North American Lumber Sizes
Understanding the difference between nominal and actual lumber dimensions is crucial for accurate project planning.
Why the Difference?
Lumber is sold by nominal size (e.g., 2×4), but the actual dimensions are smaller after planing and surfacing. Always use actual dimensions when planning your woodworking projects to avoid costly mistakes.
Plywood & Sheet Goods
1/4" Plywood
Actual: 6.35mm
Metric: 6mm
1/2" Plywood
Actual: 12.7mm
Metric: 12mm
5/8" Plywood
Actual: 15.9mm
Metric: 16mm
3/4" Plywood
Actual: 19.05mm
Metric: 18mm
How to Read a Fractional Inch Ruler
Understanding ruler markings is essential for accurate woodworking measurements.
Ruler Marking Guide
1/2" marks (longest lines)
Major divisions
1/4" marks (medium lines)
Quarter inch divisions
1/8" marks (shorter lines)
Eighth inch divisions
1/16" marks (shortest lines)
Finest common divisions
✓ Common Measuring Tools
- • Tape Measure - standard 25ft or 7.5m
- • Combination Square - for precise angles
- • Digital Calipers - switch mm/inch modes
- • Folding Rule - traditional carpenter's tool
💡 Pro Tips
- • Always measure from the 1" mark, not the end
- • Use the same tape for the entire project
- • Mark with a sharp pencil for accuracy
- • Measure twice, cut once!
Project-Specific Conversion Guides
Tailored measurement guidelines for different woodworking projects.
Furniture Making
Common Table Heights
- • Dining table: 29-30" (737-762mm)
- • Coffee table: 16-18" (406-457mm)
- • Desk: 28-30" (711-762mm)
- • Counter height: 36" (914mm)
Joinery Measurements
- • Mortise depth: 1/3 to 1/2 stock thickness
- • Tenon thickness: 1/3 of stock
- • Dovetail angle: 1:6 for softwood, 1:8 hardwood
- • Drawer slide clearance: 1/32" (0.8mm)
Hardware & Tool Conversions
Essential conversion charts for screws, drill bits, and router bits.
Drill Bit Sizes
| Fraction | Decimal (in) | Metric (mm) | Common Use |
|---|---|---|---|
| 1/16" | 0.0625 | 1.6mm | Pilot holes for small screws |
| 1/8" | 0.125 | 3.2mm | Small screws, dowels |
| 3/16" | 0.1875 | 4.8mm | #8 screw pilot holes |
| 1/4" | 0.25 | 6.4mm | #12 screws, bolts |
| 5/16" | 0.3125 | 7.9mm | Dowels, large screws |
| 3/8" | 0.375 | 9.5mm | Large screws, bolts |
| 1/2" | 0.5 | 12.7mm | Bolts, large holes |
Wood Screw Sizes
#4
2.8mm
#6
3.5mm
#8
4.2mm
#10
4.8mm
#12
5.5mm
1/4"
6.35mm
Router Bit Shank Sizes
1/4" Shank
= 6.35mm
Small routers, trim work, light-duty applications
1/2" Shank
= 12.7mm
Standard routers, heavy-duty work, reduced vibration
8mm Shank
= 0.315"
Metric routers, European tools, requires 8mm collet
Woodworking Tolerance Guide
Understanding how precise your measurements need to be for different types of work.
Rough Carpentry
±1/16"
(1.6mm)
Framing, deck building, rough construction
Cabinet Making
±1/32"
(0.8mm)
Face frames, cabinet boxes, door frames
Fine Furniture
±1/64"
(0.4mm)
Joinery, drawer fitting, precision work
Instrument Making
±0.001"
(0.025mm)
Musical instruments, specialty work
💡 Golden Rule
Measure twice, cut once! Always verify critical dimensions before cutting. Use the same measuring tool throughout your entire project to maintain consistency.
Wood Thickness Standards
Understanding rough lumber thickness versus finished dimensions.
Why S2S (Surfaced 2 Sides)?
Planing and surfacing removes approximately 1/4" of material to create smooth, flat surfaces. S2S lumber has both faces surfaced, while S4S (surfaced 4 sides) has all four sides planed smooth. Always specify the finished thickness you need when ordering lumber.
Frequently Asked Questions
Why do European and American lumber sizes differ?
The differences stem from historical measurement systems and manufacturing standards. North American lumber uses nominal sizing (e.g., 2×4 is actually 1.5" × 3.5"), which accounts for planing and surfacing in the nominal dimension. European lumber uses actual metric dimensions (e.g., 50×100mm is close to the actual finished size). This can cause confusion when working with international plans or importing materials.
How do I convert metric router bits to imperial?
Common conversions:
- 6mm ≈ 1/4" (actually 6.35mm, but 6mm bits fit 1/4" collets with minimal clearance)
- 8mm ≈ 5/16" (close fit, some wobble possible - not recommended)
- 12mm ≈ 1/2" (actually 12.7mm, use proper 1/2" collet for safety)
Best practice: Use router bits designed for your router's collet system. Never force metric bits into imperial collets as this can be dangerous.
Can I use metric measuring tools for imperial woodworking plans?
Yes, but follow these guidelines:
- Convert plans first: Use this converter to change all dimensions to metric
- Work in one system: Stick to either metric or imperial throughout the entire project
- Keep a reference chart: Post common conversions in your workshop
- Round sensibly: Use practical increments (5mm, 10mm, etc.)
- Double-check critical dimensions: Verify measurements with both systems for important cuts
What's the best way to convert a European woodworking plan?
Follow this step-by-step process:
- Convert all dimensions using the calculator above
- Round to practical fractions (1/16" or 1/8" typically work best)
- Adjust for lumber availability - European sizes may not match North American stock dimensions
- Check hardware compatibility - metric screws vs imperial screws may differ
- Note critical dimensions where precision matters most
- Create a new cut list with all measurements in your preferred system
How accurate should measurements be for different types of joints?
Required tolerances by joint type:
- Butt joints: ±1/16" - gaps can be filled with wood filler
- Pocket hole joints: ±1/32" - affects alignment and fit
- Mortise & tenon: ±1/64" - must fit snugly for strength
- Dovetails: ±0.005" - hand-fit precision required
- Drawer slides: ±1/32" - ensures proper operation
- Miter joints: Angle accuracy more important than dimension
How do I read a dual-scale (metric/imperial) tape measure?
Tips for dual-scale tape measures:
- Top edge: Usually inches (fractional markings)
- Bottom edge: Usually centimeters and millimeters
- Don't mix scales: Read from one edge only for accuracy
- Mark clearly: Indicate which measurement system on your cut list
- Common mistake: Reading 30cm as 30 inches (actually 11.8")
- Pro tip: Use painter's tape to mark the scale you're using