tripod camera mount |
| 5/16" | 0.3125" | 7.9375 mm | Standard M8 bolt equivalent |
| 3/8" | 0.3750" | 9.5250 mm | 3/8" socket drive ratchet, 10mm wrench near-fit |
| 7/16" | 0.4375" | 11.1125 mm | 11mm hex nut sizing |
| 1/2" | 0.5000" | 12.7000 mm | 1/2" plumbing copper pipe, 1/2" drive sockets |
| 5/8" | 0.6250" | 15.8750 mm | 16mm spark plug sockets |
| 3/4" | 0.7500" | 19.0500 mm | 3/4" garden hose fittings, 19mm lug nuts |
| 7/8" | 0.8750" | 22.2250 mm | Motorcycle/bicycle handlebar diameter |
| 1" | 1.0000" | 25.4000 mm | International exact definition: 1 inch = 25.4 mm |
Inches to Millimeters Quick Reference Table
| Inches (in) | Millimeters (mm) |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 5 in | 127 mm |
| 10 in | 254 mm |
| 25 in | 635 mm |
| 50 in | 1,270 mm |
| 100 in | 2,540 mm |
| 250 in | 6,350 mm |
| 500 in | 12,700 mm |
| 1,000 in | 25,400 mm |
| 5,000 in | 127,000 mm |
⚠️ 5 Fatal Metrology & Dimensional Analysis Traps
Unit conversion errors have historically caused catastrophic aerospace accidents, structural overloads, and pharmacological dosage failures. Watch out for these 5 traps:
Linear conversions do not scale linearly into area or volume. While 1 meter = 100 centimeters, 1 square meter = 10,000 cm² (100²), and 1 cubic meter = 1,000,000 cm³ (100³). Applying linear conversion ratios to surface area, HVAC ductwork, or tank volume causes severe 100× to 1,000× estimation errors.
US fluid ounces (29.57 mL) and British Imperial fluid ounces (28.41 mL) are different. Even more drastically, a US liquid gallon is 128 US fl oz (3.785 L), while a UK Imperial gallon is 160 UK fl oz (4.546 L)—a massive 20.1% volume discrepancy that causes critical miscalculations in aviation fueling and chemical synthesis.
Standard formulas (°F = °C × 1.8 + 32) apply strictly to absolute scale readings. When converting temperature differentials or temperature changes (ΔT), the +32 offset MUST be omitted: a temperature change of 1°C equals a change of exactly 1.8°F. Adding 32 to heat pump delta-T metrics, building insulation R-values, or thermal expansion calculations corrupts engineering calculations.
Hardware manufacturers sell storage in decimal SI prefixes (1 GB = 1,000,000,000 bytes = 10⁹), whereas operating systems like Windows calculate filesystem allocation in binary IEC prefixes (1 GiB = 1,073,741,824 bytes = 2³⁰). This causes a 1 TB drive to register as approximately 931 GB in OS file explorers, leading to unexpected disk partition shortfalls.
Truncating decimal places during intermediate conversion steps compounds floating-point error across multi-stage formulas. In aerospace telemetry, CNC metal fabrication, or pharmaceutical microgram compounding, conversions must retain full IEEE 754 64-bit double precision until final display formatting.