Construction teams are comfortable with cubic feet and cubic metres — concrete, excavation and fill are all priced that way. Timber is the odd one out. North American lumber yards and sawmills quote in board feet, while many drawings, imported stock and international suppliers work in cubic metres. Knowing how the units relate saves time on take-offs and avoids costly misunderstandings with suppliers.
Cubic feet: the general volume unit
A cubic foot is the volume of a cube 12 inches on each side — 1,728 cubic inches. For any rectangular piece:
Cubic feet = length (ft) × width (ft) × height (ft)
If dimensions are in inches, multiply them and divide by 1,728. One cubic foot equals about 0.0283 m³.
Board feet: the timber unit
A board foot is a slice of a cubic foot: 1 inch × 12 inches × 12 inches, or 144 cubic inches. That means:
- 12 board feet = 1 cubic foot
- 1 board foot ≈ 0.00236 m³
- 1 m³ ≈ 424 board feet
For a piece of lumber, the standard formula is:
Board feet = thickness (in) × width (in) × length (ft) ÷ 12
A worked comparison
Take a timber beam 6 × 8 inches and 14 feet long:
- Board feet: 6 × 8 × 14 ÷ 12 = 56 bf
- Cubic feet: 56 ÷ 12 = 4.67 ft³
- Cubic metres: 56 × 0.00236 ≈ 0.132 m³
All three describe the same piece of wood — they are just different ways of expressing it.
Why the difference matters on site
- Pricing. Lumber is priced per board foot or per thousand board feet (MBF). Converting a cubic-metre estimate incorrectly can make a quote look 12 times cheaper or more expensive than it is.
- Nominal sizes. Softwood lumber is tallied on nominal dimensions (a 2×4 is counted as 2 × 4 even though it measures about 1½ × 3½). Cubic measurements of the actual board will not match the invoice.
- Logs versus lumber. Standing timber and logs are measured with log rules that estimate the board feet a log will yield after sawing. The log’s geometric volume is always larger than its lumber yield, because bark, slabs and sawdust are lost.
Speeding up the take-off
For repetitive members, build a small table of common sections — 2×4, 2×6, 2×8, 4×4, 6×6 — with the board feet per foot of length. Multiply by total running length and you have the volume. When sizes vary or the drawings are metric, it is quicker to calculate board feet online: enter thickness, width, length and quantity in inches or millimetres, add a price per board foot, and get both board feet and cubic metres at once.
Checklist for timber quantities
- Confirm the unit the supplier quotes in.
- Use nominal sizes for softwood, rough thickness for hardwood.
- Add 10–15% wastage for cutting and defects.
- Keep a conversion note on the take-off sheet: 12 bf = 1 ft³; 424 bf ≈ 1 m³.
For teams that also buy logs or work with small sawmills, a set of free sawmill calculators covering board feet, log scale and log loads helps keep estimates consistent from the forest to the framing crew.
Conclusion
Cubic feet and board feet are not competing systems; one is simply a twelfth of the other. Keep that relationship in mind, be clear about nominal versus actual sizes, and timber quantities will reconcile cleanly with supplier invoices.
