JAS Log Scaling: The Japanese Agricultural Standard Explained
How JAS scales round logs: the 1 cm / 2 cm diameter rounding rule, V = D² × L ÷ 10,000, the long-log adjustment, the oval-log rule, and why Asia-Pacific plantation trade quotes JAS cubic metres.
The volume formula
JAS measures the small-end diameter under bark and rounds it down: below 14 cm to the nearest 1 cm, at 14 cm and above to the nearest even 2 cm step — 25.9 cm scales to 24. For logs up to 6 m, volume is V = D² × L ÷ 10,000 (D in cm, L in m), rounded to 3 decimals. A 24 cm × 4 m log therefore scales 576 × 4 ÷ 10,000 = 0.230 m³.
Logs longer than six metres
Longer logs add a taper allowance to the diameter: D_eff = D + (⌊L⌋ − 4) ÷ 2 centimetres, then the same formula. A 24 cm log 8 m long gets D_eff = 24 + (8 − 4)/2 = 26 → 26² × 8 ÷ 10,000 = 0.541 m³. The add-on approximates the taper a small-end measurement otherwise misses.
Oval logs
When a log is oval, both axes are scaled separately and averaged. On top of that, an ovality correction may apply: with a short axis of 14–40 cm and a long-short difference of 6 cm or more, add 2 cm per full 6 cm of difference; above 40 cm short axis, 2 cm per 8 cm. Example: axes 22 and 30 scale to 22 and 30, average 26, difference 8 ≥ 6 → +2 → D = 28; at 5 m: 784 × 5 ÷ 10,000 = 0.392 m³.
Why D² (and no π/4)
True cylinder volume would be π/4 × D² × L, so a bare D² × L reads ≈ 27% (4/π) higher than a cylinder of the same D. That is deliberate: D is a small-end diameter rounded down, and the missing π/4 approximates the taper between the small end and the rest of the log. For typical plantation logs the two effects largely cancel — which is why JAS cubic metres track commercial reality closely enough to price shipments on.
Grading beyond volume
JAS also standardises quality: rules for knots, sweep, splits and ovality, with defect deductions agreed contract by contract. Vietnam, Japan and much of the Asia-Pacific plantation trade quote roundwood in JAS m³ precisely because the scale is unambiguous — but mixing scales inside one deal (JAS for intake, Hoppus for stock, true m³ for export) is where margin leaks; pick one and reconcile the rest with conversion (Hoppus Formula Explained: Quarter-Girth Log Measurement for Hoppus).
Scale a load in seconds
JAS log scale implements the exact rounding, ovality and long-log rules above — including the per-axis averaging — and multiplies by piece count for a truck or container load. For true-volume scaling by mid-diameter, Log volume (Huber/Smalian) applies Huber, Smalian and Newton; comparing the two on the same log is the fastest way to understand your own taper bias.