Cube-Out vs Weigh-Out: Which Limit Stops Your Container First?
Balanced density per container — about 1.0 t/m³ in a 20GP, 0.46 in a 40GP, 0.39 in a 40HC — why timber sometimes fills by volume and sometimes by weight, with worked examples.
Two ceilings on every load
Every container shipment hits one of two limits first. Cube-out: the space is full before the weight limit. Weigh-out: the payload limit is hit with space to spare. The decider is balanced density — payload divided by usable volume — compared against your cargo's density. Cargo denser than balanced density weighs out; lighter cargo cubes out.
Container reference table
| Type | Nominal | Usable | Payload | Balanced density |
|---|---|---|---|---|
| 20GP | 33.2 m³ | ≈ 28 m³ | 28,200 kg | ≈ 1.01 t/m³ |
| 40GP | 67.7 m³ | ≈ 58 m³ | 26,700 kg | ≈ 0.46 t/m³ |
| 40HC | 76.4 m³ | ≈ 68 m³ | 26,500 kg | ≈ 0.39 t/m³ |
Timber cargo: which limit wins?
Kiln-dried pine lumber at 450–500 kg/m³ sits right at a 40GP's balanced density — one extra bundle decides between cube-out and weigh-out. Green hardwood at 950–1,150 kg/m³ weighs out a 20GP after roughly 24–28 m³. Plywood (500–600 kg/m³) fills a 40HC by volume; loose chips (250–350 kg/m³) always cube out. The same product can flip limits with moisture alone.
Worked example
Sawn pine at 470 kg/m³ into a 40GP: 58 m³ usable × 470 = 27.3 t — over the 26,700 kg payload, so the load weighs out: max volume = 26,700 ÷ 470 ≈ 56.8 m³. The same pine into a 20GP: 28 m³ × 470 = 13.2 t, far under 28,200 kg — the box cubes out at 28 m³. Two containers, same cargo, two different binding constraints.
Practical checks before the truck leaves
Road limits at both ends are usually tighter than the container plate — verify axle and gross weights for the destination country. Heavy bundles raise floor-load questions (kg per m²) and weight-distribution rules (≈ 60/40 front/rear). Dunnage and lashing cost volume: budget 5–10% of the cube. Container loading computes fill volume, weight and flags which limit binds; the sea-leg view continues in Stowage Factor for Timber Cargo: IMO Ranges and How to Use Them.