Every woodworking plan you’ll ever follow involves a tape measure, a fraction, and a number that might be confusing at first glance. Measurements sit at the heart of every successful project — get them wrong and nothing fits, get them right and everything clicks into place. This guide covers everything a beginner needs: imperial fractions, metric equivalents, nominal vs actual lumber sizes, and how to handle Australian hardware stores when the plan in your hand is written for North America.
Imperial vs Metric: Which System Are You Working In?
The woodworking world runs on two parallel systems that rarely agree with each other. In Australia, tape measures are sold with metric as the primary scale (millimetres and centimetres), but the overwhelming majority of woodworking plans — particularly from North America where most published plans originate — are written in imperial inches and fractions.
As an Australian beginner, the practical approach is this: keep a metric tape for measuring your space and timber, but learn to read imperial fractions when following a plan. The conversion table below will save you a lot of mental effort.
Reading Imperial Fractions
Imperial measurements divide an inch into fractions: halves, quarters, eighths, and sixteenths. A tape measure’s smallest markings are usually 1/16 inch. Woodworking plans typically use these common fractions:
- 1/8″ — the thickness of a thin sheet or veneer layer
- 1/4″ — plywood backing panels, small dadoes
- 3/8″ — dowel diameter, tenon thickness in lighter joinery
- 1/2″ — common plywood sheet thickness for shelving
- 5/8″ — less common but appears in some cabinet carcass work
- 3/4″ — the most important dimension in woodworking; standard plywood and many solid timber thicknesses
- 1-1/2″ — the actual thickness of a “2×” lumber piece (2×4, 2×6, etc.)
- 3-1/2″ — the actual width of a 2×4
Fraction to Decimal to Millimetre Conversion Table
Use this table whenever a plan gives you a fraction and you need to mark it on metric timber or confirm a measurement in millimetres.
| Imperial Fraction | Decimal Inches | Millimetres (mm) | Nearest AU Timber Size |
|---|---|---|---|
| 1/16″ | 0.0625″ | 1.6 mm | — (tolerance/gap only) |
| 1/8″ | 0.125″ | 3.2 mm | 3 mm (veneer/ply) |
| 3/16″ | 0.1875″ | 4.8 mm | 4.5 mm (ply) |
| 1/4″ | 0.25″ | 6.35 mm | 6 mm (ply) |
| 3/8″ | 0.375″ | 9.5 mm | 9 mm (ply) |
| 1/2″ | 0.5″ | 12.7 mm | 12 mm (ply) |
| 5/8″ | 0.625″ | 15.9 mm | 16 mm (ply) |
| 3/4″ | 0.75″ | 19.05 mm | 19 mm (standard dressed timber) |
| 1″ | 1.0″ | 25.4 mm | 25 mm (ply) or 19 mm dressed |
| 1-1/4″ | 1.25″ | 31.75 mm | 35 mm (nearest dressed) |
| 1-1/2″ | 1.5″ | 38.1 mm | 38 mm (dressed 2× lumber) |
| 1-3/4″ | 1.75″ | 44.5 mm | 45 mm (nearest dressed) |
| 2″ | 2.0″ | 50.8 mm | 50 mm |
| 3″ | 3.0″ | 76.2 mm | 75 mm |
| 3-1/2″ | 3.5″ | 88.9 mm | 90 mm (actual width of 2×4) |
| 5-1/2″ | 5.5″ | 139.7 mm | 140 mm (actual width of 2×6) |
Nominal vs Actual Lumber Sizes
This is the source of more beginner confusion than anything else in woodworking. When a plan says “use a 2×4,” it does not mean the timber is 2 inches by 4 inches. Lumber is sold by its nominal (rough-sawn) size, but by the time it’s dried, planed, and dressed, the actual size is smaller.
Always build your project using the actual dimensions — the nominal size is just a label used at the hardware store.
| Nominal Size (inches) | Actual Size (inches) | Actual Size (mm) | AU Equivalent (dressed pine) |
|---|---|---|---|
| 1×2 | 3/4″ × 1-1/2″ | 19 × 38 mm | 19 × 38 mm |
| 1×3 | 3/4″ × 2-1/2″ | 19 × 63 mm | 19 × 63 mm |
| 1×4 | 3/4″ × 3-1/2″ | 19 × 90 mm | 19 × 90 mm |
| 1×6 | 3/4″ × 5-1/2″ | 19 × 140 mm | 19 × 140 mm |
| 1×8 | 3/4″ × 7-1/4″ | 19 × 184 mm | 19 × 184 mm |
| 1×10 | 3/4″ × 9-1/4″ | 19 × 235 mm | 19 × 235 mm |
| 1×12 | 3/4″ × 11-1/4″ | 19 × 286 mm | 19 × 285 mm |
| 2×2 | 1-1/2″ × 1-1/2″ | 38 × 38 mm | 38 × 38 mm |
| 2×4 | 1-1/2″ × 3-1/2″ | 38 × 90 mm | 38 × 90 mm (or 35 × 90 mm) |
| 2×6 | 1-1/2″ × 5-1/2″ | 38 × 140 mm | 38 × 140 mm |
| 2×8 | 1-1/2″ × 7-1/4″ | 38 × 184 mm | 38 × 184 mm |
| 2×10 | 1-1/2″ × 9-1/4″ | 38 × 235 mm | 38 × 235 mm |
| 4×4 | 3-1/2″ × 3-1/2″ | 90 × 90 mm | 90 × 90 mm |
How to Read a Tape Measure
A standard metric tape measure shows millimetres as the smallest division and centimetres as the numbered markings. On an imperial tape, the numbered marks are inches, and the lines between them represent fractions:
- The longest line between inch marks = 1/2″
- The next longest = 1/4″ and 3/4″
- The medium lines = 1/8″, 3/8″, 5/8″, 7/8″
- The shortest lines = 1/16″ increments
When measuring timber, always hook the tape’s end tab on the end of the board and read from the face side. The end tab has a small amount of intentional play — exactly 1/16″ — that compensates for whether you’re measuring inside or outside a corner.
The Australia-Specific Problem
Most AU hardware stores label timber in millimetres, and the dressed sizes align closely (but not perfectly) with the actual sizes in the conversion tables above. When you’re following an American plan that calls for a 1×6, you need a board that measures 19 mm × 140 mm. That’s exactly what Bunnings stocks as “19 × 140 DAR pine” (Dressed All Round). The same logic applies across the range.
Where you’ll encounter a gap is with plywood. American plans call for 3/4″ plywood (19.05 mm). Australian plywood sheets are typically sold in 17 mm or 18 mm thickness, not 19 mm. This 1–2 mm difference matters for dadoes and rabbets — adjust your router bit depth accordingly.
The cleanest workflow for Australian builders: convert the entire cut list from imperial to millimetres using the table above, then purchase metric-labelled timber. You’ll spend a few minutes on the conversion once, and from that point on everything is in the same system.
If you want plans that come with both imperial and metric dimensions — and detailed cut lists for every project — Ted’s Woodworking is worth bookmarking. 16,000 plans across every skill level, and the measurement conversions are already done for you.
