Board Foot Calculator: How to Measure Lumber

This board foot calculator works out the volume of lumber in board feet from its dimensions. You enter the thickness, width, and length of a board (or multiple boards), and it returns the total board feet, which is the standard unit for buying and selling hardwood lumber in North America.

Formula

The standard formula for board feet is:

Board Feet = (Thickness (in) × Width (in) × Length (ft)) / 12

Where:

• Thickness is in inches (in)
• Width is in inches (in)
• Length is in feet (ft)
• 12 is a constant that converts the product of inches×inches×feet into board feet

If you have multiple boards of the same size, multiply the result by the number of boards. If boards have different sizes, calculate each one separately and add the totals.

Worked Example

Using the default values from the calculator above:

• Thickness = 1 inch
• Width = 6 inches
• Length = 8 feet
• Number of boards = 1

Step 1: Multiply thickness by width: 1 × 6 = 6

Step 2: Multiply by length in feet: 6 × 8 = 48

Step 3: Divide by 12: 48 / 12 = 4

So one board that is 1″ thick, 6″ wide, and 8′ long equals 4 board feet. The calculator gives the same result (4.00 bd ft).

Why Board Feet? A Quick Background

Board feet have been used in the lumber trade for over a century. The unit reflects the volume of a board before it is milled or dried, not its final surfaced dimensions. For example, a “2×4� actually measures about 1.5″ × 3.5″ after planing, but the board foot calculation uses the nominal (rough) dimensions of 2″ × 4″. This is why a 2×4 that is 8 feet long is considered 5.33 board feet (2 × 4 × 8 / 12), even though the physical board is smaller.

Hardwood lumber is almost always sold by the board foot, while softwood (like pine or fir) is typically sold by the linear foot or by pre-cut lengths. Knowing the difference helps you compare prices accurately when shopping for wood.

How to Measure a Board Correctly

To get an accurate board foot count, you need three measurements:

1. Thickness – measure in inches. For rough lumber, use the actual rough thickness (e.g., 4/4 = 1 inch, 6/4 = 1.5 inches). For surfaced lumber, use the nominal thickness if you are buying by the board foot, or the actual thickness if you are calculating for a project.
2. Width – measure in inches. Again, use nominal width for rough stock, actual width for surfaced.
3. Length – measure in feet. If your board is 96 inches long, convert to 8 feet.

For boards with tapered widths (like a log or a live-edge slab), use the average width: measure the widest and narrowest points, add them, and divide by 2. For tapered thickness, use the average thickness the same way.

Common Mistakes and Misconceptions

One frequent error is mixing units. The formula requires inches for thickness and width, but feet for length. If you measure length in inches, you must divide by 144 instead of 12 (because 12×12 = 144). For example, a board 1″ × 6″ × 96″ (all inches) would be 1 × 6 × 96 / 144 = 4 board feet – same result as before.

Another misconception is confusing board feet with square feet. Square feet measure area (length × width), while board feet measure volume (thickness × width × length). A 1″ thick board has the same number of board feet as square feet, but a 2″ thick board has twice as many board feet for the same area.

Also remember that lumber yards often round up to the nearest whole board foot when pricing. A board that is 4.2 board feet might be billed as 5 board feet. Always confirm the pricing method before buying.

How to Choose the Right Size and Quantity

When planning a project, calculate the board feet needed for each piece, then add 10–20% extra for waste, defects, and cutting errors. For example, if your project requires 50 board feet, buy 55–60 board feet. This is especially important with hardwoods, which can have knots, splits, or sapwood that you might need to cut around.

For a simple table top of 1″ thick, 30″ wide, and 72″ long (2.5 ft × 6 ft), the board feet would be 1 × 30 × 6 / 12 = 15 board feet. If you buy 4/4 lumber (1″ thick) that is 6″ wide, you would need five 6″-wide boards to get 30″ of width, each 6′ long – that’s 5 × 6 = 30 board feet, because each board is 1 × 6 × 6 / 12 = 3 board feet. The extra 15 board feet covers the glue joints and edge trimming.

Limits of the Formula

The board foot formula is a simple volume calculation – it does not account for moisture content, grain direction, or defects. A board that is 8/4 (2 inches) thick but has a large knot may have less usable wood than a clear 4/4 board of the same volume. Also, the formula assumes a rectangular cross-section; irregular shapes like logs or curved pieces require more complex methods (e.g., the Doyle or Scribner log rules, which estimate usable lumber from log diameter).

For surfaced lumber, using nominal dimensions overestimates the actual volume. If you need precise material for a project, measure the actual dimensions after surfacing, not the nominal ones.

Frequently Asked Questions

What is a board foot?

A board foot is a unit of volume equal to a board that is 1 inch thick, 12 inches wide, and 1 foot long (144 cubic inches). It is the standard measure for hardwood lumber in the US and Canada.

How many board feet are in a 2x4x8?

A 2×4×8 (nominal dimensions) has 2 × 4 × 8 / 12 = 5.33 board feet. In practice, lumber yards often round this to 5 or 6 board feet for pricing.

How do I convert board feet to cubic feet?

One board foot equals 1/12 of a cubic foot (since 12 board feet = 1 cubic foot). To convert, divide board feet by 12. For example, 24 board feet = 2 cubic feet.

Is board foot the same as linear foot?

No. A linear foot is simply a length of 12 inches, regardless of width or thickness. A board foot accounts for volume. For example, a 1×6 board that is 8 feet long is 4 board feet but 8 linear feet.

Why does my lumber yard charge more than the board foot calculation?

Lumber yards often add a surcharge for surfacing, planing, or cutting to size. They may also round up to the nearest whole board foot, or charge a minimum (e.g., 1 board foot) for small pieces. Always ask for the final price per board foot after any processing.

Related

• Calculate Bricks for a Wall: Formula and Example
• Board Foot Calculator: Formula and Examples
• Calculate a Wood Cutting Angle: Formula and Examples

in t | Thickness | in | 1 | 0
in w | Width | in | 6 | 0
in l | Length | ft | 8 | 0
in n | Number of boards | - | 1 | 1
out bdft | Board feet | bd ft | (t * w * l / 12) * n | 2