Inches to Millimeters Converter
Enter inches to convert to millimeters, or enter millimeters to convert to inches.
Enter a value to see the conversion
How Inches to Millimeters Conversion Works
An inch is an imperial unit of length equal to exactly 25.4 millimeters. This precise, exact relationship was established by the 1959 International Yard and Pound Agreement, which defined the yard as 0.9144 meters, thereby making 1 inch = 25.4 mm with no rounding or approximation. The factor 25.4 is one of the cleanest conversion factors between imperial and metric systems.
According to the National Institute of Standards and Technology (NIST), the inch is defined solely in terms of the meter (and thus the millimeter) for all legal and scientific purposes. The US officially retired the slightly different "survey inch" (used in land surveys) in 2023, making 25.4 mm the only definition of an inch in the United States.
This conversion is essential in engineering (mechanical drawings may specify dimensions in either system), woodworking (US lumber in inches, hardware sometimes in mm), electronics (PCB trace widths in mm, panel cutouts in inches), and CNC machining. According to the BIPM, the metric system is used by over 95% of the world's countries, making the inch-to-mm conversion one of the most frequently needed in international commerce. Our Length Converter handles all length units for complex multi-unit conversions.
The Inches to Millimeters Formula
The conversion formula is exact:
Millimeters = Inches x 25.4
To convert the other direction:
Inches = Millimeters / 25.4
The factor 25.4 is exact by international definition. No rounding is involved. For centimeters: multiply inches by 2.54 (also exact). For meters: multiply inches by 0.0254 (also exact).
Worked example: A US woodworker needs to drill a hole for a 3/8-inch dowel. The metric drill bit set shows sizes in mm. Convert: 3/8 = 0.375 inches. 0.375 x 25.4 = 9.525 mm. The nearest standard metric drill bit is 9.5 mm (slightly undersize for a press fit) or 10 mm (slightly oversize for a clearance fit).
Key Terms You Should Know
- Inch (in or ") -- An imperial unit equal to exactly 25.4 mm. There are 12 inches in a foot and 36 in a yard. The inch is subdivided into fractions (1/2, 1/4, 1/8, 1/16, 1/32, 1/64) or decimals.
- Millimeter (mm) -- A metric unit equal to 1/1,000 of a meter or 1/10 of a centimeter. The standard precision measurement unit in engineering and manufacturing worldwide. There are 25.4 mm in one inch.
- Centimeter (cm) -- Equal to 10 millimeters or 0.3937 inches. One inch = 2.54 cm exactly. Commonly used for clothing measurements and body dimensions.
- Gauge -- A traditional system for measuring wire, sheet metal, and needle thickness. Wire gauges do not have a simple linear relationship to inches or mm. For example, 14-gauge steel is 1.897 mm (0.0747"), while 20-gauge is 0.912 mm (0.0359").
- Tolerance -- The acceptable range of variation from a specified dimension. Expressed as +/- a value in mm or inches. Tight tolerances in precision manufacturing may be +/- 0.01 mm (0.0004 inches).
Inches to Millimeters Reference Table
Fractional and decimal inch equivalents in millimeters. All values exact per the NIST definition.
| Fraction | Decimal (in) | Millimeters |
|---|---|---|
| 1/16" | 0.0625 | 1.5875 |
| 1/8" | 0.1250 | 3.1750 |
| 3/16" | 0.1875 | 4.7625 |
| 1/4" | 0.2500 | 6.3500 |
| 5/16" | 0.3125 | 7.9375 |
| 3/8" | 0.3750 | 9.5250 |
| 1/2" | 0.5000 | 12.7000 |
| 5/8" | 0.6250 | 15.8750 |
| 3/4" | 0.7500 | 19.0500 |
| 7/8" | 0.8750 | 22.2250 |
| 1" | 1.0000 | 25.4000 |
| 2" | 2.0000 | 50.8000 |
| 6" | 6.0000 | 152.4000 |
| 12" (1 ft) | 12.0000 | 304.8000 |
Practical Examples
Plywood thickness: Standard US plywood comes in nominal thicknesses of 1/4", 3/8", 1/2", 5/8", and 3/4". In millimeters: 6.35, 9.525, 12.7, 15.875, and 19.05 mm respectively. However, actual plywood thickness is often slightly less than nominal -- a "3/4-inch" sheet typically measures 23/32" (18.26 mm). European plywood comes in metric sizes: 6, 9, 12, 15, 18, and 21 mm.
Pipe and tubing: US plumbing uses nominal pipe sizes (NPS) in inches, while much of the world uses metric. A 1/2-inch copper pipe has an outer diameter of 15.88 mm. A 15 mm metric pipe is very close but not identical. When connecting US and metric plumbing, adapters are needed because the slight dimensional difference (0.88 mm) prevents a proper seal. Our mm to Inches Converter handles the reverse calculation.
