Board feet is the unit hardwood lumber is sold in, and it is the reason a quote for timber can surprise you. It is not complicated, but it uses nominal dimensions, which means the number you work out is not the volume of wood you take home.
The definition
One board foot is 144 cubic inches. In practice that is a board one inch thick, twelve inches wide and twelve inches long.
The formula:
board feet = (thickness in inches x width in inches x length in inches) / 144
For a single piece. Multiply by quantity for several.
A 1 x 6 that is 8 feet long:
- 1 x 6 x 96 = 576 cubic inches
- 576 / 144 = 4 board feet
A 2 x 4 that is 8 feet long:
- 2 x 4 x 96 = 768 cubic inches
- 768 / 144 = 5.33 board feet
The catch: nominal versus actual
That 2 x 4 is not 2 inches by 4 inches. It is about 1.5 by 3.5, because it was sawn at the nominal size and then planed smooth. The actual volume is 1.5 x 3.5 x 96, which is 504 cubic inches, or 3.5 board feet.
So the board is billed at 5.33 board feet and contains 3.5 board feet of wood. That is not a trick, it is the convention: hardwood is priced on the rough size the mill produced, and dressing is a service that removes material.
This matters when you are estimating cost. If you work out your cut list in actual dressed dimensions and then multiply by a price per board foot, you will underestimate the bill by roughly 35 percent for nominal 2 inch stock, and by similar amounts for other sizes.
| Nominal | Actual dressed | Board feet per 8 ft (nominal) | Actual volume |
|---|---|---|---|
| 1 x 4 | 3/4 x 3 1/2 | 2.67 | 1.75 |
| 1 x 6 | 3/4 x 5 1/2 | 4.00 | 2.75 |
| 2 x 4 | 1 1/2 x 3 1/2 | 5.33 | 3.50 |
| 2 x 6 | 1 1/2 x 5 1/2 | 8.00 | 5.50 |
| 2 x 8 | 1 1/2 x 7 1/4 | 10.67 | 7.25 |
| 4 x 4 | 3 1/2 x 3 1/2 | 10.67 | 8.17 |
Hardwood is priced differently again
Construction lumber, the softwood sold in 2 by 4s at a builders merchant, is priced per piece. Board feet barely comes into it.
Hardwood sold at a timber yard is priced per board foot, often by species and grade. Red oak and walnut are not the same price, and FAS grade and a lower grade are not the same price either. Ask what the price per board foot is for the species and grade you are being shown, and ask whether the price is for random width boards or a specific width.
Widths in hardwood are random: you might be offered boards 5 to 8 inches wide, and you cannot specify exactly what you get. Allow for that in your plan, because a project needing an 11 inch wide panel cannot be cut from 8 inch boards in one piece.
Turning a cut list into an order
- Write every part with its finished length, width and thickness.
- Add the extra for milling. If you are starting from rough lumber, allow about 1/4 inch on thickness and width and 1 inch on length for squaring up.
- Add cutting allowance for each part. Allow 1 inch of length on every part for trimming to final size after the ends are squared.
- Add the kerf. A table saw blade removes about 1/8 inch per cut. That is not much per cut and it adds up across twenty parts.
- Group by stock thickness. You buy by thickness, so a project with 3/4 inch and 1 1/2 inch parts is two purchases.
- Multiply by a waste factor. 15 to 20 percent is realistic for a first project, because defects, knots and grain run-out mean some of what you buy cannot be used for the part you planned.
- Convert to board feet using the formula above, with nominal dimensions if you are being quoted nominal.
- Confirm the price per board foot before ordering, and ask for the figure including any milling charge.
What to do when it is too expensive
Timber cost is usually driven by the species rather than by the design. Substituting poplar for walnut in a painted piece, or ash for oak in a stained one, can halve the bill with no visible difference. Ash takes a stain very like oak. Poplar paints beautifully and is the cheapest hardwood most yards stock.
The other lever is grade. The next grade down often has one face that is perfectly good, and if that face is the one people will see, the saving is real. Look at the boards before you accept them.