Convert Inches to Millimeters
Instantly convert Inches (in) to Millimeters (mm) with our free online calculator.
Formula: in to mm — multiply by 25.4
Reference Table
| Inches (in) | Millimeters (mm) |
|---|---|
| 1 | 25.4 |
| 5 | 127 |
| 10 | 254 |
| 25 | 635 |
| 50 | 1270 |
| 100 | 2540 |
Inches-to-millimeters conversion is critical for any US manufacturing operation using international machinery, fasteners, or tooling. 1 inch = 25.4 mm exactly. CNC mills from Japanese or German manufacturers expect mm; 3D printer slicers typically work in mm; and international fastener suppliers list metric equivalents of imperial sizes. A 1/4-inch rod is 6.35 mm — very close to a 6 mm metric rod but not identical, which is why international contract manufacturing requires unambiguous unit specification.
How to Convert Inches to Millimeters
Formula
To convert Inches (in) to Millimeters (mm): multiply by 25.4
Step-by-Step
- Start with your value in Inches (in).
- Multiply by 25.4 to perform the conversion.
- The result is your value expressed in Millimeters (mm).
Conversion Factor
1 in = 25.4 mm
Reverse Factor
1 mm = 0.0393701 in
Worked Example
Convert 25 Inches to Millimeters: 25 in = 635 mm
About Inch (in)
An imperial unit of length equal to exactly 1/12 of a foot = 25.4 mm exactly per the 1959 International Yard and Pound Agreement (and equally per NIST SP 811). Inches are the dominant US working unit for: body height ('5 foot 10' = 70 inches; US adult male mean 175 cm = 69 in per CDC NHANES data); screen and TV diagonal sizes (the ubiquitous '55-inch TV', '24-inch monitor', '15.6-inch laptop' product specs are universal globally even in metric markets); paper margins (US Letter 8.5 × 11 inches; Legal 8.5 × 14 inches per ANSI/NISO Z39.18); plumbing pipe diameters in US construction per ASME B36.10M (1/2-inch, 3/4-inch, 1-inch, 1½-inch nominal pipe sizes); thread sizing per ASME B1.1 / ANSI Unified Inch Screw Thread system (1/4-20 UNC, 1/2-13 UNC, 3/8-16 UNC fasteners by inch diameter × threads-per-inch); woodworking + cabinetry (typical lumber 'nominal 2x4' actually 1.5 × 3.5 inches finished); jewelry chain length (16, 18, 20, 24 inches standard necklace lengths); rifle barrel length (typical 16-22 inches rifle, 26-32 inches shotgun); and tire wheel diameter (US-spec 14-22 inch rim). Despite most industries adopting millimeters, inches remain dominant in US manufacturing, woodworking, plumbing, and most consumer electronics. Convert inches to mm by multiplying by 25.4; to cm by multiplying by 2.54.
About Millimeter (mm)
A metric unit of length equal to exactly 0.001 m = 10⁻³ m (about the thickness of a credit card or 1/16 of an inch). The millimeter is the default unit in mechanical engineering drawings worldwide per ISO 128 (technical drawing) + ISO 286 (GD&T tolerances): tight machining tolerances are typically ±0.01 to ±0.10 mm; press-fit interference fits ±0.025 mm class IT6; semiconductor wafer flatness < 0.001 mm = 1 μm. mm is also the working unit for: rainfall measurement per WMO (1 mm rain = 1 L/m² per AMS Glossary; heavy rainfall warning ≥ 25 mm/24h in most jurisdictions); camera lens focal lengths (typical full-frame 24-70 mm zoom; portrait 85 mm; telephoto 200-600 mm; macro 100 mm); bullet calibers outside the US (9 mm Parabellum, 7.62 mm AK-47 + NATO, 5.56 mm M16); tire sidewall heights and rim widths (e.g., 225/45R17 means 225 mm wide × 45% aspect-ratio sidewall × 17-inch rim diameter); precision machining + 3D printing layer heights (typical FDM 0.1-0.3 mm; SLA 0.025-0.1 mm); and lumber and architectural-millwork specs in DIN-standard markets. Convert mm to inches by dividing by 25.4; to cm by dividing by 10.
