Inches to mm Converter

The inch and the millimeter belong to two different measurement systems, yet they meet constantly in engineering drawings, product specifications, woodworking plans, and international manufacturing. Knowing exactly how to convert between them — and how much precision your specific task requires — saves time and prevents costly errors.

This guide covers the exact conversion formula, a complete fractional inches chart, a whole-number table from 1 to 100 inches, and clear guidance on when rounding is acceptable and when it is not.

The Conversion Formula

The formula to convert inches to mm is a single multiplication:

mm = inches × 25.4

To convert mm back to inches, divide by 25.4:

inches = mm ÷ 25.4

Worked examples:

• 3 inches × 25.4 = 76.2 mm
• 12 inches × 25.4 = 304.8 mm
• 0.5 inches × 25.4 = 12.7 mm
• 100 mm ÷ 25.4 = 3.937 inches

Quick mental estimate: Multiply inches by 25 instead of 25.4. This gives a result about 1.5% low. Example: 4 inches × 25 = 100 mm (exact answer is 101.6 mm). Useful for rough checks — never for precision work.

Why 1 Inch = 25.4 mm Exactly

The conversion is not an approximation. It is a legal definition.

On July 1, 1959, representatives from six nations — the United States, the United Kingdom, Canada, Australia, New Zealand, and South Africa — signed the International Yard and Pound Agreement. That agreement defined the yard as exactly 0.9144 meters. Since one inch is 1/36 of a yard, one inch became exactly 0.0254 meters, which equals precisely 25.4 mm.

In practice, the 25.4 mm value had already been widely adopted before 1959. The American Standards Association adopted it in 1933. By 1935, industry across 16 countries used the same figure. The 1959 treaty simply made it the universal legal standard.

The previous US inch had been approximately 25.400 050 8 mm — a difference of about 2 parts per million. For everyday use that gap was invisible, but for precision metrology and scientific work it mattered. The 1959 agreement eliminated it.

Fractional Inches to mm Chart

Most workshop drawings, hardware specs, and imperial tools use fractional inch measurements. This table covers all common fractions in a single-inch span. All values use the exact factor of 25.4.

Whole Inches to mm Table

How Precise Does Your Conversion Need to Be?

The right level of precision depends entirely on what you are building or measuring. Using too little precision in the wrong context causes parts to not fit. Using unnecessary precision adds work without benefit.

General DIY and home improvement

Round to the nearest whole millimeter. A difference of 0.4 mm between 25 mm and 25.4 mm (1 inch) has no practical effect when cutting timber, hanging shelves, or measuring room dimensions.

Woodworking and furniture making

Use one decimal place (0.1 mm precision). Joinery, dovetails, and fitted cabinet work benefit from tighter tolerances. A 0.5 mm gap in a drawer slide is noticeable; 0.1 mm is not.

CNC machining and precision engineering

Use the full 25.4 factor with no rounding during intermediate calculations. Machining tolerances specified in thousandths of an inch (0.001″) convert to hundredths of a millimeter (0.025 mm). Rounding at any earlier step introduces cumulative error that compounds across a stack of tolerances.

Technical drawings and CAD

Store model units in one system only. Apply the conversion factor only at the annotation stage, when the drawing must show dimensions in both units. Round the displayed value to match the precision implied by the tolerance callout on the drawing.

Precision inspection and metrology

Retain at least one additional decimal place beyond your working precision before the final rounding step. This guard digit prevents truncation bias from accumulating across multiple calculated values.

Where Inches to mm Conversion Is Used

The need to convert between inches and millimeters comes up across a wide range of industries and everyday situations.

Engineering and manufacturing

Engineering drawings produced in the United States often use decimal inches. Metric-system countries use millimeters. When US-designed parts are manufactured overseas, or when American engineers work with metric suppliers, every dimension on the drawing requires conversion. A wrong unit in a machined part specification results in a component that does not fit — and a costly remake.

Construction and architecture

The US construction industry works primarily in feet and inches. Building materials sourced internationally, imported fixtures, and European technical standards use millimeters. Architects and contractors regularly convert between the two when specifying pipe diameters, structural member sizes, window openings, and tile dimensions.

Woodworking

Many woodworking plans from the US use fractional inches. Router bits, drill bits, and dowels are often sold in metric sizes in countries outside the US. A woodworker following an American plan with metric tools needs to convert each measurement. The fractional inches chart above covers the values that appear most often.

