mm to Inches Converter: Convert Millimeters to Inches Instantly (Free Tool)
You have a part dimension in millimeters. Your tools are in inches. You need to convert.
Or you are reading a European blueprint. It uses millimeters. Your workshop uses inches.
Millimeters and inches are both units of length. But they come from different systems. The metric system uses millimeters. The US customary system uses inches.
Getting the conversion wrong can ruin your project. A machined part might not fit. A 3D print might be the wrong size.
Today, I give you a free mm to Inches Converter.
You type millimeters. The calculator shows inches. It works both ways.
Let me explain what millimeters and inches are and why they matter.
What are Millimeters and Inches? (Simple Explanation)
Millimeter (mm) is a metric unit of length. One millimeter is one-thousandth of a meter. It is about the thickness of a credit card.
Inch (in) is a US customary unit of length. One inch is 1/12 of a foot. It is about the length of the top part of your thumb.
The relationship:
1 inch = 25.4 millimeters (exactly)
1 millimeter = 0.0393701 inches
Why This mm to Inches Converter Matters
Here is why you need to convert between millimeters and inches.
Reason 1: Machining and manufacturing
CNC machines use millimeters (metric) or inches (US). Blueprints may use either. Convert to program correctly.
Reason 2: 3D printing
STL files use millimeters. Printer software may use inches. Convert to avoid scaling issues.
Reason 3: Automotive
European car parts use millimeters. US tools measure in inches. Convert for proper fit.
Reason 4: Electronics
PCB dimensions are often in millimeters. Component leads are in inches (0.1 inch spacing). Convert to design.
Reason 5: Home improvement
European furniture has dimensions in millimeters. US rooms are measured in inches and feet. Convert to see if it fits.
The Formula (Very Simple)
Millimeters to inches:
inches = mm × 0.0393701
Or more simply:
inches = mm ÷ 25.4
Inches to millimeters:
mm = inches × 25.4
Example 1: 100 mm to inches
100 ÷ 25.4 = 3.937 inches
Example 2: 2 inches to mm
2 × 25.4 = 50.8 mm
Example 3: 1 mm to inches
1 ÷ 25.4 = 0.03937 inches
Example 4: 1 inch to mm
1 × 25.4 = 25.4 mm
LIVE mm to Inches Converter
Type in either box. The other box updates instantly. Watch the ruler move.
📏 mm to Inches Converter
📐 Millimeters (mm)
📏 Inches (in)
📏 mm to Inches Converter
Convert millimeters to inches instantly • Works both ways
📐 Millimeters (mm)
Metric unit (1 mm = 0.03937 in)
⇄
📏 Inches (in)
US customary unit (1 in = 25.4 mm)
📏 VISUAL RULER
0 mm / 0 in
025 mm50 mm75 mm100 mm125 mm
01 in2 in3 in4 in5 in
Common lengths:1 mm (0.039 in)10 mm (0.394 in)1 inch (25.4 mm)50 mm (1.97 in)100 mm (3.94 in)1 m (39.37 in)
💡 1 inch = 25.4 millimeters exactly. 1 millimeter = 0.03937 inches.
⚡ Formula: inches = mm ÷ 25.4 • mm = inches × 25.4
How to Use This mm to Inches Converter
It could not be simpler.
To convert millimeters to inches: Type a number in the Millimeters box. The Inches box updates instantly.
To convert inches to millimeters: Type a number in the Inches box. The Millimeters box updates instantly.
To swap values: Click the swap button (⇄) between the boxes.
