Woodworking Unit Converter
Convert inches to mm, fractions to decimals, hardwood quarter-thickness to actual dimensions, area to sheet counts, and estimate project weight by species — all in one tool built for woodworkers.
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Board feet, lumber cost, compound miter angles, and unit conversion — all four calculators in one hub. Once you have your dimensions converted, use the board feet calculator to work out your lumber order quantity.
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On this page: Converter · Fraction reference · Hardwood thickness guide · Metric vs imperial in woodworking · FAQs
Unit converter
Enter a value in any field — all others update instantly. Use the fraction helper below for tape-measure readings.
Pick a whole-inch measurement and a fraction from the dropdowns to see the decimal and metric equivalents instantly.
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Fraction to decimal to mm — complete reference table
The single most useful table in a woodworking shop. Imperial plans use fractions; machines and digital calipers use decimals; European plans and most CNC software use millimetres. Keep this table bookmarked or print it and pin it above the bench.
| Fraction | Decimal inches | Millimetres | Common use |
|---|---|---|---|
| 1/64″ | 0.0156″ | 0.397 mm | Finest router bit step |
| 1/32″ | 0.0313″ | 0.794 mm | Tight tolerance joinery |
| 1/16″ | 0.0625″ | 1.588 mm | Scroll saw blade clearance |
| 1/8″ | 0.125″ | 3.175 mm | Small mortise width, drawer gap |
| 3/16″ | 0.1875″ | 4.763 mm | Standard shelf pin hole |
| 1/4″ | 0.25″ | 6.35 mm | Cabinet back panel, dado width |
| 5/16″ | 0.3125″ | 7.938 mm | Pocket hole, 8mm mortise approx |
| 3/8″ | 0.375″ | 9.525 mm | Tenon thickness, spline slot |
| 7/16″ | 0.4375″ | 11.113 mm | Hinge recesses |
| 1/2″ | 0.5″ | 12.7 mm | Plywood, MDF sheet thickness |
| 9/16″ | 0.5625″ | 14.288 mm | — |
| 5/8″ | 0.625″ | 15.875 mm | Cabinet side wall thickness |
| 11/16″ | 0.6875″ | 17.463 mm | — |
| 3/4″ | 0.75″ | 19.05 mm | Standard plywood, 4/4 S4S hardwood |
| 13/16″ | 0.8125″ | 20.638 mm | — |
| 7/8″ | 0.875″ | 22.225 mm | 4/4 S2S hardwood actual thickness |
| 15/16″ | 0.9375″ | 23.813 mm | — |
| 1″ | 1.000″ | 25.4 mm | 4/4 rough nominal, full-inch stock |
| 1 1/4″ | 1.250″ | 31.75 mm | 5/4 rough nominal |
| 1 1/2″ | 1.500″ | 38.1 mm | 6/4 rough nominal, 2×4 actual width |
| 1 3/4″ | 1.750″ | 44.45 mm | 8/4 S4S typical actual |
| 2″ | 2.000″ | 50.8 mm | 8/4 rough nominal |
| 3 1/2″ | 3.500″ | 88.9 mm | 2×4 actual width |
| 5 1/2″ | 5.500″ | 139.7 mm | 2×6 actual width, 1×6 actual width |
Bold rows are the most frequently needed values in a typical shop. Use the converter above to calculate any value not listed here.
Hardwood thickness guide: quarters notation explained
Hardwood lumber is sold by the quarter-inch of rough-sawn thickness, described as a fraction with 4 as the denominator — "four-quarter", "five-quarter", and so on. This notation is confusing at first because the size you receive after surfacing is significantly smaller than the number suggests.
The quarter-inch system describes the thickness of the board as it left the saw at the mill — before drying, before planing. By the time you buy it at a hardwood dealer, a 4/4 board has already lost about 1/16″ to 1/8″ in drying and preliminary flattening. Then you plane it further in the shop.
