Scaling board feet refers to the process of measuring and estimating the usable lumber volume from a log, or directly calculating the volume of sawn timber. This crucial step is fundamental for valuing timber, planning milling operations, and trading wood products, ensuring fair and accurate transactions.
What is a Board Foot?
A board foot (BF) is a standard unit of volume for lumber, representing a piece of wood 12 inches long, 12 inches wide, and 1 inch thick. It is equivalent to 144 cubic inches.
- Formula for Sawn Lumber:
Board Feet = (Length in feet × Width in inches × Thickness in inches) / 12
Scaling Logs for Board Feet
Estimating board feet from a log involves measuring its dimensions and applying a "log rule." Log rules are standardized tables or mathematical formulas that predict the approximate lumber yield from logs based on their diameter and length. This process is essential for foresters, loggers, and sawyers to determine the volume of timber before it is milled.
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Measure Log Length:
- Measure the log from the butt (larger end) to the tip (smaller end).
- Lengths are typically recorded in even-foot increments (e.g., 8, 10, 12, 16 feet).
- Always add a few extra inches (e.g., 4-6 inches) for trim allowance to square off ends or remove defects after milling.
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Measure Log Diameter:
- This is the most critical measurement and is typically taken at the small end of the log.
- Crucially, when measuring the diameter, it is important that you measure from the inside of the bark. This ensures you're measuring the actual wood that will be milled, not the bark.
- For irregular logs: If a log isn't perfectly round, two measurements might be necessary. These measurements should be taken at right angles to each other across the smallest end, and the average of these two measurements should be used to determine the log's diameter.
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Apply a Log Rule:
Once dimensions are known, a specific log rule is used to estimate the board foot volume. Different log rules exist, each with its own methodology and historical context. The most common include:- Doyle Log Rule: Tends to under-scale small logs and over-scale large logs. Its simplicity has made it popular in some regions.
- Scribner Log Rule: A diagrammatic rule based on actual saw kerf and log taper. It is widely used and generally more accurate than Doyle, though it may still slightly under-estimate the yield from smaller logs.
- International 1/4-inch Log Rule: Considered the most consistent and accurate for all log sizes because it accounts for a fixed saw kerf (1/4 inch) and a standard taper (1/2 inch per 4 feet of length). It is often preferred for scientific or comparative purposes due to its consistent predictions.
Example Comparison of Log Rules
Let's consider a 16-foot log with an 8-inch small-end diameter (measured inside bark) to illustrate how different rules yield varying board foot estimates:
| Log Rule | Board Feet Estimate (16' long, 8" diameter) | Notes |
|---|---|---|
| Doyle Log Rule | 16 BF | Formula: (D - 4)$^2$ L / 16, where D=diameter in inches, L=length in feet. For an 8" log: (8-4)$^2$ 16 / 16 = 4$^2$ = 16 BF. This rule typically under-estimates small log volumes significantly. |
| Scribner Log Rule | 24 BF | Based on diagrams of actual lumber recovery, it provides a more realistic yield for average logs compared to Doyle, though it can still be conservative for smaller logs. |
| International 1/4-inch Log Rule | 40 BF | This rule is based on a more detailed mathematical formula accounting for fixed saw kerf and log taper, making it the most consistent and often the highest yielding for a given log size, especially for smaller logs. It provides a more accurate representation of potential lumber recovery. |
Note: The actual lumber yield from a log can vary based on milling efficiency, log defects, saw kerf, and timber species. Log rules provide an estimate of sound, usable lumber.
Calculating Board Feet for Sawn Lumber
Once logs are milled into sawn lumber, calculating board feet becomes a direct measurement using the formula mentioned earlier. This is common for inventory management, sales, and purchasing.
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Steps for Sawn Lumber:
- Measure the actual length of the board in feet.
- Measure the actual width of the board in inches.
- Measure the actual thickness of the board in inches.
- Apply the formula: (Length in feet × Width in inches × Thickness in inches) / 12
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Examples:
- A nominal 2" x 4" x 10' board (actual dimensions might be 1.5" x 3.5" for kiln-dried, but calculation uses nominal for BF):
(10 ft × 4 in × 2 in) / 12 = 80 / 12 = 6.67 board feet - A 1" x 6" x 8' board:
(8 ft × 6 in × 1 in) / 12 = 48 / 12 = 4 board feet - A 4" x 4" x 12' beam:
(12 ft × 4 in × 4 in) / 12 = 192 / 12 = 16 board feet
- A nominal 2" x 4" x 10' board (actual dimensions might be 1.5" x 3.5" for kiln-dried, but calculation uses nominal for BF):
Practical Insights for Scaling
- Defects: Log rules estimate the volume of clear lumber. Significant defects like rot, excessive sweep (curve), large knots, or splits will reduce the actual recoverable board feet. Skilled timber scalers make deductions for these defects.
- Overrun/Underrun: The actual lumber recovered from a log can be more (overrun) or less (underrun) than predicted by a log rule. This is influenced by factors such as mill efficiency, saw kerf width, and the specific quality of the log.
- Moisture Content: While not directly part of scaling, it's important to remember that lumber typically shrinks as it dries. Board foot calculations are usually based on nominal green (undried) dimensions, though dry dimensions are what you'll work with in projects.
Understanding how to scale board feet is essential for anyone involved in forestry, logging, or lumber purchasing, ensuring fair trade and efficient resource utilization throughout the timber supply chain.
