Workshops are where unit systems collide hardest: American plans in feet and PSI meet European tools in millimeters and bars. This guide covers the conversions that cause real damage when guessed.
Lumber: nominal vs actual (the classic trap)
A US "2x4" is neither 2 inches nor 4 inches thick. Those are nominal (rough-sawn) dimensions; after planing and drying it measures about 1.5 × 3.5 in (38 × 89 mm). European lumber is sold in actual millimeters: 45 × 95, 45 × 120. When a plan says "2x4 framing", it means spacing and function, not exact size: buy the local equivalent (45 × 90 mm or similar).
| US nominal | US actual (in) | Metric actual (mm) |
|---|---|---|
| 2x4 | 1.5 × 3.5 | 38 × 89 |
| 2x6 | 1.5 × 5.5 | 38 × 140 |
| 1x6 board | 0.75 × 5.5 | 19 × 140 |
| 4x4 post | 3.5 × 3.5 | 89 × 89 |
Board feet and volume pricing
Hardwood is priced in board feet (US/Canada): 1 BF = 144 in³ = a board 12 in × 12 in × 1 in = 2.36 liters. Formula: (thickness_in × width_in × length_ft) ÷ 12. A 1-inch-thick, 8-inch-wide, 10-foot board: (1 × 8 × 10) ÷ 12 = 6.7 BF. Europe prices by m³ or per piece; 1 m³ = 423.8 board feet. Convert precisely with volume tools.
Pressure: PSI, bar, and the compressor sticker
- 1 bar = 14.5 psi = 100 kPa
- Nail guns: 4-8 bar (60-120 psi) typical
- Compressors in Europe max at 8-10 bar; US equivalents quote 115-150 psi, same machines
- Tires: car 2.2-2.5 bar (32-36 psi); bicycle road 5-7 bar (75-100 psi)
- Never exceed a tool's rated pressure; the rating is a burst-safety number, not a target. Convert with bar ↔ psi
Torque: where mistakes break bolts
| Fastener | Nm | lb-ft |
|---|---|---|
| Bicycle stem bolts | 5 | 3.7 |
| Spark plugs | 25-30 | 18-22 |
| Wheel bolts (car) | 110-140 | 81-103 |
| Truck wheel | 600 | 443 |
The fatal confusion: lb-ft vs lb-in (12x different). An inch-pound wrench clicking at "120" is 10 lb-ft, car-wheel territory it has no business touching. Metric-side confusion Nm vs kgf·m (9.81x) is rarer but exists on old Japanese specs.
The small stuff: fasteners and drill bits
- Imperial fractions drill bits: 1/16 in steps = 1.59 mm. Closest metric match to 5/32 in (3.97 mm) is 4 mm, not 4.5 mm
- M-series bolts are named by major diameter: M8 = 8 mm shaft. An 8 mm bolt needs a 14 mm hex or a 6.8-7 mm pilot hole in wood
- Imperial bolt heads measure fractions across flats: 1/2 in hex = 12.7 mm, so a 13 mm wrench fits, and this is why every metric set "works" on most American bolts
- Pipe threads (NPT vs BSP) are NOT interchangeable despite similar diameters; they seal differently
Room and material math
- Paint: 1 L covers ~10 m² (100 sq ft) per coat. A 4×4 m room, walls only: ~40 m² = 2.5 L for two coats, so buy 2.5-3 L
- Concrete: one 25 kg bag ≈ 12 liters of mixed concrete ≈ 0.0125 m³
- Screws: US #8 screw ≈ 4.2 mm metric; #10 ≈ 4.8 mm (M5 territory)
- Area conversions for plans: sq ft ↔ m²
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