Inches to Millimeters Converter (in to mm) – Fast & Accurate Tool
Convert inches to millimeters (in to mm) and millimeters to inches (mm to in) with ultra-high engineering accuracy. Features dual decimal and fractional input modes down to 1/64″, real-time step-by-step formula calculations, secondary metric units, CNC machining bolt charts, and electronics component pitch guides.
Instant Inches ↔ Millimeters Calculator
Equivalent Measurements in Other Units:
Quick Common Benchmark Values:
How to Convert Inches to Millimeters (Formula & Step-by-Step Calculation)
To convert inches to millimeters, multiply the length in inches by exactly 25.4. This factor is universally fixed by the 1959 International Yard and Pound Agreement, establishing that one international inch is defined as exactly 25.4 millimeters.
Direct Mathematical Formula:
For the opposite calculation, use our companion Millimeters to Inches Converter with the reciprocal formula: in = mm ÷ 25.4 (or mm × 0.0393700787).
Step-by-Step Worked Calculation Examples
Example 1: Decimal Inches (3.25″)
- Identify the length: 3.25 inches.
- Apply the multiplier:
3.25 × 25.4. - Perform the multiplication:
3.25 × 25.4 = 82.55. - Result: 3.25 in = 82.55 mm (or 8.255 cm).
Example 2: Fractional Inches (3/8″)
- Convert fraction to decimal:
3 ÷ 8 = 0.375 inches. - Apply the exact standard:
0.375 × 25.4. - Calculate the product:
0.375 × 25.4 = 9.525. - Result: 3/8 in = 9.525 mm (commonly rounded to 9.5 mm in carpentry).
For centimeter conversions, see our Inches to Centimeters Converter and Centimeters to Inches Converter.
Inches to Millimeters Quick Conversion Chart (Fractional & Decimal)
The reference table below provides exact metric conversions across standard Imperial fractional increments, decimal inches, and corresponding centimeters:
| Fractional Inch | Decimal Inch (in) | Millimeters (mm) | Centimeters (cm) |
|---|---|---|---|
| 1/64″ | 0.0156 in | 0.3969 mm | 0.0397 cm |
| 1/32″ | 0.0313 in | 0.7938 mm | 0.0794 cm |
| 1/16″ | 0.0625 in | 1.5875 mm | 0.1588 cm |
| 1/8″ | 0.1250 in | 3.1750 mm | 0.3175 cm |
| 3/16″ | 0.1875 in | 4.7625 mm | 0.4763 cm |
| 1/4″ | 0.2500 in | 6.3500 mm | 0.6350 cm |
| 5/16″ | 0.3125 in | 7.9375 mm | 0.7938 cm |
| 3/8″ | 0.3750 in | 9.5250 mm | 0.9525 cm |
| 1/2″ | 0.5000 in | 12.7000 mm | 1.2700 cm |
| 5/8″ | 0.6250 in | 15.8750 mm | 1.5875 cm |
| 3/4″ | 0.7500 in | 19.0500 mm | 1.9050 cm |
| 7/8″ | 0.8750 in | 22.2250 mm | 2.2225 cm |
| 1″ | 1.0000 in | 25.4000 mm | 2.5400 cm |
| 1-1/2″ | 1.5000 in | 38.1000 mm | 3.8100 cm |
| 2″ | 2.0000 in | 50.8000 mm | 5.0800 cm |
| 3″ | 3.0000 in | 76.2000 mm | 7.6200 cm |
| 4″ | 4.0000 in | 101.6000 mm | 10.1600 cm |
| 6″ | 6.0000 in | 152.4000 mm | 15.2400 cm |
| 10″ | 10.0000 in | 254.0000 mm | 25.4000 cm |
| 12″ (1 ft) | 12.0000 in | 304.8000 mm | 30.4800 cm |
Precision Engineering, Machining & Bolt Specifications Guide
In precision CNC milling, toolroom lathe operations, and additive 3D manufacturing, imperial drawings frequently specify tolerances in thousandths of an inch (commonly nicknamed thou or mils). Understanding the exact millimeter equivalent is crucial to avoiding expensive machining scrap:
Imperial Bolt Diameters vs. Closest ISO Metric Fasteners