Paper sizes: US Letter paper is 8.5 x 11 inches (215.9 x 279.4 mm). The international A4 standard is 210 x 297 mm (8.27 x 11.69 inches). A4 is slightly narrower but taller than Letter. When printing US documents on A4 paper or vice versa, margins need adjustment. Our Area Converter can help calculate the area difference between paper sizes.
Tips for Converting Inches to Millimeters
- Multiply by 25.4 for exact results: Since the factor is exact, every inch-to-mm conversion produces a precise value. For mental math, multiply by 25 and add 1.6% -- close enough for most estimates.
- Memorize key fractional equivalents: 1/4" = 6.35 mm. 1/2" = 12.7 mm. 3/4" = 19.05 mm. 1" = 25.4 mm. These anchor points help you estimate quickly for common hardware sizes.
- For fractional inches, convert to decimal first: Divide the numerator by the denominator, then multiply by 25.4. Example: 7/16" = 7 / 16 = 0.4375" x 25.4 = 11.1125 mm.
- Use our Length Converter for simultaneous multi-unit results: When you need inches, mm, cm, feet, and meters all at once, the comprehensive converter handles everything in one step.
- Check tool compatibility before substituting: A 12.7 mm metric socket is not quite the same as a 1/2-inch socket (both are 12.7 mm nominally, but manufacturing tolerances differ between metric and imperial tool standards).
US vs. International Standard Sizes
Many everyday items come in different dimensions depending on whether they were designed to US imperial or international metric standards. The table below shows common examples where inches-to-mm conversion is needed.
| Item | US (inches) | Metric (mm) | Interchangeable? |
|---|---|---|---|
| Letter Paper | 8.5 x 11" | 215.9 x 279.4 mm | A4 is 210 x 297 mm (close) |
| 1/2" Bolt | 0.500" | 12.70 mm | No (12 mm or 13 mm differ) |
| 3/4" Plywood | 0.719" actual | 18.26 mm actual | 18 mm metric is close |
| 1/2" Copper Pipe | 0.625" OD | 15.88 mm OD | No (15 mm metric differs) |
| 3.5" Floppy Disk | 3.5" | 88.9 mm | 90 mm (same item) |
Frequently Asked Questions
How many millimeters are in an inch?
There are exactly 25.4 millimeters in one inch. This is an exact definition, not an approximation, established by the 1959 International Yard and Pound Agreement between six English-speaking countries. The agreement defined 1 yard = 0.9144 meters exactly, which gives 1 inch = 25.4 mm exactly. This precise relationship is the foundation for converting between imperial and metric length units.
How do I convert inches to millimeters?
Multiply the inch value by 25.4 to get millimeters. For example, 3 inches x 25.4 = 76.2 mm. For fractional inches, first convert to decimal: 3/8 inch = 0.375 inches, then 0.375 x 25.4 = 9.525 mm. The multiplication by 25.4 is exact, so there is no rounding error in the conversion itself. This is the standard method used by NIST and all international standards organizations.
What is 1/4 inch in millimeters?
One quarter inch (1/4 inch) equals exactly 6.35 millimeters. This is calculated as 0.25 x 25.4 = 6.35 mm. Other common fractional conversions: 1/8 inch = 3.175 mm, 3/8 inch = 9.525 mm, 1/2 inch = 12.7 mm, 3/4 inch = 19.05 mm. These fractional-to-millimeter conversions are essential for US machinists working with metric-specified parts.
What is the difference between inches and millimeters for screen size?
Screen diagonal size is measured in inches globally (a 27-inch monitor, a 6.7-inch phone), while component dimensions like bezel width and pixel pitch are often in millimeters. For example, a 27-inch monitor has a diagonal of 685.8 mm. The iPhone 15 Pro has a 6.7-inch (170.18 mm) diagonal display. Pixel pitch on a 27-inch 4K monitor is about 0.16 mm (0.0063 inches). Both units serve different precision needs in the same product.
How do I convert fractional inches to millimeters?
Convert the fraction to a decimal, then multiply by 25.4. For example, 5/16 inch: 5 divided by 16 = 0.3125, then 0.3125 x 25.4 = 7.9375 mm. Common conversions to memorize: 1/16 inch = 1.5875 mm, 1/8 inch = 3.175 mm, 1/4 inch = 6.35 mm, 1/2 inch = 12.7 mm. Many machinists keep a printed conversion chart at their workbench for quick reference.
Why is the inch defined as exactly 25.4 mm?
The inch was defined as exactly 25.4 mm by the 1959 International Yard and Pound Agreement, signed by the US, UK, Canada, Australia, New Zealand, and South Africa. Before this agreement, different countries had slightly different inch definitions that varied by a few parts per million. The agreement standardized the international inch to simplify trade and engineering. The US retained a slightly different "survey inch" for land surveying, but NIST officially retired the survey inch in 2023.