Lumber Calculator
This lumber calculator works out board feet, actual dimensions and cost for any run, plus how many pieces a framing layout needs at 16 or 24 inches on centre.
Board Feet
Rough or Custom Stock
For rough-sawn, hardwood or anything that is not a standard nominal size, enter the dimensions you are actually being charged for.
Studs, Joists and Rafters
How many pieces a wall, floor or roof needs at a given spacing, and what that comes to in board feet.
Lumber Cost
How to use the Lumber Calculator
Pick the nominal size you buy it as, a 2 × 4, a 1 × 6, then the length of each piece and how many you need. Board feet are billed on the nominal size, so that is what the calculation uses, but the actual milled dimensions are shown alongside, because those are what you build to.
Use Custom for rough-sawn or hardwood, where the stock really is the size you measure. Hardwood is often quoted in quarters of an inch: 4/4 is one inch, 8/4 is two.
Framing answers a different question, not what a pile of lumber measures, but how many pieces a wall, floor or roof needs at 16 or 24 inches on centre.
Lumber formula
board feet = (nominal thickness in × nominal width in × length ft) ÷ 12total board feet = board feet per piece × piecespieces at spacing = floor(run ÷ spacing) + 1
The ÷ 12 is a unit conversion hiding inside the formula. A board foot is 144 cubic inches; two of the three dimensions are already in inches, so dividing by 12 converts the third from feet.
Nominal against actual, for softwood dried and surfaced on four sides:
| Nominal | Actual | Nominal | Actual |
|---|---|---|---|
| 1 × 4 | ¾ × 3½ in | 2 × 4 | 1½ × 3½ in |
| 1 × 6 | ¾ × 5½ in | 2 × 6 | 1½ × 5½ in |
| 1 × 8 | ¾ × 7¼ in | 2 × 8 | 1½ × 7¼ in |
| 1 × 10 | ¾ × 9¼ in | 2 × 10 | 1½ × 9¼ in |
| 1 × 12 | ¾ × 11¼ in | 2 × 12 | 1½ × 11¼ in |
| 4 × 4 | 3½ × 3½ in | 6 × 6 | 5½ × 5½ in |
Boards 6 inches and wider lose ¾ inch rather than ½, which is why a 2 × 6 is 5½ and not 5¾.
Worked example
Twenty 2 × 4s, 8 ft long:
- Board feet per piece: (2 × 4 × 8) ÷ 12 = 5.33 bd ft
- Total: 5.33 × 20 = 106.67 bd ft
- Linear feet: 8 × 20 = 160 lf
- Actual size of each: 1½ × 3½ in, so the timber really present is 160 × 1.5 × 3.5 ÷ 144 = 70 bd ft
You pay for 106.67 board feet and receive 70 board feet of wood. That gap is normal and is priced in, it is the reason nominal sizing survives.
What the result means
A board foot is a volume, not a length. One board foot is 144 cubic inches: a piece 1 inch thick, 12 inches wide and 1 foot long. An 8 ft 2 × 4 is 5.33 board feet, not 8.
Linear feet and board feet get quoted differently by different suppliers. Trim, moulding and decking usually go by the linear foot, framing lumber by the piece, hardwood by the board foot. The cost tab accepts whichever your supplier quotes, so the comparison is like for like.
The framing count always has one more member than it has spaces. That extra piece at the end of the run is the single most commonly forgotten item on a framing take-off.
Common mistakes
- Using actual dimensions for board feet. Board feet are billed nominal. Using 1.5 × 3.5 instead of 2 × 4 understates the bill by a third.
- Forgetting the last piece. A 24 ft wall at 16 in on centre needs 19 studs, not 18, there is one at each end.
- Ordering exact lengths. Lumber comes in 2 ft increments. Needing 9 ft means buying 10 ft, and the offcut is not free.
- Leaving out corners, tees and openings. This count covers the field studs only. Corners, channels, headers, cripples and blocking are extra.
Frequently asked questions
How do you calculate board feet?
Multiply nominal thickness in inches by nominal width in inches by length in feet, then divide by 12. An 8 foot 2 by 4 is 2 times 4 times 8 divided by 12, which is 5.33 board feet. Multiply by the number of pieces for the total.
Is a 2×4 really 2 inches by 4 inches?
No. A modern surfaced 2 by 4 measures 1.5 by 3.5 inches. The nominal size refers to the rough green dimension before drying and planing. You are billed on the nominal size but you build with the actual one, which is why both are shown here.
How many studs do I need for a 24 foot wall?
At 16 inches on centre, 19 studs. Twenty-four feet is 288 inches, 288 divided by 16 is 18 spaces, and a run always needs one more member than it has spaces. Add extras for corners, wall intersections and openings.
What is the difference between a board foot and a linear foot?
A linear foot is a length, ignoring thickness and width. A board foot is a volume of 144 cubic inches. Ten linear feet of 1 by 12 is 10 board feet; ten linear feet of 1 by 6 is only 5 board feet, even though both are ten feet long.
What does 4/4 mean in hardwood?
Quarters of an inch of rough thickness. 4/4 is one inch, 5/4 is an inch and a quarter, 8/4 is two inches. After surfacing, a 4/4 board finishes around 13/16 of an inch but is still sold as 4/4. Use the Custom tab and enter the thickness you are quoted.
Why is lumber sold in even lengths?
Mills cut to 8, 10, 12, 14, 16 and 20 feet, which suits standard framing spacing and transport. Odd lengths are special order. Round your requirement up to the next even length and expect an offcut.
Sources and assumptions
Nominal-to-actual dimensions follow the American Softwood Lumber Standard PS 20, for dry lumber surfaced on four sides. Green or rough-sawn stock measures larger.
A board foot is defined as 144 cubic inches and is computed on nominal dimensions, which is the convention US lumber yards bill on.
Framing counts assume one member at each end of the run. Corners, wall intersections, openings, headers, cripples and blocking are additional and depend on the layout.