Electronics and screen sizes

Screen sizes for phones, tablets, monitors, and televisions are stated diagonally in inches internationally, including in countries that otherwise use metric. Circuit board dimensions, component spacing, and PCB trace widths are specified in metric. Engineers working across hardware and display specifications switch between inches and millimeters constantly.

Plumbing and pipe specifications

Pipe thread sizes follow a separate convention. A nominal 1-inch pipe thread does not have an internal diameter of 25.4 mm. Due to historical wall thickness changes, the internal diameter of a 1-inch nominal pipe thread is approximately 30 mm. This is a known exception — when working with pipe threads, always verify against the relevant standard (NPT, BSP, or equivalent) rather than relying on a simple inch-to-mm conversion.

3D printing

3D printing software works in millimeters. Print files derived from inch-based CAD models require conversion of every dimension. A model designed as 2 inches × 3 inches × 1 inch must enter the slicer as 50.8 mm × 76.2 mm × 25.4 mm. Importing the wrong unit scale produces a print that is 25.4 times too large or too small.

Common Conversion Mistakes

1. Multiplying by 2.54 instead of 25.4

Multiplying by 2.54 converts inches to centimeters, not millimeters. It is a 10-times error. A 6-inch measurement would give 15.24 cm (correct) but only 15.24 mm instead of the correct 152.4 mm. Always use 25.4 when your target unit is millimeters.

2. Rounding fractional inches before converting

If a drawing shows 7/16″ and you round it to 0.44 inches before multiplying, you get 11.176 mm instead of the exact 11.1125 mm. Always convert the exact decimal equivalent of the fraction first, then round the result if needed.

3. Assuming 25 mm equals 1 inch

25 mm is 0.984 inches, not 1 inch. The shortcut of multiplying by 25 introduces a 1.57% error. Over short distances this is negligible. Over 10 feet (3,048 mm), the error reaches 48 mm — nearly 2 inches.

4. Rounding at each step in a calculation chain

When converting multiple dimensions in sequence, round only at the final step. Each intermediate rounding introduces a small error. Those errors stack. In a tolerance analysis with five converted dimensions, five rounded intermediate values can produce a cumulative error large enough to cause a fit problem.

3. Treating nominal pipe sizes as direct conversions

Nominal pipe sizes in inches do not convert directly to actual internal diameters in mm using the 25.4 factor. A 1-inch nominal pipe has an internal diameter of approximately 26–30 mm depending on the schedule. Always consult the relevant pipe standard rather than converting the nominal designation.

Frequently Asked Questions

How many mm is 1 inch?

1 inch equals exactly 25.4 mm. This is not an approximation — it is the internationally agreed definition in force since July 1, 1959, established by the International Yard and Pound Agreement signed by six nations.

How do I convert inches to mm?

Multiply the number of inches by 25.4. Example: 6 inches × 25.4 = 152.4 mm. To go the other direction, divide mm by 25.4. Example: 100 mm ÷ 25.4 = 3.937 inches.

How many mm is 1/2 inch?

1/2 inch equals exactly 12.7 mm. Calculated as 0.5 × 25.4 = 12.7.

How many mm is 1/4 inch?

1/4 inch equals exactly 6.35 mm. Calculated as 0.25 × 25.4 = 6.35.

How many mm is 1/8 inch?

1/8 inch equals exactly 3.175 mm. Calculated as 0.125 × 25.4 = 3.175.

Is 25 mm the same as 1 inch?

No. 25 mm equals 0.984 inches. One full inch is 25.4 mm. The 0.4 mm difference is irrelevant for rough estimates but significant in machining, precision fitting, and engineering tolerances.

What is the difference between inches and millimeters?

The inch belongs to the imperial measurement system, used primarily in the United States. The millimeter belongs to the metric system, used by most of the world. One inch equals 25.4 mm. Millimeters are a smaller unit and better suited to precision measurement in engineering, manufacturing, medicine, and science.

How precise does an inches to mm conversion need to be?

It depends on the task. General DIY work tolerates rounding to the nearest whole millimeter. Woodworking benefits from 0.1 mm precision. CNC machining and precision engineering require using the exact 25.4 factor with no intermediate rounding. Technical drawings should match the precision implied by the tolerance callout.