Real Examples: Everyday Conversions
Example 1: Machining tolerance
- Tolerance: 0.1 mm
- Convert to inches: 0.1 ÷ 25.4 = 0.00394 inches (about 0.004″)
Example 2: 3D printing
- Print size: 200 mm × 150 mm × 100 mm
- Convert to inches: 7.87″ × 5.91″ × 3.94″
Example 3: Bolt diameter
- M10 bolt: 10 mm diameter
- Convert to inches: 10 ÷ 25.4 = 0.394 inches (about 3/8″)
Example 4: Electronics spacing
- PCB grid: 2.54 mm spacing
- Convert to inches: 2.54 ÷ 25.4 = 0.1 inches (common IC pin spacing)
mm to Inches Conversion Table
| mm | inches | inches (fraction approx) |
|---|---|---|
| 0.5 | 0.0197 | 1/64″ |
| 1 | 0.0394 | 1/32″ |
| 2 | 0.0787 | 5/64″ |
| 3 | 0.1181 | 1/8″ |
| 4 | 0.1575 | 5/32″ |
| 5 | 0.1969 | 3/16″ |
| 6 | 0.2362 | 15/64″ |
| 7 | 0.2756 | 9/32″ |
| 8 | 0.3150 | 5/16″ |
| 9 | 0.3543 | 11/32″ |
| 10 | 0.3937 | 3/8″ |
| 12 | 0.4724 | 15/32″ |
| 13 | 0.5118 | 1/2″ |
| 15 | 0.5906 | 19/32″ |
| 16 | 0.6299 | 5/8″ |
| 18 | 0.7087 | 45/64″ |
| 19 | 0.7480 | 3/4″ |
| 20 | 0.7874 | 25/32″ |
| 22 | 0.8661 | 55/64″ |
| 24 | 0.9449 | 15/16″ |
| 25 | 0.9843 | 63/64″ |
| 25.4 | 1.0000 | 1″ |
| 30 | 1.1811 | 1-3/16″ |
| 40 | 1.5748 | 1-9/16″ |
| 50 | 1.9685 | 1-31/32″ |
| 60 | 2.3622 | 2-23/64″ |
| 70 | 2.7559 | 2-3/4″ |
| 80 | 3.1496 | 3-5/32″ |
| 90 | 3.5433 | 3-35/64″ |
| 100 | 3.9370 | 3-15/16″ |
mm to Inches Fraction Conversion
Many US applications use fractions of an inch, not decimals.
Common fractions:
| Fraction | Decimal (in) | mm |
|---|---|---|
| 1/64″ | 0.015625 | 0.397 |
| 1/32″ | 0.03125 | 0.794 |
| 1/16″ | 0.0625 | 1.588 |
| 1/8″ | 0.125 | 3.175 |
| 3/16″ | 0.1875 | 4.763 |
| 1/4″ | 0.25 | 6.35 |
| 5/16″ | 0.3125 | 7.938 |
| 3/8″ | 0.375 | 9.525 |
| 7/16″ | 0.4375 | 11.113 |
| 1/2″ | 0.5 | 12.7 |
| 9/16″ | 0.5625 | 14.288 |
| 5/8″ | 0.625 | 15.875 |
| 11/16″ | 0.6875 | 17.463 |
| 3/4″ | 0.75 | 19.05 |
| 13/16″ | 0.8125 | 20.638 |
| 7/8″ | 0.875 | 22.225 |
| 15/16″ | 0.9375 | 23.813 |
| 1″ | 1.0 | 25.4 |
Common mm to Inches Conversions in Machining
| mm | inches | Common use |
|---|---|---|
| 1 | 0.039 | Fine tolerance |
| 2 | 0.079 | Small clearance |
| 3 | 0.118 | Small part |
| 4 | 0.157 | Medium-small |
| 5 | 0.197 | Small fastener |
| 6 | 0.236 | Medium-small |
| 8 | 0.315 | Medium |
| 10 | 0.394 | Common size |
| 12 | 0.472 | Medium |
| 16 | 0.630 | Large |
| 20 | 0.787 | Very large |
| 25 | 0.984 | 1″ (close) |
| 25.4 | 1.000 | Exactly 1″ |
Why 25.4 is Exact
The inch was redefined in 1959. The international yard and pound agreement set the yard as exactly 0.9144 meters.
Therefore:
1 yard = 0.9144 m
1 foot = 0.9144 ÷ 3 = 0.3048 m
1 inch = 0.3048 ÷ 12 = 0.0254 m = 25.4 mm
So 1 inch = 25.4 mm exactly. No rounding.
