How do you use this tool?
- Type a value in the Millimeters field, the Inches result updates instantly.
- Or type a value in the Inches field to convert to millimeters.
- For fractional inches (e.g. 3/8"), use the decimal equivalent (0.375) or the fraction input mode.
- Refer to the reference tables for screen sizes, tire widths, and drill bit standards.
What This Tool Does
This converter handles millimeters ↔ inches in both directions using the exact conversion factor. It supports decimal inch output and fractional inch display (to the nearest 1/64”), making it useful for both engineering work (decimals) and carpentry or hardware (fractions).
How Do You Convert Millimeters to Inches?
Inches = Millimeters ÷ 25.4
Millimeters = Inches × 25.4
The inch is defined as exactly 25.4 mm since the International Yard and Pound Agreement of 1959. This makes the conversion between mm and inches perfectly exact, no rounding in the base factor.
What Are Common Screen Sizes?
US monitors and TVs are sold by diagonal inches; manufacturing specs and panel measurements use millimeters.
| Screen Size | Inches (diagonal) | Millimeters (diagonal) | Approx. Width (16:9) |
|---|---|---|---|
| Small monitor | 24 in | 609.6 mm | 530 mm (20.9”) |
| Standard monitor | 27 in | 685.8 mm | 597 mm (23.5”) |
| Large monitor | 32 in | 812.8 mm | 708 mm (27.9”) |
| Widescreen | 34 in | 863.6 mm | 798 mm (31.4”) |
| Small TV | 43 in | 1,092.2 mm | 952 mm (37.5”) |
| Standard TV | 55 in | 1,397.0 mm | 1,218 mm (48.0”) |
| Large TV | 65 in | 1,651.0 mm | 1,439 mm (56.7”) |
| XL TV | 75 in | 1,905.0 mm | 1,661 mm (65.4”) |
| Projector screen | 100 in | 2,540.0 mm | 2,215 mm (87.2”) |
What Are Common Tire Width Values?
US tires are measured in millimeters for width but inches for rim diameter, a hybrid metric/imperial system.
| Tire Width Code | Width in mm | Width in inches |
|---|---|---|
| 195 | 195 mm | 7.68” |
| 205 | 205 mm | 8.07” |
| 215 | 215 mm | 8.46” |
| 225 | 225 mm | 8.86” |
| 235 | 235 mm | 9.25” |
| 245 | 245 mm | 9.65” |
| 255 | 255 mm | 10.04” |
| 275 | 275 mm | 10.83” |
| 305 | 305 mm | 12.01” |
How Do You Convert Drill Bit Sizes?
US workshops stock drill bits in both metric (mm) and imperial (fractional inches) sizes.
| Fraction | Decimal Inches | Millimeters |
|---|---|---|
| 1/16” | 0.0625” | 1.588 mm |
| 3/32” | 0.0938” | 2.381 mm |
| 1/8” | 0.125” | 3.175 mm |
| 5/32” | 0.1563” | 3.969 mm |
| 3/16” | 0.1875” | 4.763 mm |
| 7/32” | 0.2188” | 5.556 mm |
| 1/4” | 0.25” | 6.350 mm |
| 5/16” | 0.3125” | 7.938 mm |
| 3/8” | 0.375” | 9.525 mm |
| 7/16” | 0.4375” | 11.113 mm |
| 1/2” | 0.500” | 12.700 mm |
What Are Common Use Cases?
Electronics and Screen Sizes
When comparing monitors or TVs between US retail listings (in inches) and international or manufacturer specs (in mm or cm), exact conversion prevents buying the wrong bezel size, mount hardware, or desk setup.
Phone screen sizes: the iPhone 16 Pro Max has a 6.9” screen = 175.3 mm diagonal. Replacement screen glass is specified in mm by suppliers.
Woodworking and Carpentry
US lumber dimensions are in inches (nominal and actual), but European hardware, Festool rails, IKEA furniture, European router bits and jigs, uses millimeters. Converting between the two is constant work for anyone using both systems. A 19 mm shelf is 0.748”, close to 3/4” (19.05 mm) but not identical.
Automotive and Tire Specs
Tire width in millimeters is the most visible metric measurement for most US drivers. Wheel offsets and brake rotor specs are also in millimeters. Knowing that 225mm = 8.86” helps when cross-shopping parts or understanding fitment guides.
Screw and Bolt Sizes
Metric screws (M4, M6, M8) are specified in millimeters. US screws use numbered sizes (No. 6, No. 8) or fractional inches. Common conversions:
| Metric | Approx. Imperial |
|---|---|
| M3 (3 mm) | No. 4 screw |
| M4 (4 mm) | No. 8 screw |
| M5 (5 mm) | No. 10 screw |
| M6 (6 mm) | 1/4” |
| M8 (8 mm) | 5/16” |
| M10 (10 mm) | 3/8” |
Photography and Camera Lenses
Lens filter thread sizes are in millimeters (e.g., 77 mm filter thread). Lens focal lengths are in millimeters. Sensor sizes use both systems: a “1-inch” sensor is actually 13.2 × 8.8 mm, the “1 inch” designation is a legacy tube size, not a diagonal measurement.
3D Printing
FDM printers work entirely in millimeters. Designing parts for US hardware (screws, standoffs, enclosures) requires constant mm ↔ inch conversion. A #6-32 screw has a 3.505 mm (0.138”) major diameter and a 0.794 mm (0.03125”) pitch.
What Are Common Fractional Inch Values?
When precision work requires expressing results as fractions of an inch:
| Decimal Inches | Nearest Fraction | mm Equivalent |
|---|---|---|
| 0.0625” | 1/16” | 1.588 mm |
| 0.125” | 1/8” | 3.175 mm |
| 0.1875” | 3/16” | 4.763 mm |
| 0.25” | 1/4” | 6.350 mm |
| 0.375” | 3/8” | 9.525 mm |
| 0.5” | 1/2” | 12.700 mm |
| 0.625” | 5/8” | 15.875 mm |
| 0.75” | 3/4” | 19.050 mm |
| 0.875” | 7/8” | 22.225 mm |
| 1.0” | 1” | 25.400 mm |
Frequently Asked Questions
Why do car tires use millimeters for width but inches for rim diameter?
US tire standards evolved incrementally. Rim diameter remained in inches (a long-established US auto industry standard) while tire cross-section dimensions adopted metric in the 1970s when the US auto industry began sourcing from metric-standard international suppliers. The mixed system stuck.
Is 1 mm equal to 1/25 of an inch?
Close: 1 mm = 1/25.4 inch = approximately 0.03937 inches. Saying “about 1/25 of an inch” introduces a 1.6% error, which matters in precision machining but is acceptable for rough estimation.
What is a mil in manufacturing?
In US manufacturing, a “mil” is 0.001 inch (one thousandth of an inch), not a millimeter. 1 mil = 0.0254 mm. Plastic sheeting, paint coatings, and wire insulation are often specified in mils. This is a common source of confusion in cross-border manufacturing.