Two people can measure the same log honestly and write down volumes that differ by more than a fifth. Neither is cheating. They are using different conventions, and if that is not settled before the contract, somebody loses money.
The two measurements
True volume (Huber’s formula) treats the log as a cylinder at its middle:
True CBM = mid girth (cm)² × length (m) ÷ 125,660
Hoppus volume takes a quarter of the girth and squares it:
Hoppus CBM = (mid girth (cm) ÷ 4)² × length (m) ÷ 10,000
Where 21.5% comes from
Divide one by the other and the log’s own size drops out:
- True volume divides by 4π ≈ 12.566
- Hoppus divides by 16
So Hoppus / True = 12.566 ÷ 16 = 0.7854, which is π/4. Hoppus is always 78.54% of true volume, which is 21.46% less — for every log, every species, every size.
| Convert | Multiply by |
|---|---|
| Hoppus → true volume | 1.2732 |
| True volume → Hoppus | 0.7854 |
| Hoppus feet → cubic feet | 1.2732 |
| Hoppus CBM → Hoppus feet | 35.315 |
Worked example
A teak log: mid girth 150 cm, length 4 m.
- True volume: 150² × 4 ÷ 125,660 = 0.716 CBM (25.3 CFT)
- Hoppus volume: (150 ÷ 4)² × 4 ÷ 10,000 = 37.5² × 4 ÷ 10,000 = 0.563 CBM (19.9 Hoppus feet)
- Difference: 0.154 CBM on one log.
On a 30 m³ container, that gap is roughly 6.4 m³. At $400 per m³, it is about $2,570 on a single container — which is why the convention belongs in the contract, not in a conversation.
Which market uses what
| Market | Usual convention |
|---|---|
| India, Myanmar (teak especially) | Hoppus, often quoted in Hoppus feet |
| UK and Commonwealth tradition | Hoppus, historically |
| Europe, Middle East | True volume (Huber), metric |
| Japan, Korea, Taiwan | JAS, which is Huber with rounding rules |
| North America | Doyle or Scribner board-foot scales |
How to avoid the argument
- Write the convention into the contract, in words: “Volumes are Hoppus (quarter girth), measured at mid length, over bark deducted.”
- Put both numbers on the tally sheet. True volume and Hoppus, side by side. Then nobody has to redo the arithmetic under pressure.
- Never mix Hoppus feet and cubic feet in one document. They differ by 27%.
- Agree the bark rule at the same time. Over bark and under bark differ by 2–6% on many species.
- Say which end the length is measured to, particularly on tapered logs.
Keeping the raw measurement, not just the result
Whichever convention your contract uses, the sheet has to keep the girth and length you actually measured. Once only a volume is written down, nobody can re-check it against the other convention.
LumberLinq records girth and length as your staff tally on the phone and calculates round log volume with the Hoppus formula — the convention used across the Indian and teak trade — in CBM and CFT. Nothing is re-keyed afterwards, so the sheet your buyer sees and the stock in your system always agree, and the raw measurement stays on file if a buyer asks for true volume.
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Frequently asked questions
How do I convert Hoppus to true volume? Multiply by 1.2732. The other way, multiply true volume by 0.7854.
Why does Hoppus exist if it under-measures? It was created in 1736 to estimate the squared beam that could be cut from a round log, so the buyer paid for usable timber, not the round section. The custom outlived the hand-squaring.
Which markets still use Hoppus? India, Myanmar and parts of the UK and Commonwealth trade, especially for teak.
Is a Hoppus foot the same as a cubic foot? No. One Hoppus foot is about 1.273 cubic feet of actual wood.
Related: Huber’s formula explained · Free timber tally sheet template