Fractions, Decimals and Millimeters Conversion Chart | Inch to mm Table

Fractions to Decimals and Millimeters Chart

1/4 inch equals 6.35 mm. 3/8 inch equals 9.53 mm. 1/2 inch equals 12.70 mm. This chart lines up inch fractions, decimal inches, and millimeters in one table so you can convert between all three formats in a single lookup. It is part of the reference charts on this site, alongside the fraction to decimal chart and the decimal to fraction inches chart.

Use this when your tools, plans, and parts mix Imperial and metric units — tape measures in fractions, calipers in decimals, and imported hardware in millimeters.

Use the precision filter to switch between 16ths (general work), 32nds (finish carpentry), and 64ths (machining). For dedicated precision charts, see the 16ths, 32nds, and 64ths pages.

How to Convert Between Fractions, Decimals and Millimeters

Three conversions cover every direction you will need:

• Fraction to decimal: Divide the numerator by the denominator. For example, 5/8 = 5 ÷ 8 = 0.625 inches.
• Decimal inches to millimeters: Multiply the decimal by 25.4. Continuing the example, 0.625 × 25.4 = 15.875 mm.
• Millimeters to inches and fraction: Divide millimeters by 25.4 to get decimal inches, then find the nearest fraction in the chart or use the quick converter above.

Worked example — fraction to mm: Convert 5/8 inch. Divide: 5 ÷ 8 = 0.625. Multiply: 0.625 × 25.4 = 15.875 mm. Look up 5/8 in the chart to confirm: 0.6250 in, 15.88 mm.

Worked example — mm to fraction: Convert 16 mm to inches. Divide: 16 ÷ 25.4 = 0.6299 inches. The nearest fraction in the chart is 5/8 inch (0.6250 in, 15.88 mm). The difference is 0.12 mm — close enough for general woodworking, but verify tolerances for precision work.

You can skip the math by entering values into the quick converter at the top of the chart.

When to Work in Inches vs Millimeters

Most US construction and woodworking uses inches and fractions. Lumber, plywood, drywall, and building hardware are all sized in inches. Tape measures, framing squares, and layout tools are marked in fractions. If your project uses standard building materials, inches are the native system.

Metalworking, manufacturing, and imported components often use millimeters. Drill bits, bearings, Euro cabinet hinges, CNC tooling, and fasteners from metric-system countries are all specified in mm. When you install metric hardware into an Imperial-dimensioned project, you need to cross-reference between the two systems to confirm fit.

Use these rules of thumb when choosing a system:

• Use inches and fractions when matching lumber, plywood, drywall, and standard US building materials.
• Use millimeters for tools, fasteners, and components specified in metric, including CNC tooling and imported hardware.
• Use decimal inches or mm for CAD, CAM, CNC programming, and machining where fractions are impractical.
• Check “close enough” equivalents before substituting one system for the other. A 3/8-inch bolt is not 10 mm — it is 9.53 mm. That gap can matter in a press fit.

Practical Examples Using the Chart

Matching a drill bit to metric specs. You need a 10 mm hole but only have fractional drill bits. Look up 10 mm in the converter: 10 ÷ 25.4 = 0.3937 inches. The nearest fraction is 25/64 inch (0.3906 in, 9.92 mm). For a clearance hole, a 13/32 inch bit (0.4063 in, 10.32 mm) gives slight oversize.

Checking a bolt substitution. A plan calls for a 3/8-inch bolt. You have 10 mm bolts on hand. Look up 3/8 in the chart: 9.53 mm. A 10 mm bolt is 0.47 mm larger in diameter. For through-bolting with clearance, this is usually acceptable. For a threaded insert or tap, use the exact size.

Converting a cabinet panel thickness. European cabinet sides are often 19 mm. Divide: 19 ÷ 25.4 = 0.7480 inches. The nearest fraction is 3/4 inch (0.7500 in, 19.05 mm). The difference is 0.05 mm — effectively identical for cabinetry.

Translating a CAD dimension. Your flooring calculator output shows 0.4375 inches for a plank detail. Scan the chart: 0.4375 = 7/16 inch = 11.11 mm. Mark your tape at 7/16 or set your metric stop to 11.1 mm.

Related Tools

• Fraction to Decimal Chart — Full interactive conversion chart with 16ths, 32nds, and 64ths views plus a quick converter for any fraction.
• Decimal to Fraction Inches Chart — Convert decimal inch values from calculators and software into standard inch fractions with rounding guidance.
• Fraction to Decimal Chart (16ths) — Focused chart for general carpentry where 1/16-inch accuracy is enough.
• Fraction to Decimal Chart (32nds) — Precision chart for finish carpentry and cabinetry.
• Lumber Dimensions Chart — Look up nominal vs actual lumber sizes where fractional and metric dimensions intersect.
• Drywall Calculator — Estimate sheets and materials for drywall projects where 1/2” and 5/8” thickness fractions are standard measurements.