Tag Archive | feet
March 20, 2012  

Calculate Board Feet? Go Figure.

Figuring out board feet is a way of life for me. Both logs and the lumber produced are figured in board feet, so I can’t really escape it, whether I am in the shop or at the sawmill. It is easy for me to forget that this isn’t an everyday occurrence for everyone else, even full-time woodworkers. I know that most woodworkers can explain what a board foot is, or at least won’t admit that they don’t know what it is, but I am still surprised when I ask someone how much lumber they need and they respond along the lines of, “Oh, six or seven boards.” This doesn’t tell me much because I don’t know how thick, long or wide those six or seven boards are in someone else’s mind. That’s where board feet come into play.

Board feet is a measure of the volume of wood, not just the surface area. A single board foot measures one square foot x 1″ thick. By the way, the thickness is based on the rough-cut thickness, not the finish-planed thickness. So, 4/4 thick hardwood lumber (rough-sawn at 1-1/8″ thick and finish-planed to 3/4″ thick) that has one square foot of surface is equal to one board foot. If that same square foot was 2″ thick, it would measure two board feet.

It would be easy enough to figure it out if all wood came in 12″ x 12″  1″ chunks, but it doesn’t. Random widths and random lengths are standard for hardwoods, which creates the need for some math and a better understanding of the calculations. Let’s start with just a single board. Here are three ways to approach it:

• Length (in inches) x Width (in inches) x Thickness (in inches) ÷ 144
• Length (in feet) x Width (in inches) x Thickness (in inches) ÷ 12
• Length (in feet) x Width (in feet) x Thickness (in inches)

Let’s plug in the dimensions of a specific board to see how it works. The measurements are Length (96″ or 8′)  x Width (9″ or .75′) x Thickness (1.5″)

• Length (96″) x Width (9″) x Thickness (1.5) = 1296 ÷ 144 = 9 bd. ft.
• Length (8′) x Width (9″) x Thickness (1.5) = 108 ÷ 12 = 9 bd. ft.
• Length (8′) x Width (.75′) x Thickness (1.5)  = 9 bd. ft.

When measuring a stack of boards it is time-consuming and sometimes impossible to measure each individual board, so averages and estimations come into play. When approaching a stack of wood, I start off by trying to get an average length. This is done by eye and is more accurate if the boards are closer in length. It is difficult to get a good average in a stack that ranges from 16′ to 4′. In that situation it is best to make a couple of groups if you can and then get an average. After you get an average length of the stack, measure the width of the stack. Be sure to subtract the air-spaces from the average width of the stack. Next, get the thickness of the boards. Take the time to make separate piles for different thicknesses. If the pile is a mixture of thicknesses an average can also be taken. The last bit of accounting is to determine the number of layers in the stack. Here’s how the calculations go for stacks of wood:

• Length (in inches) x Width (in inches) x Thickness (in inches)
x Layers ÷ 144
• Length (in feet) x Width (in inches) x Thickness (in inches)
x Layers ÷ 12
• Length (in feet) x Width (in feet) x Thickness (in inches)
x Layers

Here are some real dimensions to see how it plays out. The average measurements of the stack are Length (120″ or 10′)  x Width (48″ or 4′) x Thickness (1″) x 20 Layers:

• Length (120″) x Width (48″) x Thickness (1″)
x Layers (20) = 115,200 ÷ 144 = 800 bd. ft.
• Length (10′) x Width (48″) x Thickness (1″)
x Layers (20) = 9600 ÷ 12 = 800 bd. ft.
• Length (10′) x Width (4′) x Thickness (1″)
x Layers (20) = 800 bd. ft.

I normally calculate using inches divided by 144 for all the measurements because my random length lumber is very random. Lumber from large mills is usually cut to the nearest foot with packs consisting of only two lengths, 8′ and 9′, for example. In that case, using feet instead of inches is the simpler method.

The easiest to calculate is 12′ long, 1″ thick lumber. An 8″ wide board is 8 bd. ft., no calculation necessary. Just change inches to feet and go home.

October 18, 2011  

Doyle Log Scale: How To Determine Board Feet In A Log

 

(Click this image for a larger view. Click the pdf link below for a printable Doyle log scale or Doyle log rule.)

Doyle Log Scale WunderWoods pdf

Many times customers will call to discuss having a log milled and how much it will cost. The answer is often based on how many board feet (12″ x 12″ x1″) will be produced. So, the first thing I ask is, “How big is the log?” Usually the answer is, “Well, I can’t get my arms around it.” And, while this may be helpful, there is a more accurate way to determine the size of a log and how many board feet will be produced.

There are three common scales or rules used in the industry (Doyle, Scribner, and International), but the Doyle scale is the most commonly used around the St. Louis area. All three of the scales estimate logs closely in the medium to larger size range, but the Doyle underestimates footage on the smaller logs. Because of this, it is advantageous for buyers of logs to use the Doyle scale to make up for extra log handling on small logs. Since the buyers like this scale, it is what they use and therefore, what the sellers use.

Doyle folding log rule used by most log buyers in Midwest

All buyers have a Doyle scale on them at all times, usually in the form of a folding rule with the footage marked at each inch. The printable version above has more increments on it, but it is basically the same and is used in conjunction with a tape measure. I always have a tape measure on me, so I usually use the printed version (they are also cheaper).

The formula for the scale is based on a tapered cylinder, milled with a 1/4″ kerf. Straight logs, with little taper and cut on a thin-kerf bandsaw will yield more lumber than the scale predicts. It usually averages out, because logs are usually not so perfect, and often have boards that are below-grade and end up in the firewood pile.

To use the scale, first measure the average diameter of the small end of the log inside the bark (in inches). Locate that row on the scale. Next, measure the length of the log (in feet). Move over on the scale to that length column. Where those two measurements intersect, you will find the board footage for that log. The process must be repeated for each log. Deductions are made for defects, like rot and curved logs.

Since sawmills usually charge by the board foot, this scale will help you determine the amount of lumber you will have and what you can expect your bill to be. Make sure to accurately measure your log and not just guess the diameter. The logs seem bigger than the actual measurement. My customers are usually off by about a foot in diameter on good-sized logs when they guess.

A little perspective on log sizes:
• A respectable diameter on a hardwood tree is 20″.
• A large diameter on a hardwood tree  is 30+”.
• The smallest diameter most hardwood mills buy is 13″.
• The largest logs I get on a once-a-year basis is about 45″ diameter (8′-10′ from the ground).
• The largest hardwood I have ever milled is a 54″ diameter (20′ from the ground) Burr Oak.