Navigating Metric and Imperial Measurements in Woodworking
Woodworking spans the globe, and so do its measurement systems. Whether you're following plans from a European maker, using imported hardware, or working with metric plywood, the ability to convert between millimeters and inches is essential for accurate work.
The Fundamental Conversion
The relationship between inches and millimeters is defined exactly:
1 inch = 25.4 millimeters (exactly)
This makes conversion straightforward: multiply inches by 25.4 to get millimeters, or divide millimeters by 25.4 to get inches. However, the results rarely come out to clean numbers in the other system.
Why Measurements Don't Convert Cleanly
Metric lumber dimensions like 19mm or 12mm evolved independently from imperial dimensions like 3/4" or 1/2". They're not designed to be equivalent—they're designed to be round numbers in their respective systems.
For example:
- 3/4 inch = 19.05 mm (not exactly 19mm)
- 19 mm = 0.7480" (not exactly 3/4")
- 1/2 inch = 12.7 mm (not exactly 12mm)
- 12 mm = 0.4724" (not exactly 1/2")
These small differences—about 0.05mm or 0.002" in these cases—usually don't matter for most woodworking. But for precision joinery or when mixing materials from different systems, awareness of these discrepancies prevents problems.
Practical Rounding Guidelines
When converting measurements, round appropriately for your application:
- General woodworking: Round to the nearest 1/16" or 1mm
- Joinery: Keep at least 2 decimal places (0.01") or 0.1mm
- Hardware fitting: Use exact conversions—a 6mm bolt won't fit a 1/4" hole
- Mixed materials: Measure actual dimensions rather than assuming nominal sizes
Common Metric Lumber Sizes
Metric plywood and MDF from European and Asian sources typically comes in these thicknesses:
- 3mm ≈ 1/8" (backer boards, light panels)
- 6mm ≈ 1/4" (cabinet backs, drawer bottoms)
- 12mm ≈ 1/2" (shelving, cabinet sides)
- 18-19mm ≈ 3/4" (structural panels, cabinet carcasses)
- 25mm ≈ 1" (heavy-duty applications)
Note that metric plywoods are often slightly thinner than their imperial "equivalents." An 18mm panel is about 0.71"—noticeably thinner than a true 3/4" (0.75") sheet. This matters when mixing materials or using imperial dado blades.
Hardware Compatibility
Metric and imperial hardware are not interchangeable:
- Screws: M4 ≠ #8, M5 ≠ #10 (similar but different thread pitches)
- Shelf pins: 5mm ≠ 3/16" (close, but binding can occur)
- Drawer slides: Check specifications—some are metric, some imperial
- Hinges: European hinges use metric; American hinges use imperial
When in doubt, drill test holes in scrap and check actual fit before committing to a project piece.
Reading Fractional Inches
For those more familiar with metric, fractional inches can seem archaic. The system divides inches into halves, quarters, eighths, and sixteenths (sometimes 32nds or 64ths for precision work):
- 1/2" = 0.500" = 12.7mm
- 1/4" = 0.250" = 6.35mm
- 1/8" = 0.125" = 3.175mm
- 1/16" = 0.0625" = 1.5875mm
Mixed fractions like 1-3/4" mean one inch plus three-quarters (1.75" or 44.45mm).
Tips for Working Across Systems
- Stick to one system per project: Mixing systems invites errors
- Label drawings clearly: Note which measurement system is used
- Measure, don't assume: Actual material dimensions may vary from nominal
- Keep a conversion reference handy: Use our reference table for quick lookups
- Use digital calipers: They switch between mm and inches instantly
Our measurement converter handles all the math for you. Enter any value—millimeters, decimal inches, or fractional inches—and instantly see the equivalent in all three formats. The reference table shows common lumber dimensions for quick comparison.