Buying lumber is easier when you understand board feet. Instead of pricing every board by its length alone, lumber suppliers often use this measurement to account for thickness, width, and length together.
To calculate board feet, multiply thickness in inches by width in inches by length in feet, then divide by 12.
Board feet = Thickness (inches) × Width (inches) × Length (feet) ÷ 12
For example, a board measuring 2 inches thick, 6 inches wide, and 8 feet long contains:
2 × 6 × 8 ÷ 12 = 8 board feet
The calculation is straightforward. The important part is using the correct units and understanding which dimensions your lumber supplier uses for billing.
What Is a Board Foot?
A board foot is a unit of volume, not length or surface area. One board foot equals a piece of wood measuring:
1 inch thick × 12 inches wide × 12 inches long
That is equivalent to 144 cubic inches or 1/12 of a cubic foot. The University of Missouri Extension uses this definition in its guide to measuring trees and logs.
A board does not need to have those exact dimensions to contain one board foot. For example, these pieces have the same volume:
| Thickness | Width | Length | Board feet |
|---|---|---|---|
| 1 inch | 12 inches | 1 foot | 1 |
| 1 inch | 6 inches | 2 feet | 1 |
| 2 inches | 6 inches | 1 foot | 1 |
You may see board feet abbreviated as BF or bd. ft.
Board Foot Formulas
There are two convenient versions of the formula.
When Length Is Measured in Feet
Board feet = Thickness (in) × Width (in) × Length (ft) ÷ 12
Use this when the board length is something like 6 feet, 8 feet, or 10 feet.
When All Measurements Are in Inches
Board feet = Thickness (in) × Width (in) × Length (in) ÷ 144
Use this when measuring shorter pieces or working from a cutting list in inches.
Both formulas give the same answer when the units are correct. Wood-Mizer’s board-foot guide explains this calculation for lumber.
How to Calculate Board Feet Step by Step
Step 1: Record the Thickness
Measure the thickness or identify the thickness category used for the lumber sale.
Convert fractions to decimals when using a calculator:
- 3/4 inch = 0.75 inch.
- 1 1/4 inches = 1.25 inches.
- 1 1/2 inches = 1.5 inches.
- 1 3/4 inches = 1.75 inches.
For the actual physical volume, use the measured thickness. For checking an invoice, confirm the supplier’s measurement rules.
Step 2: Measure the Width
Record the width in inches.
Random-width hardwood boards should usually be calculated individually because their widths differ. A stack of boards that all look roughly 8 inches wide may contain noticeably different amounts of lumber.
Step 3: Measure the Length
Record the length in feet for the first formula.
If a board measures 8 feet 6 inches, convert the extra inches:
6 ÷ 12 = 0.5 foot
Therefore, the length is 8.5 feet, not 8.6 feet.
Step 4: Multiply and Divide
For a board measuring 1.5 inches thick, 8 inches wide, and 8.5 feet long:
1.5 × 8 × 8.5 ÷ 12 = 8.5 board feet
Keep enough decimal places during the calculation and round the final result as appropriate.
Board Foot Calculation Examples
Example 1: A Single Board
A board measures 1 inch thick, 9 inches wide, and 8 feet long.
1 × 9 × 8 ÷ 12 = 6 board feet
Example 2: A Short Piece Measured in Inches
A piece measures 2 inches thick, 10 inches wide, and 36 inches long.
2 × 10 × 36 ÷ 144 = 5 board feet
Example 3: Several Identical Boards
You have six boards, each measuring 1 inch thick, 8 inches wide, and 9 feet long.
First calculate one board:
1 × 8 × 9 ÷ 12 = 6 board feet
Then multiply by the quantity:
6 × 6 = 36 board feet total
Example 4: Boards of Different Sizes
Calculate each size separately, multiply by its quantity, and add the results.
| Quantity | Thickness | Width | Length | BF per board | Total BF |
|---|---|---|---|---|---|
| 4 | 1 inch | 6 inches | 8 feet | 4 | 16 |
| 3 | 1 inch | 8 inches | 9 feet | 6 | 18 |
| 2 | 2 inches | 10 inches | 6 feet | 10 | 20 |
| Total | 54 |
This method is useful for checking a lumber order or estimating material for a project.
Nominal vs Actual Dimensions: Which Should You Use?
Use actual dimensions to calculate physical wood volume. Use the seller’s stated tally rules to check a lumber bill.
These can produce different answers.
For example, a nominal 2×4 that measures 1.5 × 3.5 inches and is 8 feet long has an actual volume of:
1.5 × 3.5 × 8 ÷ 12 = 3.5 board feet
Using the nominal dimensions gives:
2 × 4 × 8 ÷ 12 = 5.33 board feet
The difference does not necessarily mean the invoice is wrong. Commercial lumber measurements may use nominal sizes or established tally conventions.
