International Log Rule1/4 inch Kerf Board Foot Calculator
Most scientifically accurate US log rule. Accounts for 1/4-inch saw kerf and log taper (1/2 inch per 4 ft). Standard for high-value hardwoods, research timber sales and export documentation.
Why this calculation matters
The International rule tracks real mill output most closely, which is why it is the benchmark the other rules are judged against.
International Log Rule
1/4 inch Kerf Board Foot Calculator
This picker does not change the answer. Board feet depends only on the dimensions you enter, not on the species — that is what makes it a rule rather than an estimate. The species is here so you can note it on a printout.
Measured over bark. If your figure is already inside bark (DIB), set bark to 0.
| Section | Diameter | BF This Section | Cumulative BF |
|---|
Kerf. This is the International ¼-inch rule: the saw kerf is built into the Grosenbaugh polynomial, not a setting. The rule is only published for ¼″ and ⅛″ kerfs; other widths have no published variant. If you are cutting a thin bandsaw kerf your real yield will run above this figure.
About International Log Rule
Developed by the Lake States Experiment Station. The most accurate log scaling method - calculates board feet using the Grosenbaugh polynomial form of the official table, accounting for taper and 1/4-inch saw kerf. Preferred for high-value hardwood timber sales, forestry research and international export documentation. Foresters consider International 1/4-inch the most accurate log rule in common use, and this calculator saves you paging through an international log rule table.
Where Used?
Formulas
BF = 0.04976191·L·D² + 0.006220239·L²·D − 0.1854762·L·D + 0.0002591767·L³ − 0.01159226·L² + 0.04222222·LD = small-end diameter inside bark (inches), L = log length (feet)Grosenbaugh (1948) polynomial form of the official USDA table; assumes 1/2-inch taper per 4 ft- International 1/4-inch rule — Grosenbaugh polynomial (all six coefficients)
Formula checked against the sources above by an automated regression test (tests/test-calculators.js) that derives each expected value independently of this page. Last reviewed .
FAQ
Wood species data
Density, hardness and movement for 60 timbers
How the calculation works
The log is treated as a series of four-foot sections rather than one cylinder, and each section is scaled from its own diameter. That is why the rule tracks real mill output more closely than the older diagram rules: it allows for both saw kerf and the fact that a log gets wider toward the butt.
Show the formula
Each 4 ft section: board feet = 0.905 x (0.22 d^2 - 0.71 d), where d is that section’s small-end diameter inside bark, increasing toward the butt by the taper entered. Section volumes are summed; a part-section is prorated by its length.The 0.905 factor converts the 1/8-inch kerf basis to the 1/4-inch rule. At the default 0.5 in per 4 ft taper this reproduces the standard closed-form polynomial to within 0.05%.
References and what each one provides
- USDA Wood Handbook (FPL GTR-190)USDA Forest Products LaboratoryPublished density, moisture relationships, shrinkage coefficients and mechanical properties for a wide range of species. The reference dataset behind most wood-property figures on this site.
Unverified values are marked as such rather than presented as sourced.
Limitations of this calculation
Applies the International quarter-inch rule with its built-in kerf and taper allowance. It does not account for log defect, sawing pattern, or mill-specific recovery.
Do not use this for Sale settlement without an agreed rule, even though it is the most accurate of the three.
Figures are engineering estimates from the inputs and assumptions shown. Verify against the actual material and, where the result affects structure or safety, against a qualified professional.
How to use this tool
- Enter the log measurements below, then press Calculate.
- Fill in small-end diameter, log length, number of logs, bark each side.
- Press Calculate.
Worked example
Common mistakes to avoid
- Quoting 'International rule' without saying 1/4-inch. There is also a 1/8-inch version; they differ.
- Applying the rule per log instead of per 4-foot section on long logs. The rule steps with taper.
- Assuming International BF equals sawn output. It is the closest rule to reality, but mill practice still varies.