How the lumber calculator works
A board foot is a volume, not a length: one foot long, one foot wide and one inch thick, which equals 144 cubic inches or 2.36 litres. Multiply the three dimensions of a board, convert to that unit and multiply by the number of pieces. In metric the same logic simply gives cubic metres.
Board feet = thickness (in) × width (in) × length (ft) ÷ 12 · m³ = thickness (m) × width (m) × length (m)One cubic metre is 423.8 board feet, and one board foot is 0.00236 m³. The waste allowance is applied to the volume and the price, because you pay for the offcuts, knots, splits and end checks that end up in the scrap bin.
Worked example
Twenty spruce boards of 27 × 145 mm, each 4 m long, contain 0.027 × 0.145 × 4 = 0.01566 m³ per board and 0.3132 m³ in total, the same as 132.7 board feet. Adding 10% for trimming brings the order to 0.3445 m³. At 600 per cubic metre the bill is 0.3445 × 600 = 206.71. In imperial, twenty 1 × 6 in boards 12 ft long are 1 × 6 × 12 ÷ 12 = 6 board feet each, 120 board feet together, or 132 with 10% waste; at 4.50 per board foot that costs 594.
Nominal and actual sizes
| Nominal (US softwood) | Actual dressed size | Board feet per 8 ft |
|---|---|---|
| 1 × 4 | 3/4 × 3-1/2 in (19 × 89 mm) | 2.67 |
| 1 × 6 | 3/4 × 5-1/2 in (19 × 140 mm) | 4.00 |
| 2 × 4 | 1-1/2 × 3-1/2 in (38 × 89 mm) | 5.33 |
| 2 × 6 | 1-1/2 × 5-1/2 in (38 × 140 mm) | 8.00 |
| 2 × 10 | 1-1/2 × 9-1/4 in (38 × 235 mm) | 13.33 |
| 4 × 4 | 3-1/2 × 3-1/2 in (89 × 89 mm) | 10.67 |
Board feet are traditionally counted on the nominal size, which is why a 2 × 4 × 8 ft stud is 5.33 board feet even though it is only 1.5 × 3.5 in. Hardwood is sold rough in quarters: 4/4 is 1 in thick, 5/4 is 1.25 in, 8/4 is 2 in, and surfacing removes about 1/8 to 3/16 in. European sawn timber is usually specified by its real sawn or planed size (for example 45 × 95 mm construction timber), so enter the size printed on the quote.
Buying tips
- Ask how the price is expressed. Dimensional softwood in North America is mostly sold per piece or per linear foot; divide the piece price by its board feet to compare offers.
- Rough hardwood loses more. Straightening, planing and cutting around defects often costs 20–30% of the volume in common grades.
- Buy lengths that suit your cut list. Two 1.2 m parts come cleanly from a 2.5 m board but not from a 2.4 m one once you allow for end checks and saw kerf.
- Weight check. Dry spruce weighs about 450 kg per m³ and oak about 700; 0.3 m³ of oak is over 200 kg, more than a car roof or small trailer may carry.
- Structural members such as joists, beams and posts must be graded and sized to your local code or by an engineer; this calculator only handles quantity and cost.
A reproducible calculation with complete inputs
For a reproducible lumber calculator (board feet) example, use the metric input system and enter Thickness: 27 mm; Width: 145 mm; Length: 4 m; Number of pieces: 20; Price: 600 per m³; Waste allowance: 10% (planed softwood). These settings describe one specific scenario, not a new default for every job. The result follows the method and allowances already explained above. Keep the entire set of values when comparing it with a hand calculation; omitting a selected material or an allowance can change the result even when the dimensions match. The following rows show the unrounded calculation values alongside the precision used by the interface.
| Result | Calculation value | Displayed value | Unit |
|---|---|---|---|
| Total volume | 0.3132 | 0.3132 | m³ |
| Volume incl. waste | 0.34452 | 0.3445 | m³ |
| Total price | 206.712 | 206.71 | |
| Board feet | 132.726655555 | 132.7 | board ft |
| Volume per piece | 0.01566 | 0.01566 | m³ |
How should I prepare these entries?
Thickness: For rough hardwood use the nominal quarter size: 4/4 = 1 in, 8/4 = 2 in. Width: enter the value in mm. Length: enter the value in m. Number of pieces: enter the value in the field’s stated scale. Price: Your supplier's rate in any currency: per cubic metre (metric) or per board foot (imperial). Waste allowance: choose among 10% (planed softwood), None (exact count), 20% (rough hardwood with defects), 30% (knotty or cheap grades). Choose the unit system before transferring your measurements. A depth, length, area and coverage can have different field units even within one system. Read each label individually rather than entering the same unit convention in every box. Optional openings or allowances should describe your actual scenario instead of being silently left at a sample value. The worked inputs above identify exactly which selections produced the table.
Frequently asked questions
How do you calculate board feet?
Multiply thickness in inches by width in inches by length in feet and divide by 12. A 2 × 6 that is 10 ft long is 2 × 6 × 10 ÷ 12 = 10 board feet.
How many board feet are in a cubic metre?
One cubic metre equals 423.8 board feet, and one board foot equals 0.00236 m³ or 2.36 litres.
How many board feet are in a 2x4x8?
A 2 × 4 that is 8 ft long is 2 × 4 × 8 ÷ 12 = 5.33 board feet, calculated on the nominal size.
Do you use nominal or actual size for board feet?
Lumberyards use the nominal size for dimensional softwood and the rough quarter thickness for hardwood, so a surfaced 4/4 board is still billed as 1 in thick even though it measures about 13/16 in.
How much extra lumber should I buy for waste?
Allow about 10% for planed softwood and 20–30% for rough hardwood, where knots, splits and uneven edges must be cut away.