Quick Facts
- 1 Inch equals 25.4 Millimeters
- 1 Millimeter equals 0.0393701 Inches
- Inch is a unit of length & distance
- Millimeter is a unit of length & distance
- This conversion is commonly used in construction, navigation, athletics, and everyday measurement
- The Inch belongs to the imperial system
- The Millimeter belongs to the metric system
Common Inch to Millimeter Conversions
| Inches (in) | Millimeters (mm) |
|---|---|
| 0.1 | 2.54 |
| 0.5 | 12.7 |
| 1 | 25.4 |
| 2 | 50.8 |
| 3 | 76.2 |
| 4 | 101.6 |
| 5 | 127 |
| 10 | 254 |
| 15 | 381 |
| 20 | 508 |
| 25 | 635 |
| 30 | 762 |
| 40 | 1016 |
| 50 | 1270 |
| 75 | 1905 |
| 100 | 2540 |
| 150 | 3810 |
| 200 | 5080 |
| 250 | 6350 |
| 500 | 12700 |
| 1000 | 25400 |
Understanding Inches
The Inch (symbol: in) is a unit of length & distance. An imperial unit of length equal to exactly 1/12 of a foot = 25.4 mm exactly per the 1959 International Yard and Pound Agreement (and equally per NIST SP 811). Inches are the dominant US working unit for: body height ('5 foot 10' = 70 inches; US adult male mean 175 cm = 69 in per CDC NHANES data); screen and TV diagonal sizes (the ubiquitous '55-inch TV', '24-inch monitor', '15.6-inch laptop' product specs are universal globally even in metric markets); paper margins (US Letter 8.5 × 11 inches; Legal 8.5 × 14 inches per ANSI/NISO Z39.18); plumbing pipe diameters in US construction per ASME B36.10M (1/2-inch, 3/4-inch, 1-inch, 1½-inch nominal pipe sizes); thread sizing per ASME B1.1 / ANSI Unified Inch Screw Thread system (1/4-20 UNC, 1/2-13 UNC, 3/8-16 UNC fasteners by inch diameter × threads-per-inch); woodworking + cabinetry (typical lumber 'nominal 2x4' actually 1.5 × 3.5 inches finished); jewelry chain length (16, 18, 20, 24 inches standard necklace lengths); rifle barrel length (typical 16-22 inches rifle, 26-32 inches shotgun); and tire wheel diameter (US-spec 14-22 inch rim). Despite most industries adopting millimeters, inches remain dominant in US manufacturing, woodworking, plumbing, and most consumer electronics. Convert inches to mm by multiplying by 25.4; to cm by multiplying by 2.54.
It belongs to the imperial measurement system.
Inches are commonly used in construction, navigation, athletics, and everyday measurement.
Understanding Millimeters
The Millimeter (symbol: mm) is a unit of length & distance. A metric unit of length equal to exactly 0.001 m = 10⁻³ m (about the thickness of a credit card or 1/16 of an inch). The millimeter is the default unit in mechanical engineering drawings worldwide per ISO 128 (technical drawing) + ISO 286 (GD&T tolerances): tight machining tolerances are typically ±0.01 to ±0.10 mm; press-fit interference fits ±0.025 mm class IT6; semiconductor wafer flatness < 0.001 mm = 1 μm. mm is also the working unit for: rainfall measurement per WMO (1 mm rain = 1 L/m² per AMS Glossary; heavy rainfall warning ≥ 25 mm/24h in most jurisdictions); camera lens focal lengths (typical full-frame 24-70 mm zoom; portrait 85 mm; telephoto 200-600 mm; macro 100 mm); bullet calibers outside the US (9 mm Parabellum, 7.62 mm AK-47 + NATO, 5.56 mm M16); tire sidewall heights and rim widths (e.g., 225/45R17 means 225 mm wide × 45% aspect-ratio sidewall × 17-inch rim diameter); precision machining + 3D printing layer heights (typical FDM 0.1-0.3 mm; SLA 0.025-0.1 mm); and lumber and architectural-millwork specs in DIN-standard markets. Convert mm to inches by dividing by 25.4; to cm by dividing by 10.
It belongs to the metric measurement system.
Millimeters are commonly used in construction, navigation, athletics, and everyday measurement.
Why Convert Inches to Millimeters?
Whether you are travelling internationally, working on a construction project, or studying science, converting between Inches and Millimeters is a task you will encounter regularly. Builders and architects often work with specifications that mix metric and imperial units, while athletes and coaches may need to compare race distances reported in different systems. Having an accurate, instant conversion tool removes guesswork and reduces measurement errors that can be costly in professional settings.
Frequently Asked Questions
How do I convert Inches to Millimeters?
An imperial unit of length equal to exactly 1/12 of a foot = 25. To convert Inches to Millimeters, multiply by 25.4. For example, 25 in equals 635 mm.
How many Millimeters are in 1 Inch?
There are 25.4 Millimeters in 1 Inch.
How many Inches are in 1 Millimeter?
There are 0.0393701 Inches in 1 Millimeter.
What is the formula for Inch to Millimeter conversion?
The formula is: multiply by 25.4. This means 1 in = 25.4 mm.
Is a Inch bigger than a Millimeter?
No. One Inch is smaller than one Millimeter because 1 in equals 25.4 mm, which is greater than 1.
When do you need to convert between Inches and Millimeters?
A metric unit of length equal to exactly 0. Inch and Millimeter are both length units, so conversion comes up whenever one source of information uses one unit and another uses the other — a classic cross-reference challenge in engineering, trade, travel, and everyday life.