Metric Prefixes for Length
| Unit | mm equivalent | Use |
|---|---|---|
| 1 µm (micrometer) | 0.001 mm | Precision machining |
| 1 mm | 1 mm | Common small parts |
| 1 cm | 10 mm | Everyday objects |
| 1 m | 1,000 mm | Room dimensions |
Frequently Asked Questions (FAQs)
1. How many mm in an inch?
25.4 mm exactly.
2. How many inches in 1 mm?
0.03937 inches.
3. What is 10 mm in inches?
10 ÷ 25.4 = 0.3937 inches.
4. What is 1/2 inch in mm?
0.5 × 25.4 = 12.7 mm.
5. How do you convert mm to inches quickly?
Divide by 25.4.
6. How do you convert inches to mm quickly?
Multiply by 25.4.
7. What is 100 mm in inches?
100 ÷ 25.4 = 3.937 inches.
8. What is 3/4 inch in mm?
0.75 × 25.4 = 19.05 mm.
9. Why is 25.4 the conversion factor?
Because the inch is defined as exactly 25.4 mm by international agreement.
10. What is 5 mm in inches?
5 ÷ 25.4 = 0.1969 inches (about 3/16″).
Common Mistakes
Mistake #1: Using 25.4 for mm to inches incorrectly
mm to inches: divide by 25.4. Inches to mm: multiply by 25.4. Do not reverse.
Mistake #2: Rounding too early
Keep several decimal places until the final step.
Mistake #3: Confusing mm with cm
1 cm = 10 mm. Do not mix them.
Mistake #4: Using approximate fraction conversions for precision work
0.125″ is exactly 1/8″. 3.175 mm is exactly 1/8″. But 3 mm is not exactly 1/8″. Always calculate.
Mistake #5: Forgetting to convert all dimensions
If you convert one dimension but not another, your part will not fit.
mm to Inches in Different Industries
Machining:
- Tolerances often ±0.1 mm (±0.004″)
- Precision parts ±0.01 mm (±0.0004″)
3D Printing:
- Layer height: 0.1-0.3 mm (0.004-0.012″)
- Nozzle diameter: 0.4 mm (0.016″)
Electronics:
- PCB grid: 2.54 mm (0.1″)
- IC pin spacing: 1.27 mm (0.05″)
- SMD components: 0.5-1 mm (0.02-0.04″)
Automotive:
- Bolt sizes: M6 (6 mm), M8 (8 mm), M10 (10 mm)
- Convert to inch equivalents for US tools
Fraction to mm Quick Reference
| Fraction (in) | Decimal (in) | mm |
|---|---|---|
| 1/64 | 0.015625 | 0.397 |
| 1/32 | 0.03125 | 0.794 |
| 1/16 | 0.0625 | 1.588 |
| 1/8 | 0.125 | 3.175 |
| 3/16 | 0.1875 | 4.763 |
| 1/4 | 0.25 | 6.35 |
| 5/16 | 0.3125 | 7.938 |
| 3/8 | 0.375 | 9.525 |
| 7/16 | 0.4375 | 11.113 |
| 1/2 | 0.5 | 12.7 |
| 9/16 | 0.5625 | 14.288 |
| 5/8 | 0.625 | 15.875 |
| 11/16 | 0.6875 | 17.463 |
| 3/4 | 0.75 | 19.05 |
| 13/16 | 0.8125 | 20.638 |
| 7/8 | 0.875 | 22.225 |
| 15/16 | 0.9375 | 23.813 |
| 1 | 1.0 | 25.4 |
Final Thoughts
Converting between millimeters and inches is essential for international work.
My mm to Inches Converter gives you:
- Instant conversion both ways
- Visual ruler
- Common length presets
- Exact 25.4 conversion
Bookmark this page. Use it for machining, 3D printing, electronics, and home improvement.
The next time someone asks “How many inches is 100 mm?”, you will know. 3.937 inches.
Disclaimer: This is an educational tool. For precision machining, use calibrated instruments.
External Links (Authority Backlinks):
- Wikipedia – Millimetre{:target=”_blank” rel=”noopener noreferrer”}
- Wikipedia – Inch{:target=”_blank” rel=”noopener noreferrer”}
- Wikipedia – Metric system{:target=”_blank” rel=”noopener noreferrer”}
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