Millimeters to Inches Converter
Convert MM to Inches with precision for engineering and design.
How to Convert Millimeters to Inches
Converting millimeters (mm) to inches is critical in engineering, manufacturing, woodworking, and precision work. The millimeter is a metric unit representing one-thousandth of a meter, while the inch is a standard unit in the imperial system.
The Exact Conversion
One inch equals exactly 25.4 millimeters. This internationally standardized definition means that conversions are perfectly precise.
Formula: Inches = Millimeters ÷ 25.4
This relationship was established in 1959 and is used worldwide for precision measurements.
Step-by-Step Example
- Start with millimeters: Let's convert 25 mm to inches.
- Divide by 25.4: 25 ÷ 25.4 = 0.984 inches
- Result: 25 mm is just under 1 inch
Common MM to Inches Conversions
- 1 mm = 0.0394 inches
- 5 mm = 0.197 inches
- 10 mm = 0.394 inches (about 3/8")
- 15 mm = 0.591 inches (about 5/8")
- 20 mm = 0.787 inches (about 3/4")
- 25 mm = 0.984 inches (about 1")
- 25.4 mm = 1 inch exactly
- 50 mm = 1.969 inches (about 2")
- 100 mm = 3.937 inches
Practical Applications
- Engineering: Precision parts often use mm specifications
- Woodworking: Drill bits and screws come in both mm and inches
- Screen bezels: Phone and laptop dimensions
- Camera lenses: Filter diameter in mm
- Jewelry: Ring sizes and gem measurements
- Rainfall: Weather reports may use mm or inches
MM to Fractional Inches
Engineers and machinists often need fractional inch equivalents:
- 3.175 mm = 1/8 inch
- 6.35 mm = 1/4 inch
- 9.525 mm = 3/8 inch
- 12.7 mm = 1/2 inch
- 15.875 mm = 5/8 inch
- 19.05 mm = 3/4 inch
- 22.225 mm = 7/8 inch
Understanding Precision
In precision engineering, measurements are often given to multiple decimal places. Our calculator provides 4 decimal places for accuracy. For general use, 2 decimal places (hundredths of an inch) is usually sufficient.
The "Thou" Unit
In machining, the "thou" (or "mil") refers to one-thousandth of an inch (0.001"). Converting: 1 mm ≈ 39.37 thou. This unit is common in tolerance specifications.
Frequently Asked Questions
How many inches is 10 mm?
10 mm equals approximately 0.394 inches, or roughly 3/8 of an inch.
How many inches is 25 mm?
25 mm equals approximately 0.984 inches, just under 1 inch.
What is 100 mm in inches?
100 mm equals approximately 3.937 inches, or about 4 inches.