Log Board Foot Calculator

A log is scaled from two numbers: its diameter inside the bark at the small end and its length. A 16 inch log 16 feet long scales 144 board feet by Doyle, 160 by Scribner Decimal C and 180 by the International ¼ inch rule. The rules disagree because each makes its own allowance for slabs, saw kerf and taper, and the gap is widest on small logs.

How the log board feet are calculated

Doyle is a formula. Subtract 4 inches for slabs and edgings from the small-end diameter D, square it, and scale by length L over 16 feet:

Doyle board feet = (D − 4)² × L ÷ 16

International ¼ inch was built by Judson Clark from 4 foot sections, each assumed ½ inch fatter than the one above it, with a ¼ inch saw kerf. Purdue Extension gives the 16 foot version as 0.796D² − 1.375D − 1.230, with every value rounded to the nearest 5 board feet; logs longer than 20 feet are scaled as two or more logs.

Scribner was drawn from diagrams of 1 inch boards sawn from each log size. Its Decimal C form, the one most buyers use, rounds to the nearest 10 board feet. Because it comes from drawings, not a formula, the calculator reads Scribner Decimal C and International ¼ inch straight from Purdue Extension's published tables (FNR-191). Where Purdue prints no value (a 39 inch log, or odd lengths for International), it falls back to the Bruce and Schumacher formula for Scribner and Grosenbaugh's formula for International, and the math box tells you so.

Worked example: one 16 inch log

A 16 foot log measures 16 inches across the small end inside the bark.

• Doyle: (16 − 4)² × 16 ÷ 16 = 12² = 144 board feet
• Scribner Decimal C: table value 16, meaning 160 board feet
• International ¼ inch: table value 180 board feet

At $100 per thousand board feet (MBF) the same log is worth $14.40, $16.00 or $18.00 depending on the rule. Virginia Cooperative Extension uses exactly this log and price to show landowners why the rule named in a timber sale matters.

Log rules compared, 16 foot logs

Values in board feet from Purdue FNR-191, Tables 1 to 3. Purdue calls Doyle a rule that "seriously under scales small logs and over scales large logs", and the USDA Forest Service's collection of log rules notes it gives buyers a large overrun on small logs. International ¼ inch is the most consistent of the three and is the usual yardstick for comparison. Scribner is fairly consistent on logs 16 feet or shorter and under 28 inches.

How to measure a log

Measure the small end, inside the bark. If it is out of round, Purdue recommends two measurements at right angles, averaged. Round to the whole inch the way your buyer does; always rounding down gives a smaller scale. For length, Ohio State Extension notes that logs are sold in 2 foot steps and need 6 to 8 inches of trim: a log 16 feet 6 inches long is a 16 foot log, one of 16 feet 1 inch is a 14 foot log.

In metric mode, enter centimeters and meters. The rules only exist in inches and feet, so the calculator converts, rounds the diameter to the whole inch and the length down to the whole foot, and shows both under the fields.

Board foot calculator for a standing tree

Switch to Standing tree to estimate before the saw comes out. Measure the diameter at breast height (DBH), 4½ feet up, outside the bark, and count the merchantable height in 16 foot logs up to where the stem narrows or branches. The calculator reads the Mesavage and Girard volume tables for form class 78, which Purdue and Southern Regional Extension Forestry both publish; Purdue's version runs from 10 to 40 inch DBH and from 1 to 6 logs in half-log steps.

Ohio State Extension's example: an 18 inch tree with three 16 foot logs holds about 215 board feet Doyle. The same tree is 280 board feet Scribner and 314 International.

Form class is the diameter inside bark at the top of the first log as a percentage of DBH. A rule of thumb in the Purdue and SREF tables adds or removes 3% of the volume per form class, so the 18 inch tree at form class 80 comes to about 228 board feet Doyle. Small trees with many logs are blank in the published tables; the calculator tells you instead of guessing.

From logs to lumber

The log scale predicts what a mill will saw; what you get depends on the sawyer, kerf and grade. Once the boards are stacked, tally them with the board foot calculator, and plan the cuts for a build with the lumber calculator. For a beam sawn from your own logs, check the size with the beam span calculator before you cut it; if your project mixes solid wood and sheet goods, the plywood calculator lays out the panels. More wood tools are under framing and lumber.