Before buying, ask whether the quote uses nominal thickness, surfaced dimensions, and any width or length rounding.
How to Calculate Board Feet for 4/4 and 8/4 Lumber
Hardwood lumber thickness is often described in quarters of an inch.
| Thickness designation | Nominal thickness |
|---|---|
| 4/4 | 1 inch |
| 5/4 | 1.25 inches |
| 6/4 | 1.5 inches |
| 8/4 | 2 inches |
| 12/4 | 3 inches |
These designations do not guarantee the finished thickness after drying and surfacing.
For example, if a supplier tallies an 8/4 board as 2 inches thick, 7 inches wide, and 9 feet long:
2 × 7 × 9 ÷ 12 = 10.5 board feet
A surfaced board sold under that thickness category may measure less than 2 inches. Confirm the basis of the sale rather than substituting measurements without checking.
How to Calculate Lumber Cost per Board Foot
Once you know the billable board footage, multiply it by the quoted price:
Lumber cost = Board feet × Price per board foot
Suppose your order totals 54 board feet and the example price is $7 per board foot:
54 × $7 = $378
This is an illustration, not a current market price. Delivery, tax, surfacing, and other charges may be additional.
To calculate the effective price per board foot from a total price:
Price per board foot = Lumber price ÷ Board feet
For example:
$120 ÷ 15 BF = $8 per board foot
Compare prices using the same measurement basis and equivalent lumber quality.
Board Feet vs Square Feet vs Linear Feet
These measurements describe different things.
| Measurement | What it measures | Dimensions needed |
|---|---|---|
| Linear feet | Length | Length |
| Square feet | Surface area | Width and length |
| Board feet | Wood volume | Thickness, width, and length |
A board measuring 1 inch thick, 6 inches wide, and 8 feet long has:
- 8 linear feet of length.
- 4 square feet of face area.
- 4 board feet of volume.
If the thickness increases to 2 inches, its length and face area remain unchanged, but its volume becomes 8 board feet.
You cannot convert linear feet to board feet without knowing the width and thickness.
How Much Extra Lumber Should You Buy?
The volume of your finished parts is not necessarily the amount of rough lumber you need to purchase.
Allow for saw cuts, end trimming, jointing, planing, defects, and grain selection. The required allowance depends on the boards and your cutting layout.
For a simple budgeting example, adding 20% to a 40-board-foot estimate gives:
40 × 1.20 = 48 board feet
That is an added allowance, not a guarantee of sufficient material.
If you instead expect to lose 20% of the purchased wood, the calculation is different:
40 ÷ 0.80 = 50 board feet
Most importantly, check that the boards can produce your required part sizes. Enough total volume does not guarantee enough long, wide, or clear pieces.
Measuring Live-Edge or Irregular Boards
For a rough volume estimate, measure the width at several evenly spaced positions and calculate an average.
Then use:
Estimated board feet = Thickness × Average width × Length ÷ 12
This is only an approximation for irregular shapes. Deep curves, taper, bark, and voids can make the usable quantity different from the estimate.
For a purchase, ask how the seller measures the slab. Suppliers may use specific tally methods or sell individual slabs at a fixed price.
Common Board Foot Calculation Mistakes
- Mixing units: Use the divisor 12 when length is in feet, or 144 when all dimensions are in inches.
- Forgetting thickness: Width multiplied by length gives area, not volume.
- Misreading feet and inches: Eight feet six inches equals 8.5 feet.
- Mixing nominal and actual dimensions: Establish the measurement basis first.
- Ignoring waste and part sizes: Plan the cuts as well as the total volume.
- Using board dimensions for a log estimate: Logs require separate yield-estimation methods.
Frequently Asked Questions
How Many Board Feet Are in One Cubic Foot?
There are 12 board feet in one cubic foot.
How Many Board Feet Are in a 1×6×8 Board?
Using the stated dimensions:
1 × 6 × 8 ÷ 12 = 4 board feet
A surfaced board may have smaller actual dimensions, so distinguish its physical volume from its nominal tally.
How Do You Calculate Board Feet From Metric Measurements?
When thickness and width are in millimeters and length is in meters:
Board feet = Thickness (mm) × Width (mm) × Length (m) ÷ 2,359.737
For example:
25 × 150 × 2.4 ÷ 2,359.737 ≈ 3.81 board feet
Can You Use This Formula for Standing Trees?
No. The rectangular-board formula measures sawn lumber. Standing trees and logs require methods that estimate recoverable lumber, accounting for factors such as taper, defects, and sawing losses.