What you practice
Each set draws new logs. On a log end the scale stick lies across the short and the long axis, and you read both diameters to the half inch and apply the rounding rules. On a logger's tape you read the length and drop the trim allowance. Then you scale the log: Scribner Decimal C from the table, Doyle and International 1/4 inch by their rules. Some questions put one log through all three rules, and the truckload asks for five logs and a load total. The rules follow the Idaho Forestry Contest manual (revised 2025).
Example: two logs from the Idaho manual
A 10 in log measuring 16 ft 6 in scales 16 ft, and the Decimal C table gives 6 for 10 in by 16 ft: 60 board feet. A 17 in log measuring 12 ft 6 in scales 12 ft; the table gives 14, so 140 board feet. A log measuring 20 ft 10 in has more than 8 in of trim, so it scales 21 ft.
Questions
How do I measure the scaling diameter of a log?
Measure the small end inside the bark, on the short axis and the long axis, both through the true center rather than the pith. Take each to the nearest inch and average them. If the average ends in a half inch, round down.
What if a diameter falls exactly on the half inch?
The Idaho contest manual says: if only one of the two is on the half inch, round it up. If both are, round one up and the other down. Then average the two.
What is trim allowance?
Logs are cut a little long so the ends can be squared. Up to 8 in over the foot is free, so a log measuring 16 ft 8 in scales 16 ft. At 16 ft 9 in the scaling length becomes 17 ft.
Why do I multiply Scribner Decimal C by 10?
The Decimal C table drops the last zero: 80 in the table means 800 board feet, rounded to the nearest 10. A 10 in by 16 ft log reads 6, so it holds 60 board feet.
How do I work out the Doyle log rule?
Take 4 in off the small-end diameter, square it, multiply by the length in feet and divide by 16. A 14 in by 16 ft log is 10 × 10 × 16 ÷ 16 = 100 board feet.
Which log rule gives the most board feet?
On small logs Doyle gives the least, because taking 4 in off the diameter costs a small log a large share of its volume. International 1/4 in follows lumber tally most closely. On logs near 30 in, Doyle overtakes Scribner.
Sources, read October 27, 2026: Idaho Forestry Contest manual, chapter 1 Log Scaling (revised 2025: the scaling rules and its Scribner Decimal C table); Cornell, Tree and Log Scales (PDF): Doyle and International 1/4 in rules; Mesavage and Girard (1946), Tables for estimating board-foot volume of timber, USDA Forest Service, Tables 5 and 7: 16 ft log scales to the nearest board foot.