Inches to Millimeters Conversion Table for Engineering Use
Converting linear dimensions from inches to millimeters is a routine but critical activity in engineering and manufacturing. This page defines the exact conversion factor, shows the conversion formula, presents integer and common fractional rows for quick lookup, and explains rounding and tolerance considerations for CAD, inspection, and procurement workflows.
Exact conversion factor and conversion formula
The international definition sets one inch equal to exactly 25.4 millimetres. Use that exact factor when converting between units to preserve traceability and compatibility with standards. The basic formula is simple: millimetres = inches × 25.4. For reverse conversions: inches = millimetres ÷ 25.4. When calculations enter dimensioning or tolerance stacks, retain at least one extra digit beyond the working precision before final rounding to avoid cumulative rounding error.
Compact conversion table: integers and common fractions
The table below lists whole-inch values from 0–12 and frequently used fractional-inch values in a single-inch span. Column notes: the “Fraction” column shows the common fractional representation; “Inches (decimal)” gives the exact decimal equivalent for conversion; “Millimetres (exact)” multiplies the decimal inches by 25.4; “Rounded mm” shows a practical rounded value, typically to two decimal places for machining and three decimals for high-precision inspection contexts.
| Fraction | Inches (decimal) | Millimetres (exact) | Rounded mm |
|---|---|---|---|
| 0 | 0.0000 | 0.0000 | 0.00 |
| 1/64 | 0.015625 | 0.396875 | 0.397 |
| 1/32 | 0.03125 | 0.79375 | 0.794 |
| 1/16 | 0.0625 | 1.5875 | 1.59 |
| 1/8 | 0.1250 | 3.1750 | 3.18 |
| 3/16 | 0.1875 | 4.7625 | 4.76 |
| 1/4 | 0.2500 | 6.3500 | 6.35 |
| 3/8 | 0.3750 | 9.5250 | 9.53 |
| 1/2 | 0.5000 | 12.7000 | 12.70 |
| 5/8 | 0.6250 | 15.8750 | 15.88 |
| 3/4 | 0.7500 | 19.0500 | 19.05 |
| 7/8 | 0.8750 | 22.2250 | 22.23 |
| 1 | 1.0000 | 25.4000 | 25.40 |
| 2 | 2.0000 | 50.8000 | 50.80 |
| 3 | 3.0000 | 76.2000 | 76.20 |
| 4 | 4.0000 | 101.6000 | 101.60 |
| 5 | 5.0000 | 127.0000 | 127.00 |
| 6 | 6.0000 | 152.4000 | 152.40 |
| 7 | 7.0000 | 177.8000 | 177.80 |
| 8 | 8.0000 | 203.2000 | 203.20 |
| 9 | 9.0000 | 228.6000 | 228.60 |
| 10 | 10.0000 | 254.0000 | 254.00 |
| 11 | 11.0000 | 279.4000 | 279.40 |
| 12 | 12.0000 | 304.8000 | 304.80 |
Common engineering ranges and appropriate precision
Different manufacturing operations use different length scales and precision targets. For rough woodworking or sheet metal layout, round to the nearest 0.5 mm or 0.1 mm may be acceptable. For general machining, two decimal places (0.01 mm) is a common practical target. For precision grinding, optical measurement, or CMM inspection, three decimal places (0.001 mm) or more may be required. Choose the rounding that matches the process capability and the tolerance zones called out on drawings.
Rounding conventions and significant-figure guidance
Begin rounding decisions by identifying the required tolerance band. Keep at least one guard digit during intermediate calculations to prevent truncation bias. When converting a specified inch dimension to millimetres for a drawing note, round to the same number of significant digits implied by the tolerance statement. Use symmetric rounding (half-to-even) in spreadsheets to reduce systematic rounding drift across many values.
Using conversion tables in CAD and inspection workflows
In CAD, store model units consistently and use the conversion factor only for annotation or drawing dimension calls when the drawing must display both units. For inspection, import or convert coordinate data with software that preserves floating-point precision and documents rounding steps. When generating printed reference tables for shop floors, export as CSV or PDF to retain exact numeric values and ensure consistent decimal formatting across tools.
Measurement constraints and trade-offs
Unit conversion by itself does not change measurement uncertainty. The conversion factor is exact, but measurement error comes from instruments, fixturing, and environmental conditions. When converting and rounding, be mindful that tighter tolerances demand traceable measurement methods and may make rounded values misleading if presented without stated measurement uncertainty. Accessibility considerations include ensuring printed tables use sufficient font size and contrast and that exported digital tables are machine-readable for automated inspection systems.
Which converter format fits CAD workflows?
How to use converter table for inspection?
Where to export mm converter data?
Conversion tables are practical tools for comparing dimensions quickly and aligning cross‑discipline communication. The exact factor, 1 in = 25.4 mm, provides a consistent baseline; combine it with clear rounding rules and an awareness of process capability to maintain specification intent. For critical dimensions, preserve extra precision during calculations, document the rounding approach, and verify converted values against calibrated measurement equipment or drawing callouts before final approval.
This text was generated using a large language model, and select text has been reviewed and moderated for purposes such as readability.
