-What is a Board Foot?

      Explanation and glossary of lumber terms by Professor Eugene Wengert. May 2, 2001

The following is an article by Professor Gene Wengert that defines just what a board foot is. The original questions which spurred this post follow the article.

The definition of a board foot is rather simple--one board foot is a piece of lumber that is 1 foot wide, 1 foot long and 1 inch thick, or its volumetric equivalent. But, when it comes to applying this definition to the real world, we have problems. Let's separate the footage into two parts--softwood BF and hardwood BF. (Actually, some people will use fbm for the log footage and will use BF or bfm for lumber measurement. Instead of using a lot of zeros, the abbreviation M is used to represent 1000. So, 6 MBF is 6000 BF; 4 MMBF is 4 million BF.)

Hardwood BF
The formula for calculating hardwood lumber board footage is BF = SM x T (inches) and SM = [L feet) x W (inches) / 12] where L = nominal length (truncated to closest foot, not rounded upwards), W = actual width including fractions of inches, and T = nominal thickness. The SM is rounded to the closest whole number (no fractions) before multiplying by the thickness. To calculate the board footage for a bundle of lumber containing many pieces, the SM is determined for each piece of lumber individually. The sum of the SM for all pieces of the same nominal thickness is determined and then multiplied by the nominal thickness to obtain the board footage, rounding to the closest board foot. (In practice, to measure the SM, a scaling stick is used, similar to a log scaling stick, with SM numbers for various lengths of lumber. The stick is placed across the width of the lumber.)

The nominal thickness of hardwood lumber is traditionally the minimum thickness when green or air-dried. So, 4/4 lumber is at least 1.00-inch thick in the portion of the lumber used to establish the grade of the piece; 6/4 is 1.50 inches thick; and so on. Pieces under 1.00-inch thick (such as 3/4) use a nominal thickness of 1-inch when calculating board footage. In order to achieve the required minimum thickness, hardwood lumber usually has an average thickness 1/8-inch greater than the nominal thickness. However, this over-thickness is not required (although it may be expected). In fact, many modern mills can and do meet the minimum thickness requirement by producing lumber only 1/16-inch thicker than the nominal thickness. Often there are special rules or agreements between buyer and seller concerning average thickness and minimum thickness. Such specifications are especially important for lumber that is measured after kiln drying, where actual sizes are below the minimum sizes specified for green or air-dried lumber.

Softwood BF
The formula for calculating softwood board footage is BF = L (feet) x W (inches) / 12 x T (inches) where L = nominal length (truncated to closest even foot), W = nominal width, and T = nominal thickness. Do not use the actual thickness or width. The product is then rounded to the closest 1/100 board foot (i.e., xx.xx).



Some nominal softwood thicknesses and widths (inches) after drying are:
Thickness
Width
Nominal
Actual
Nominal
Actual
in.
in.
in.
in.
4/4
.75
2
1.50
6/4
1.25
4
3.50
8/4
1.50
6
5.50
8
7.25
12
11.25



Terminology

Lumber
A wooden rectangular parallelepiped

Softwood lumber

Board
A piece of lumber that is less than 2" thick and 2" wide or wider. The thickness and width measurements refer to nominal size, not actual. (See below for a discussion of actual softwood lumber sizes.) Sometimes called "Shop Lumber" when it is intended to be cut up later into smaller pieces.

Dimension lumber
A piece of lumber that is 2" up to .5" thick and 2" wide or wider, nominal size.

Light framing
A piece of softwood dimension lumber that is 2 or 4" wide, nominal width.

Joist or plank
A piece of softwood dimension lumber that is 6" wide or wider.

Structural lumber
A piece of dimension lumber that is evaluated, graded, and marketed based on its strength and/or stiffness.

Timber
A piece of lumber that is 5" wide or wider.

Hardwood Lumber

Lumber
A piece of lumber that is at least " thick, 3" wide and 4' long.

Board
A piece of hardwood lumber that is less than 2" thick. Sometimes in the industry, board refers to any piece of lumber, regardless of thickness.

Flitch
A partially processed piece of wood that is intended to produce lumber or veneer. Usually only one (or sometimes two) side has been processed.

Cant
A partially processed piece of wood that is intended to produce lumber or veneer. Usually two or more sides have been processed.

Hardwood Dimension
A processed piece of wood that has been cut into a rough size for furniture, cabinets, millwork, etc.; usually dry.

Gene Wengert, forum technical advisor



In a recent post, there was an example of 10 boards measuring 1" x 11.51" x 10'5" being sold as 100 bd. ft. Am I correct in my understanding that this is if the ten boards were sold as 100 bd. ft., that would be the correct measure (the extra 5" in length does not matter). However, if you sold 10 boards - 1" x 12" x 10'5", the NHLA rules for width (up to 10% scant by 1/4") would apply?


You have two things combined together. There is no 1/4 inch scant variation for measuring board footage. So, any piece of hardwood lumber that is 11.40" to 12.59" wide and 10.00 to 10.99' long and 1.00 to 1.24 inches thick will be considered as having 10 BF. If it is 11.37", then the footage drops to 9 BF.

However, the NHLA rules indicate that you should have an assortment of widths (as well as quality) within the grade. If all the pieces are at 11.39" wide, this could be considered as not adhering to the spirit and perhaps letter of the rules.

Where the 1/4 inch scantness comes is in determining whether a piece of lumber is wide enough for a particular rule--such as 6" minimum width for FAS. You are permitted a few, but very few, narrow pieces by 1/4 inch. Specifically, you are permitted 10% of the 6" wide pieces that can be up to 1/4 inch narrow. (Some people think that this 10% rule is 10% of the load; not true.) But this is in regarding to grading and not measuring the footage.

Gene Wengert, forum technical advisor



Your response brought up another question when I saw the range of widths from 11.40" to 12.59" rather than 11.5" to 12.49", but I figured that one out myself. I realized that I have been guilty of looking at the width and rounding before multiplying by length and dividing by 12. I will remember to trust my board rule rather than my tape measure.


One thing I would like to add to the softwood BF calculations is that this is for surfaced softwood, not softwood in the rough. In the rough, it is calculated the same as it is for hardwood.


When taking roughsawn pine to the pressure treating plant, why is a 6/4 board considered 8/4 when you pay by the BF? A 30"x30"x192" cube has 1200 BF if you consider a BF as 144 cubic inches. I've had it go as high as 1600 BF for a cube of 6/4x6"x192" boards.


I think there are a lot of guys in the industry that consider a situation like that a "windfall". That's why I figure 6/4 when I have someone ask for dressed sizes. It's the only way I get a good night's sleep.

Our treater has two prices, one for dressed lumber and one for rough cut lumber. At least he is making an effort but he still doesn't seem to understand sizes other than 1", 2", 4", etc.



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  • KnowledgeBase: Lumber & Plywood: Buying

  • KnowledgeBase: Primary Processing: General

  • KnowledgeBase: Primary Processing

  • KnowledgeBase: Primary Processing: Lumber Grading

  • KnowledgeBase: Primary Processing: Yield Formulas

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