Measuring and Understanding Wood Volumes

Using Rule-of-Thumb Wood Volume Conversions

"Theoretically, one cubic foot (of wood volume) contains 12 board feet. For average values 6 should be used, though 10 is a conventional figure for approximations. When the conversion applies to trees, ratios of 3 to 8 should be applied (U.S. Department of Agriculture, 1935)."
- taken from Converting Factors for Southern Pine Products, Williams and Hopkins, USDA, 1968

Measuring wood is part science, part art; you use many different units, you face many potential problems.

The above quote illustrates how confusingly insane measuring and converting wood volumes can be. Measuring and estimating wood volume is not for the faint of heart.

When marketing your timber you must either know how to measure forest products or get someone to do it for you. At best you can be very confused when talking to a wood buyer; at worst you can lose a significant portion of the value of your wood.

To make the situation even more problematic, some buyers use this ignorance of volumes to ruse the seller. They have every opportunity to do so and a few use this to their financial advantage. Knowing tree measuring units is very complicated and even foresters have a hard time when talking volumes. Three hundred dollar per thousand logs using Doyle log rule is not the same as three hundred dollars per thousand logs using Scribner log rule.

Most mensurationists and foresters would agree that there is an advantage to weighing wood and weight is the measurement of choice.

In the real world, however, it is impractical to totally convert to weight. A history of wrestling with the problem of measuring logs to determine how much usable product might be manufactured from them created numerous measuring units. These units are self-perpetuating because of many factors including foreign trade, standing timber volume, accepted taxing units, regional custom, buying and selling advantages.

The Pulpwood Measurement

The standard measurement unit for wood used for paper and fuel is the cord. This is a stack of wood 4 ft. x 4 ft. x 8 ft. containing approximately 128 cubic feet of bark, wood and air space. Air space can actually be as high as 40 percent but usually averages 25 percent. You can see where weight can be advantageous here.

Pulpwood purchases by weight are very common and weight per cord varies widely with species and geography. A hardwood pulpwood cord generally weighs between 5,400 pounds and 6,075 pounds. A pine pulpwood cord weighs between 4,700 pounds and 5,550 pounds. You really need to determine your local average weight by species when measuring cordwood.

Purchasing mills or men who harvest pulpwood can give you wood weights for your area. The U.S. Forest Service or your State Forester also has a wealth of information on regional average weights. Pulpwood bought in the form of chips are separate issue and for another discussion.

The Sawtimber Measurement

A round log, generally, must be made into square or rectangular pieces to be able to determine wood volume and value. Three systems, or log rules and scales, have been developed to do just this. They are called the Doyle rule, Scribner rule, and International rule.

They were developed to estimate board foot mill tally, usually quoted as thousand board feet or MBF.

Our problem when using these log rules or scales is that they will give you three different volumes for the same pile of logs.

Measuring average sized logs - Doyle, Scribner, and International rules - will give volumes that may vary as much as 50%. This "overrun" is greatest using Doyle and the least using International. Buyers like to purchase using Doyle log rule while sellers like to sell using Scribner or International.

There will always be a difference in volumes estimated from scaler to scaler. They get into trouble when decreasing actual number of measurements and start estimating; they measure at inappropriate points on the log, miss estimate roundness, and don't deduct for defect. Accurate scaling of trees and logs requires skill and experience.

The Conversion Factor

Mensurationists cringe at the word conversion factor. They correctly feel that conversion from one unit of measure to another unit of measure of wood is too imprecise to depend on. Their job is to be precise.

But you have to have some way to estimate volumes and be able to cross over to differing units.

You now have an idea of how complicated this volume issue can become. To add a conversion factor to volumes may distort actual volumes even more.

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