inch → millimeter | Converter

Convert inch to millimeter with the formula below. 1 in ≈ 25.4 mm

Quick Insight

1 inch = 25.4 millimeter

0.375 in ≈ 9.525 mm; 18 in ≈ 457.2 mm.

💡 Tip: Divide the fraction’s numerator by its denominator, then add any whole inches before converting.

Product dimensions and fractions

Quick Values:

From

25.4

Formula: mm ≈ in × 25.4

Rounded factors

Relative rounding difference; not measurement uncertainty.

2 significant digitsΔ 1.6%

25.4

6 significant digits

25.4

12 significant digits

Visual Comparisons

📏

0.394 in → 10.0076 mm

📏

1 in → 25.4 mm

📏

3.94 in → 100.076 mm

📏

5 in → 127 mm

📏

39.4 in → 1,000.76 mm

Mathematical Facts

📐Conversion formula: mm ≈ in × 25.4

🔄Reverse conversion: in ≈ mm × 0.0393700787402

Convert inch to millimeter with the formula below. mm ≈ in × 25.4. 1 in ≈ 0.0254 m; 1 mm ≈ 0.001 m Compare a product dimension or drawing length with an inch measurement, including fractions that must first be written as decimal values. For product dimensions in in, compare the same dimension: a diagonal, width and height are different lengths. A small object dimension can be recorded in mm. Extra decimal places after conversion do not improve the original measurement. For this direction: 0.375 in ≈ 9.525 mm.

Why This Conversion Matters

A fraction such as 3/8 in represents a part of an inch. Reading its numerator and denominator as decimal digits changes the length before conversion. mm is the smaller unit, so a positive length has a larger numerical value in mm than in in. The calculator uses the full stored factor and rounds displayed results to 12 significant digits. Display precision is not a guarantee of measurement accuracy.

Real-World Examples

Product dimensions and fractions

Compare a product dimension or drawing length with an inch measurement, including fractions that must first be written as decimal values. The following values are illustrative measurements, not product specifications.

Examples:

0.375 in ≈ 9.525 mm
18 in ≈ 457.2 mm

Pro Tips:

✓For a display specification, check whether the stated inch size is the diagonal; do not substitute it for width or height.

Quick Reference Table

inches	millimeters
1 in	25.4 mm
5 in	127 mm
10 in	254 mm
50 in	1,270 mm
100 in	2,540 mm

Formula: mm = in × conversion factor

Common Mistakes to Avoid

Reading an inch fraction as decimal digits

3/8 in is 0.375 in, not 0.38 in. A mixed value such as 1 3/8 in is 1.375 in.

✓ Divide the fraction’s numerator by its denominator, then add any whole inches before converting. For this direction: 0.375 in ≈ 9.525 mm.

Pro Tips

✓Divide the fraction’s numerator by its denominator, then add any whole inches before converting.
✓mm is the smaller unit, so a positive length has a larger numerical value in mm than in in. For this direction: 0.375 in ≈ 9.525 mm.
✓For a display specification, check whether the stated inch size is the diagonal; do not substitute it for width or height.

Sources and unit definitions

Frequently Asked Questions

How do I calculate the conversion?

mm ≈ in × 25.4. 1 in ≈ 0.0254 m; 1 mm ≈ 0.001 m

What is an example?

1 in ≈ 25.4 mm; 10 in ≈ 254 mm; 100 in ≈ 2,540 mm

How should I enter a fractional inch measurement?

3/8 in is 0.375 in, not 0.38 in. A mixed value such as 1 3/8 in is 1.375 in. Divide the fraction’s numerator by its denominator, then add any whole inches before converting. For this direction: 0.375 in ≈ 9.525 mm.

Which exact definitions are used for this pair?

The meter equivalents are 1 in = 0.0254 m; 1 mm = 0.001 m. These definitions are exact; displayed conversion results may be rounded.