Verification: factors follow standard unit definitions; round for display only.

Inches to millimeters

Convert in to mm when a US drill chart, machinist drawing, fractional hardware catalog, or PCB layout lists inches but you need metric dimensions for CAD models, ISO documentation, CNC toolpaths, or overseas fabrication.

Inches and millimeters both measure length, but inches remain the default in US machining, fractional drill charts, and electronics (where thousandths of an inch are called mils or thou). Since 1959, the international inch has been defined as exactly 25.4 mm. That fixed ratio makes every inch-to-millimeter conversion exact; display rounding is the only source of visible error.

millimeters = inches × 25.4

Multiplication by 25.4 is preferred when you want an exact rational result. Because the inch is legally defined in millimeters, this direction carries no conversion uncertainty— precision limits come from your input measurement, not from the factor.

Step-by-step conversion (worked example 1)

Convert 1 in to millimeters—the exact definition anchor:

1. Write the formula: mm = in × 25.4
2. Multiply: 1 × 25.4 = 25.4 mm exactly
3. Round-trip check: 25.4 ÷ 25.4 = 1 in ✓

Step-by-step conversion (worked example 2)

Convert 0.125 in (1/8″) to millimeters—a common fractional drill size and end-mill diameter:

1. Apply: 0.125 × 25.4 = 3.175 mm
2. Context: 1/8″ appears on machinist scales, tap-drill charts, and router bit catalogs; 3.175 mm is the metric equivalent for CNC post-processors
3. Verify: 3.175 ÷ 25.4 = 0.125 in ✓

Step-by-step conversion (worked example 3)

Convert 0.1 in (100 mil) to millimeters—the standard through-hole IC pitch on many PCBs:

1. Multiply: 0.1 × 25.4 = 2.54 mm
2. In thou: 100 thou = 0.1 in = 2.54 mm—useful when a datasheet mixes mil spacing with metric pad dimensions
3. Round-trip: 2.54 ÷ 25.4 = 0.1 in ✓

Quick mental estimate (no calculator)

For a fast approximation, multiply inches by 25 instead of 25.4. Example: 0.5 in × 25 = 12.5 mm (exact: 12.7 mm—about 1.6% low). For slightly finer rough work, add roughly 1 mm per inch: 0.5 in ≈ 12.5 + 0.2 = 12.7 mm. Use the exact factor when tolerances, drill charts, or PCB land patterns are involved.

Anchor values every converter should match

Where inches → millimeters comes up

• Fractional hardware & drill charts: US drill indexes list sizes in fractional inches (1/64″ steps) and letter/number gauges. Converting 0.1875 in (3/16″) → 4.7625 mm lets you pick the closest metric drill from an overseas supplier without guessing.
• Machinist drawings: Legacy prints dimension in inches with tolerances like ±0.002 in; ISO shops and metric CNC equipment need millimeters. Converting 0.002 in → 0.0508 mm shows the tolerance is 50.8 μm—a critical sanity check before rejecting a part.
• PCB mils (thou): US fab notes and component datasheets often specify trace width, clearance, and pad diameter in mils. Converting 8 mil → 0.008 in → 0.2032 mm aligns imperial PCB rules with metric CAD libraries and DRC checks.
• Metrology & inspection: Dial indicators and micrometers in US shops read thousandths of an inch; quality reports for global customers require millimeters. Converting 0.0005 in → 0.0127 mm preserves sub-thou precision in the final report.

Millimeters to inches

Convert mm to in when a CAD export, PCB datasheet, ISO drawing, or metric supplier cut list lists millimeters but you need inches for a machinist scale, fractional drill chart, or US fabrication quote.

Reverse the inch-to-millimeter process: divide millimeters by 25.4. Because the inch is defined in millimeters, this direction is equally exact—precision limits come from your input measurement, not from the conversion factor.

inches = millimeters ÷ 25.4  ·  equivalently   inches = millimeters × 0.03937007874

Step-by-step conversion (worked example)

Convert 6.35 mm to inches—the metric equivalent of 1/4″ fractional hardware:

1. Write the formula: in = mm ÷ 25.4
2. Divide: 6.35 ÷ 25.4 = 0.25 in exactly
3. Fractional form: 0.25 in = 1/4″—confirms the drill chart entry matches the metric stock
4. Round-trip check: 0.25 × 25.4 = 6.35 mm ✓

Quick mental estimate (reverse direction)

Approximation: divide millimeters by 25 instead of 25.4. Example: 50 mm ÷ 25 = 2 in (exact: 1.9685 in). Good for rough shop checks; use 25.4 for tolerance stacks, drill selection, and CNC post-processors.

Quick reference (mm → in)

Reverse conversion is essential when you receive metric documentation from an overseas vendor but must verify fit against inch-first tooling, or when you normalize mixed-unit bills of material for CAD tools that expect one unit system throughout.

Units compared, common mistakes, and related tools

Both directions share one exact factor. Avoid shortcut errors, verify with anchor points, and keep unit symbols explicit on every drawing, drill chart, and toolpath.

Inches vs millimeters at a glance

Common mistakes to avoid

• Using 25 instead of 25.4—fine for mental estimates, but roughly 1.6% off; unacceptable for CNC, PCB land patterns, or quality inspection.
• Multiplying when you should divide (or vice versa)—in → mm means multiply by 25.4; mm → in means divide by 25.4. A sanity check: millimeters are smaller units, so the same physical length yields a larger mm number than inch number (e.g. 25.4 mm = 1 in, not 25.4 in).
• Confusing mils with millimeters—1 mil = 0.001 in = 0.0254 mm exactly. Treating “25 mil” as 25 mm is a catastrophic order-of-magnitude error on a PCB layout.
• Mixing fractional and decimal inches without converting—3/16″ = 0.1875 in, not 0.316 in. Always reduce fractions before multiplying by 25.4.
• Rounding too early in multi-step work—convert at full precision, then round once for the toolpath or inspection report. Stacked rounding causes fit problems in assemblies and misaligned PCB pads.
• Assuming drill chart numbers are metric—US fractional and letter/number drills are inch-based. A “#7 drill” is 0.2010 in (5.1054 mm), not 7 mm.

Exactness and round-trip verification

The factor 25.4 is exact by international definition, not a measured approximation. Converting 0.25 in → 6.35 mm → 0.25 in should recover the original within floating-point limits. Test anchors: 1 in = 25.4 mm exactly; 0.1 in = 2.54 mm exactly; 1 thou = 0.0254 mm exactly.

Related length converters

For the inverse of this page, open millimeters to inches. For coarser metric/imperial steps, see centimeters to inches, inches to centimeters, centimeters to millimeters, meters to feet, and millimeters to centimeters.