How to Measure Logs
Accurate log measurement is the foundation of fair timber transactions. All major log scaling rules — Doyle, Scribner Decimal C, and International 1/4" — require two inputs: small-end diameter inside bark and log length in feet. This guide covers how to measure each correctly.
Small-End Diameter Inside Bark (DIB)
The key measurement for all US log rules is the small-end diameter inside bark (DIB). You measure the smaller end of the log, and you measure inside the bark — not including the bark thickness.
Why the small end?
A log is tapered — wider at the butt than at the top. Using the small end is the conservative measurement: it captures the narrowest point where lumber can be sawn, which limits the actual yield of the log.
How to measure DIB
- Stand at the small end of the log.
- Estimate and subtract the bark thickness from the outside diameter. Bark is typically 0.5–2" thick per side depending on species. For rough estimation: most scalers subtract 1" from each side (2" total) for thin-barked species, 1.5–2" per side for thick-barked species like Douglas Fir.
- Measure the resulting inside-bark diameter with a diameter tape, log calipers, or a scale stick.
- For an out-of-round log, use the shortest diameter across the small end. Some agencies average two perpendicular measurements — confirm the protocol with your buyer.
Bark deduction reference
| Species group | Typical bark thickness (per side) | Total deduction for DIB |
|---|---|---|
| Oak, Maple, Cherry, Birch | 0.5–0.75" | 1–1.5" |
| Poplar, Ash, Elm | 0.5–1" | 1–2" |
| Douglas Fir, Larch | 1–1.5" | 2–3" |
| Southern Yellow Pine | 0.5–1" | 1–2" |
| Western Red Cedar | 0.5–0.75" | 1–1.5" |
| Ponderosa Pine | 1–2" | 2–4" |
Measuring Log Length
Log length is measured along the long axis of the log, from end to end. Use a straight measurement — not along any curve. Standard commercial log lengths are multiples of 4 feet: 8, 12, 16, and 20 feet are most common.
Trim allowance
Sawmill equipment requires a slight overrun to square the log ends. Industry convention adds a trim allowance to the targeted log length before cutting:
- Short logs (8–12 ft): add 3–4 inches trim allowance
- Long logs (14–20 ft): add 4–6 inches trim allowance
So a 16-foot log is cut to approximately 16'4"–16'6" in the woods. The extra length is wasted at the mill when the ends are squared up. For scaling purposes, the recorded length is the nominal length (16 feet), not the actual cut length.
Length rounding
Log length for scaling purposes is rounded down to the nearest foot. A log measuring 16'10" is scaled as 16 feet.
Scaling Tools
Diameter tape
A diameter tape (D-tape) measures circumference and converts to diameter automatically — it's marked so that a circumference reading gives you the equivalent diameter directly. Useful for large logs where you can't span the full diameter with calipers.
Log calipers
Mechanical calipers with two parallel arms measure log diameter directly across the face of the log. Accurate and fast. They do not require you to see all the way around the log.
Scale stick (Biltmore stick)
A flat stick held horizontally at arm's length against the log. It's calibrated so that when held at a fixed eye-to-stick distance (typically 25"), reading the stick at the left edge of the log gives you the diameter at the point where the right edge aligns — a trigonometric trick. Scale sticks often have separate columns for different log rules. Accuracy depends on technique and arm-length calibration.
Which Log Rule to Use?
Three major rules govern North American log scaling:
| Rule | Where used | Characteristic |
|---|---|---|
| Doyle | Appalachia, Southeast US | Underscales small logs (<16"); overscales large logs (>28"). Formula: (D−4)²×L/16 |
| Scribner Decimal C | Pacific Northwest, national forests | Table-based; more accurate than Doyle for most sizes |
| International 1/4" | Northeast US, Canada | Most accurate formula-based rule; accounts for saw kerf and slabs |
The rule used in your region or sale contract determines which measurement is used for payment. Always confirm with the buyer.
→ Full guide: Log Rules Explained
→ Log Scale Calculator — compute all three rules side by side
Defect Deductions
Experienced scalers also deduct for defects that reduce the usable lumber volume in a log:
- Sweep: Curvature of the log along its length. A swept log yields less clear lumber than a straight log of the same DIB.
- Crook: A sharp bend, usually near one end.
- Rot and checks: Decay, shake, or splitting reduces the usable cross-section.
- Seams and knots: Deducted by reducing the effective scaling diameter.
Defect deductions require judgment and knowledge of grading rules for your specific log sale type. For most private timber sales, a certified scaler or forester handles defect deductions.
Frequently Asked Questions
Where do you measure the diameter of a log?
For scaling purposes, measure the small-end diameter inside bark (DIB) — the narrower end of the log, measured inside the bark. This is the standard for all major US log rules (Doyle, Scribner, International).
How do you measure an out-of-round log?
Measure the shortest diameter across the small end, inside bark. Some scalers average two perpendicular measurements, but the USDA standard is to use the shortest dimension, which is more conservative.
What is a timber scale stick?
A timber scale stick (also called a log scale stick or Biltmore stick) is a calibrated measuring stick used to estimate log diameter and tree height. It is marked to give Doyle, Scribner, or International log rule board feet directly from the diameter reading.
How do you measure log length?
Measure from end to end in a straight line (not along the curve). Standard log lengths are multiples of 4 feet (8, 12, 16, 20 ft) with a trim allowance of 3–6 inches added for each end to allow squaring. Length is rounded down to the nearest foot for scaling.