| Notation | Rough nominal | Typical S2S | Typical S4S | mm (rough) | Notes |
|---|---|---|---|---|---|
| 4/4 | 1″ | 7/8″ (22.2 mm) | 3/4″ (19.1 mm) | 25.4 mm | Most common. Plan S4S at 3/4″ for cabinet work. |
| 5/4 | 1-1/4″ | 1-1/8″ (28.6 mm) | 1-1/16″ (27.0 mm) | 31.75 mm | Good for table tops and stair treads. |
| 6/4 | 1-1/2″ | 1-3/8″ (34.9 mm) | 1-1/4″ (31.8 mm) | 38.1 mm | Chair legs and turned spindles. |
| 8/4 | 2″ | 1-7/8″ (47.6 mm) | 1-3/4″ (44.5 mm) | 50.8 mm | Thick tops, legs, post stock. |
| 10/4 | 2-1/2″ | 2-3/8″ (60.3 mm) | 2-1/4″ (57.2 mm) | 63.5 mm | Heavy timber, workbench slabs. |
| 12/4 | 3″ | 2-7/8″ (73.0 mm) | 2-3/4″ (69.9 mm) | 76.2 mm | Thick slab work, large turning blanks. |
| 16/4 | 4″ | 3-7/8″ (98.4 mm) | 3-3/4″ (95.3 mm) | 101.6 mm | Live-edge slabs, large timber. |
S2S vs S4S for planning: use S4S thickness when designing joinery (mortise depth, dado width, panel thickness). Use rough nominal when calculating board feet and ordering lumber — the yard's price is based on the rough thickness, not the surfaced result. The surfacing cost may be billed separately or included; check with your dealer.
Metric vs imperial in woodworking: which to use
The answer depends on where your plans come from, what machinery you are using, and who will build from your drawings. There is no universally correct answer — but understanding when each system is more natural prevents constant conversion errors at the bench.
When imperial (inches and fractions) works better
Imperial fractions align naturally with standard lumber dimensions in North America. A 4×8 sheet of plywood is exactly 48″×96″. Standard lumber is 3-1/2″×1-1/2″ for a 2×4. Pocket screws, hinges, and hardware are specified in fractions. If you are building from an American plan, working with American-made hardware, and buying from a North American lumber yard, staying in imperial throughout avoids a conversion layer entirely.
When metric (mm) works better
Metric excels for precision layout, CNC toolpath programming, and European cabinetry plans. All modern CNC routers and laser cutters accept mm natively. European drawer slides, hinges, and system hardware are specified in millimetres. Adding and subtracting whole millimetres is faster mental arithmetic than working with fractions. If your project plans come from a European source (Blum, Häfele, IKEA-compatible cabinetry), working in mm from the start is cleaner.
Mixed-unit projects
Most real woodworking shops work in both. The practical approach: pick one system as the primary for a given project, convert any external dimensions to that system at the start, and do all layout in one unit. The converter above is fastest when used this way — a single conversion at the start, not repeated mid-project. The three values that are worth memorising because they come up constantly: 1″ = 25.4 mm, 3/4″ = 19.05 mm, 1/2″ = 12.7 mm.
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Unit converter FAQs
How do you convert inches to millimetres?
Multiply by 25.4 (exact, by international definition). So 3/4″ = 0.75 × 25.4 = 19.05 mm. To go the other way, divide mm by 25.4. The converter above handles this bidirectionally — type in either field.
What does 4/4 mean for hardwood lumber?
4/4 (four-quarter) means the board's rough-sawn thickness is 1 inch (four quarter-inches). After surfacing four sides (S4S), the actual usable thickness is typically 3/4″ (19.05 mm). After surfacing two sides (S2S) it is typically 7/8″. Use the hardwood thickness tab above for all standard quarter notations.
How do I convert a tape measure fraction to decimal inches?
Divide numerator by denominator: 3/8 = 0.375″, 7/16 = 0.4375″, 3/4 = 0.75″. For a whole-number plus fraction reading (e.g. 2-3/8″), add the decimal fraction to the whole: 2 + 0.375 = 2.375″. The fraction helper in the length converter tab does this automatically.
How many 4×8 sheets do I need?
One 4×8 sheet = 32 sq ft. Divide your total area by 32 and round up. For 85 sq ft: 85 ÷ 32 = 2.66, round up to 3 sheets. Add 10–15% for waste and miscuts. The area converter tab calculates this automatically.
What is S2S and S4S hardwood?
S2S (surfaced two sides) = top and bottom faces planed flat and parallel, edges still rough. S4S (surfaced four sides) = all four faces planed smooth and square — ready to use. S4S is thinner than S2S from the same rough stock. Use S4S thickness for joinery planning; use rough nominal for board feet ordering.
How heavy is a woodworking project?
Multiply your total board feet by the species weight per board foot. Common air-dried values: pine 1.8–2.5 lbs/bf, walnut 3.0 lbs/bf, hard maple 3.2 lbs/bf, white oak 3.4 lbs/bf. The weight by species tab calculates this from your board feet and species selection.