Fasteners designed in Imperial standards (UNC/UNF) do not interchange with ISO metric threads due to different pitch profiles (60° ISO vs 55° Whitworth or thread pitches per inch), but knowing outer shank diameters prevents clearance hole errors:
| Imperial Bolt Size | Exact Shank mm | Closest Metric Nominal | Common Applications |
|---|---|---|---|
| 1/4″-20 UNC | 6.35 mm | M6 (6.00 mm) | Standard camera tripod mounts, audio gear |
| 5/16″-18 UNC | 7.94 mm | M8 (8.00 mm) | Automotive subframes, engine brackets |
| 3/8″-16 UNC | 9.53 mm | M10 (10.00 mm) | Heavy studio equipment, machinery mounts |
| 1/2″-13 UNC | 12.70 mm | M12 (12.00 mm) | Structural steel connectors, agricultural implements |
| 5/8″-11 UNC | 15.88 mm | M16 (16.00 mm) | Heavy construction bolts, crane riggings |
Planning packaging dimensions or container freight for international equipment shipments? Calculate volume with our free CBM Calculator.
Screen Sizes, Photography & Electronic Component Measurements
Across consumer electronics and optical hardware, consumer marketing advertises diagonal dimensions in inches, while product engineers and industrial mold designers work exclusively in millimeters:
Display Diagonal vs. Millimeter Chassis
- 6.1″ Smartphone: 154.9 mm diagonal (approx. 147 mm × 71 mm body).
- 6.7″ Smartphone: 170.2 mm diagonal (approx. 161 mm × 78 mm body).
- 13.3″ Laptop: 337.8 mm diagonal (approx. 294 mm × 165 mm screen).
- 15.6″ Laptop: 396.2 mm diagonal (approx. 345 mm × 194 mm screen).
- 27″ Desktop Monitor: 685.8 mm diagonal (approx. 598 mm × 336 mm panel).
Printed Circuit Board (PCB) Pin Spacing
Electronic breadboards, headers, and IC socket pins inherit imperial grid spacing from twentieth-century vacuum tube specifications:
- 0.1″ (100 mil) Pitch: Exactly 2.54 mm (standard Arduino / Raspberry Pi GPIO pin spacing).
- 0.05″ (50 mil) Pitch: Exactly 1.27 mm (SOIC surface-mount packages).
- 0.025″ (25 mil) Pitch: Exactly 0.635 mm (high-density ribbon cable connectors).
Frequently Asked Questions (FAQ)
How many millimeters are in exactly 1 inch?
There are exactly 25.4 millimeters in one international inch. This figure is not an approximation—it is an exact definition agreed upon internationally in 1959.
How do you convert fractional inches (like 5/16″) to millimeters?
First divide the numerator by the denominator to get decimal inches (5 ÷ 16 = 0.3125 inches). Then multiply by 25.4: 0.3125 × 25.4 = 7.9375 mm.
What is the fast mental math trick to convert inches to mm?
Multiply the number of inches by 25, then add approximately 1.6% (or multiply the inches by 0.4 and add to the 25× product). For example, for 4 inches: 4 × 25 = 100, plus 4 × 0.4 = 1.6, giving exactly 101.6 mm.
What is 1/8″ and 1/4″ in millimeters?
1/8 inch equals exactly 3.175 mm. 1/4 inch equals exactly 6.35 mm.
Why do American bolts and metric sockets often not fit together?
While a 1/2″ bolt head is 12.7 mm, a 13 mm metric socket is slightly loose (+0.3 mm play), which risks rounding off fastener corners under high torque. Similarly, a 12 mm socket is 0.7 mm too small to fit over a 1/2″ bolt. Always use matched imperial or metric socket toolsets for high-torque mechanical applications.
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