Convert Inches to mm – Exact Conversion Guide & Table

Most people think inches and millimeters are just different units with no fixed relationship, but the truth is precise: one inch equals exactly 25.4 millimeters, a standard set by the National Institute of Standards and Technology (NIST) in 1959. Here’s how to convert any measurement manually, from fractions to decimals, so you never mix up your units again.

1 inch: 25.4 mm exactly ·
1/2 inch: 12.7 mm ·
1/4 inch: 6.35 mm ·
3/4 inch: 19.05 mm ·
20 mm: 0.7874 inches

The key conversion facts are summarized in the table below.

What is 1 inch in mm exactly?

The exact conversion is defined by international agreement: 1 inch equals exactly 25.4 millimeters. This standard has been in place since 1959, when English-speaking countries unified their inch definitions under the international yard and pound agreement. The National Institute of Standards and Technology (NIST) (U.S. measurement authority) codifies this in its Handbook 44, Appendix C, stating that the inch is “exactly 25.4 mm.” Wikipedia (the online encyclopedia) and Inch Calculator (home improvement resource) both confirm the same figure.

What is the exact conversion factor?

The conversion factor between inches and millimeters is a simple multiplication: multiply the number of inches by 25.4 to get millimeters. For example, 2 inches × 25.4 = 50.8 mm. The RapidTables (unit conversion tool) and Analytics-Shop (conversion resource) both provide the same formula. The reverse is also true: 1 mm equals exactly 0.0393701 inches, as listed in the NIST tables.

The implication: this definition is not a convenience — it is the legal foundation of every inch-to-millimeter conversion in the world.

How to manually convert inches to mm?

Manual conversion is straightforward once you know the formula. The key is to handle fractional inches properly — a common pitfall for DIYers and engineers alike.

What is the formula for converting inches to mm?

The formula is: mm = inches × 25.4. For example, to convert 5 inches to mm: 5 × 25.4 = 127 mm. The Inch Calculator calls this “the fastest and easiest way.” Analytics-Shop provides the same formula in a table. For decimal inches, it’s a direct multiplication.

How to convert fractional inches to mm?

Fractional inches require an extra step: convert the fraction to a decimal before multiplying. For instance, 3/4 inch becomes 0.75, then 0.75 × 25.4 = 19.05 mm. The TubingChina (engineering reference) provides a full fractional conversion chart, confirming 3/4 inch = 19.05 mm. The same logic applies to any fraction: 1/2 inch = 0.5 × 25.4 = 12.7 mm, 1/4 inch = 0.25 × 25.4 = 6.35 mm, 3/8 inch = 0.375 × 25.4 = 9.525 mm, and 5/8 inch = 0.625 × 25.4 = 15.875 mm.

The implication: once you master the decimal step, any inch measurement — from 1/16 to 63/64 — becomes a simple multiplication problem.

What is 3/4 inch in mm?

Three-quarters of an inch equals 19.05 millimeters. This is one of the most frequently searched conversions, especially in woodworking, plumbing, and automotive contexts. Below are the common fractional conversions you’ll encounter.

How many mm is 1/2 inch?

One-half inch is 12.7 mm. This is a close match to 13 mm, but the exact measurement matters in precision work. TubingChina lists 1/2 inch as 12.70 mm.

What does 1/4 inch equal in mm?

One-quarter inch is 6.35 mm. Again, the exact value is 6.35, not 6 mm. The RapidTables conversion table starts at 0.01 in = 0.254 mm and scales up.

How many mm is 3/8 inch?

Three-eighths inch is 9.525 mm. And 5/8 inch equals 15.875 mm. These are standard drill bit sizes in engineering.

The pattern: every 1/8 inch interval adds 3.175 mm. 1/8 inch = 3.175 mm, 1/4 inch = 6.35 mm, 3/8 inch = 9.525 mm, 1/2 inch = 12.7 mm, 5/8 inch = 15.875 mm, 3/4 inch = 19.05 mm, 7/8 inch = 22.225 mm, and 1 inch = 25.4 mm.

Is 20 mm the same as 1 inch?

No, 20 mm is not the same as 1 inch. A common misconception — especially among people who work with both metric and imperial fasteners — is that 20 mm is close enough to 1 inch. In reality, 20 mm is approximately 0.7874 inches, while 1 inch is 25.4 mm. The difference is more than 5 mm, which is substantial in tight tolerances.

Is 20 mm the same as 3/4 inch?

Almost, but not exactly. 3/4 inch is 19.05 mm, so 20 mm is about 0.95 mm larger. For many home projects, that difference is negligible, but for precision engineering it matters. The TubingChina chart shows 3/4 inch = 19.05 mm, confirming the gap.

Is 1/2 inch and 12mm the same?

No. 1/2 inch is 12.7 mm, so a 12 mm fastener is slightly smaller. This is a classic mix-up in metric-to-imperial conversions. The difference is 0.7 mm — enough to cause a loose fit when using a 12 mm wrench on a 1/2-inch bolt.

The trade-off: when you’re working with common hardware, knowing the exact conversion prevents stripped threads and ill-fitting parts. Use the formula, not the nearest whole number.

What size is 1 mm in inches?

One millimeter is exactly 0.0393701 inches, according to the NIST tables. For quick mental math, you can approximate 1 mm ≈ 0.04 inches (about 1/25 of an inch).

How to convert mm to inches?

The reverse formula is: inches = mm ÷ 25.4. For example, 20 mm ÷ 25.4 = 0.7874 inches. Analytics-Shop also lists the inverse conversion as mm × 0.03937, which gives the same result.

What is 1 mm in inches exactly?

The exact value is 0.03937007874 inches, but most applications round to 0.03937 inches. The UnitConverters.net (conversion tool) shows the same rounded figure.

The catch: when you’re converting from small metric dimensions to inches, the decimal places matter. A 0.1 mm difference can be critical in machining.

The common fractional conversions follow a consistent pattern.

1. Identify the inch measurement — write it as a decimal if it’s a fraction. For example, 3/4 becomes 0.75.
2. Multiply by 25.4 — the conversion factor. So 0.75 × 25.4 = 19.05 mm.
3. Round if needed — for most practical purposes, two decimal places are sufficient. The RapidTables and UnitConverters.net both provide instant results.

“The inch is defined as exactly 25.4 mm in modern usage after international standardization in 1959.”

Wikipedia (the online encyclopedia)

“ISO 370:1975 specifies methods for converting toleranced dimensions from inches into millimetres using the conversion factor 1 in = 25.4 mm.”

ISO 370:1975 (international standards body)

For anyone working with both metric and imperial tools, the conversion factor of 25.4 is your lifeline. The difference between 12 mm and 1/2 inch may be just 0.7 mm, but in a tight fit, that’s the line between a secure connection and a stripped thread. For the home DIYer, rounding to the nearest whole millimeter is often fine. For the machinist or engineer, the exact number is non-negotiable. Anyone working with both metric and imperial tools must learn the formula and check the table, because a 0.7 mm gap between 12 mm and 1/2 inch can mean the difference between a secure connection and a stripped thread.

For those who also need to work in the other direction, our guide on converting inches to centimeters provides the same level of precision for metric conversions.

Frequently asked questions