Inches to Millimeters Converter (in to mm)

Convert inches to millimeters using the calculator above. Enter a measurement in inches to see the equivalent length in millimeters.

One inch is exactly equal to 25.4 millimeters.

How to Convert Inches to Millimeters

Multiply the measurement in inches by 25.4:

Millimeters = Inches × 25.4

Example: Convert 5 inches to millimeters

Multiply 5 by 25.4:

5 in × 25.4 = 127 mm

Therefore, 5 inches equals 127 millimeters.

Accuracy and Rounding

The converter calculates through each unit’s relationship to the meter. Some results terminate exactly, while others continue as non-terminating decimals and must be rounded for display.

Keep the full conversion factor during intermediate calculations. Round only the final answer to the precision justified by the original measurement.

Common Inches to Millimeters

Fractional Inches to Millimeters

Inch measurements are often written as fractions, especially on rulers, tools, screws and building materials. Use this chart to find the metric equivalent of common fractions.

How to Convert a Fraction of an Inch

First divide the numerator by the denominator to turn the fraction into a decimal. Then multiply the decimal value by 25.4.

Example: Convert 3/4 inch to millimeters

Convert the fraction to a decimal:

3 ÷ 4 = 0.75 in

Now multiply by 25.4:

0.75 × 25.4 = 19.05 mm

Why Are Pipe Sizes Given in Inches and Millimetres?

Pipe specifications commonly use two sizing systems: Nominal Pipe Size (NPS), expressed with inch-based numbers, and Nominal Diameter (DN), expressed with metric numbers. For example, a pipe described as NPS 2 is generally identified as DN 50 in the metric system.

These numbers identify compatible pipe sizes, fittings and valves, but they do not necessarily state the pipe's exact diameter. This is why converting a nominal pipe size is different from performing an ordinary inches to millimetres calculation.

Understanding NPS and DN Pipe Sizes

NPS means Nominal Pipe Size. It is the sizing system commonly used for pipes manufactured base on North American standards. Although NPS sizes are traditionally associated with inches, the number is a nominal designation rather than an exact physical measurement.

DN means nominal diameter and is used in international and metric standards. DN values are written as whole numbers without a millimetre symbol. For example, the correct designation is DN 50, not DN 50 mm.

NPS 2 corresponds to DN 50, but neither of them gives the pipe's exact outside diameter. A standard NPS 2 pipe has an outside diameter of approximately 60.3 millimetres, or 2.375 inches.

Why is the stated pipe size different from its actual diameter?

Early pipe sizes were associated approximately with the internal space available for carrying water, steam or gas. As manufacturing methods improved, pipes could be made with thinner walls while retaining suitable strength. Standards kept the familiar nominal names so that pipes would remain compatible with existing fittings, flanges, valves and etc.

For many standard pipes, the outside diameter remains fixed while the wall thickness changes. The wall-thickness category is identified by the pipe's schedule, such as Schedule 10, Schedule 40 or Schedule 80. A thicker wall produces a smaller inside diameter even though the NPS and outside diameter remain unchanged.

Common Pipe Sizes in Inches and Millimetres

The table contains nominal designations and commonly specified outside diameters. It should not be used to assume a pipe's inside diameter or wall thickness.

A Brief History of Pipe Sizing

Inch-based pipe terminology developed as industrial piping expanded in Britain and North America. Manufacturers needed standard sizes so that pipes produced by one company could connect to fittings made by another. These established sizes remained in use as plumbing, steam systems and industrial pipelines became more widespread.

Metric pipe designations became increasingly important as international engineering and manufacturing developed. The DN system provided metric names corresponding approximately to established NPS sizes. Rather than replacing every existing size, the two systems were aligned through recognised nominal equivalents.

Today, an engineering drawing may use DN while product catalogues, imported equipment or North American specifications use NPS. Knowing both systems helps contractors, builders and engineers select compatible components across different markets.

Where Are Inch-to-Millimetre Pipe Comparisons Used?

Comparing inch and metric pipe sizes is useful in domestic plumbing, heating systems, water treatment, oil and gas installations, chemical processing, fire-protection systems and industrial maintenance. It is especially important when connecting imported equipment or replacing a component made to a different regional standard.

Always check the complete specification before buying a pipe or fitting. Material, schedule, pressure rating, thread standard and connection type can affect compatibility. Two products carrying similar inch and metric descriptions are not automatically interchangeable.

Measurement Standards and Further Reading

The inch is defined as exactly 25.4 millimeters. For authoritative information about length units, see the BIPM SI Brochure and NIST guidance on SI length units.