How to use the board foot calculator
- Choose Lumber for sawn boards, or Logs to scale a log.
- For lumber, type the thickness and width in inches, the length in feet (or switch to inches), and how many pieces. Fractions work: 3/4, 5/4 or 1 1/2.
- Press Add another size for each different size on your list. The total adds them all up.
- Type a price per board foot if you want the cost.
For a log, type the diameter inside the bark at the small end, the length and the number of logs. The board foot calculator for logs shows three log rules side by side.
The board foot formula
A board foot is a board 1 inch thick, 12 inches wide and 1 foot long, or 144 cubic inches. The American Softwood Lumber Standard (PS 20, published by NIST) gives the rule: the number of board feet "is obtained by multiplying the nominal thickness in inches or fraction of an inch by the nominal width in feet by the length in feet." With the width in inches, that is:
Board feet = thickness (in) x width (in) x length (ft) / 12, or thickness x width x length (in) / 144.
| Piece | Working | Board feet |
|---|---|---|
| 2x4, 8 ft | 2 x 4 x 8 / 12 | 5.33 |
| 2x6, 12 ft | 2 x 6 x 12 / 12 | 12 |
| 1x6, 8 ft | 1 x 6 x 8 / 12 | 4 |
| 5/4 x 6, 8 ft | 1.25 x 6 x 8 / 12 | 5 |
| 1x6, 96 in | 1 x 6 x 96 / 144 | 4 |
Press Try an example for ten 2x4s and five 1x6s, all 8 ft: 53.33 + 20 = 73.33 board feet.
Nominal and actual lumber sizes
A "2x4" does not measure 2 by 4 inches. PS 20 explains that the nominal size "is greater than the dressed size", and gives the example that "a dry 2 by 4 is surfaced to" 1 1/2 by 3 1/2 inches. Board feet are counted on the nominal size, so type 2 and 4, not 1.5 and 3.5. These are the minimum dry dressed sizes from PS 20 Table 3:
| Nominal (in) | Dry dressed (in) |
|---|---|
| 1 (boards) | 3/4 |
| 2 | 1-1/2 |
| 4 | 3-1/2 |
| 6 | 5-1/2 |
| 8 | 7-1/4 |
| 10 | 9-1/4 |
| 12 | 11-1/4 |
Thickness written in quarters, such as 4/4, 5/4 or 8/4, counts quarters of an inch: 8/4 is 2 inches. If you need the real volume of wood, measure the actual size and use the cubic yard calculator or plain cubic feet; PS 20 warns that converting board feet to cubic meters needs care for this reason.
Board foot calculator with price
For lumber sold by the board foot, multiply the total board feet by the price. Six boards of 8/4 hardwood, 8 inches wide and 10 ft long, are 2 x 8 x 10 / 12 = 13.33 board feet each, 80 board feet in all. At $9.50 per board foot (an example price), that is 80 x 9.50 = $760.00. Ask your yard whether it counts on rough or surfaced size and how it rounds lengths.
Board foot calculator for logs
A log rule estimates the board feet that a log will saw out. Purdue University Extension's guide Log and Tree Scaling Techniques (FNR-191) gives the formulas used here, where D is the diameter inside the bark at the small end in inches and L is the length in feet:
- Doyle: (D - 4)² x L / 16.
- Scribner, as a formula: (0.79D² - 2D - 4) x L / 16. Purdue notes it is "fairly consistent on 16 foot or shorter logs under 28 inches". Published Scribner Decimal C tables are drawn, not computed, and round to the nearest 10 board feet, so they can differ by a few percent.
- International 1/4-inch: a formula for each length from 4 to 20 ft, for example 0.597D² - 1.330D - 0.715 for 12 ft. Purdue says values are "rounded to the nearest multiple of 5 board feet" and longer logs "are to be scaled as two or more logs".
| Log | Doyle | Scribner (formula) | International 1/4 |
|---|---|---|---|
| 10 in x 16 ft | 36 | 55 | 65 |
| 16 in x 12 ft | 108 | 125 | 130 |
| 30 in x 16 ft | 676 | 647 | 675 |
The rules differ most on small logs. Purdue says the Doyle rule "seriously under scales small logs and over scales large logs": on the 10 inch log above it gives 36 board feet against 65 by the International rule. Purdue calls the International rule "probably the most accurate of the current rules", and adds that local tradition will most likely decide which rule is used, so ask before you compare prices. The calculator needs a diameter of at least 6 inches, where Purdue's tables start.
Board feet to square feet, and spray foam
Board feet measure volume and square feet measure area, so the conversion depends on thickness. At 1 inch thick, 1 board foot covers 1 square foot. At 2 inches thick it covers half a square foot. So square feet = board feet / thickness in inches: 80 board feet of 2 in stock covers 40 sq ft of face.
The same unit is used for spray foam insulation: one board foot is 1 sq ft sprayed 1 inch thick. DuPont says its Froth-Pak 200 kit "covers 200 square feet at 1” thickness", and explains that kit sizes are a "theoretical yield" worked out in laboratory conditions. For foam, multiply the area in square feet by the depth in inches: 100 sq ft at 3 inches is 300 board feet.
Questions and answers
How do you calculate board feet?
Multiply thickness in inches by width in inches by length in feet, then divide by 12. If the length is in inches, divide by 144 instead. Use the nominal size, so a 2x4 is 2 by 4.
How many board feet are in a 2x4x8?
2 x 4 x 8 / 12 = 5.33 board feet. A 2x4x10 is 6.67 and a 2x4x12 is 8.
Are board feet worked out on the nominal or the actual size?
On the nominal size. The American Softwood Lumber Standard multiplies the nominal thickness by the nominal width by the length. A dry 2x4 measures 1 1/2 by 3 1/2 inches but is still counted as 2 by 4.
How do I convert board feet to square feet?
Divide the board feet by the thickness in inches. At 1 inch thick, 1 board foot equals 1 square foot; at 2 inches thick, it equals half a square foot.
Which log rule should I use?
The one the buyer or mill uses, so that you compare like with like. The rules differ a lot on small logs: for a 10 inch, 16 ft log, Doyle gives 36 board feet and the International 1/4-inch rule 65.
What is a board foot of spray foam?
One square foot of foam, 1 inch thick. A kit sold as 200 board feet covers about 200 sq ft at 1 inch, or 100 sq ft at 2 inches, in ideal conditions.
Sources
Checked on 9 October 2026. If a rule has changed, please tell us.