Booking ocean freight starts with one question: how much fits in the box? This planner takes your carton or pallet size and the container’s true internal dimensions, then reports the unit count limited by both volume and payload, so you know whether your cargo cubes out or weighs out first.
Container types and their practical capacities
The ISO container family covers three standard sizes used in most ocean freight:
| Type | Internal L × W × H (approx) | Usable CBM | Max payload (approx) |
|---|---|---|---|
| 20ft GP | 589 × 235 × 239 cm | ~33 CBM | ~28,000 kg |
| 40ft GP | 1,203 × 235 × 239 cm | ~67 CBM | ~26,500 kg |
| 40ft High Cube | 1,203 × 235 × 269 cm | ~76 CBM | ~26,500 kg |
The High Cube’s extra 30 cm of height (269 vs 239 cm) adds about 13% more volume over the 40ft GP — useful for light, bulky goods that stack to ceiling height. The 20ft box has a higher payload-to-volume ratio, making it attractive for very dense cargo where the 40ft’s volume advantage doesn’t matter.
Cubing out versus weighing out
Every load is constrained by two limits: it must physically fit inside the container (volume), and it must not exceed the maximum cargo payload for safe road transport. The binding constraint depends on your product:
- Light, bulky goods (pillows, foam, apparel, hollow plastics) — run out of volume before weight. A container full of pillows barely registers on the scale.
- Dense goods (tiles, steel parts, liquids, chemicals, machinery) — reach the weight limit with space to spare inside the container.
Knowing which limit binds helps you negotiate rates: if you always cube out, a High Cube adds real value; if you always weigh out, the extra height is irrelevant.
How the planner calculates fit
The tool fills a 3D grid inside the container’s internal dimensions. For cartons it tries all six possible orientations and picks the one that maximises the unit count:
nx = floor(containerL / unitL)
ny = floor(containerW / unitW)
nz = floor(containerH / unitH)
fit by volume = nx × ny × nz (best carton orientation)
fit by payload = floor(payloadLimit / unitWeight)
units loaded = min(fit by volume, fit by payload)Pallets are treated differently: they stay upright (height is fixed) and only the footprint orientation is tried. The tool reports both the volume fit and the payload fit, flags which is binding, and shows the volume utilisation percentage.
Worked example
For illustration, consider cartons of 50 × 40 × 35 cm at 10 kg each in a 40ft GP:
- Best orientation: 50 cm along length → floor(1203/50) = 24 columns; 40 cm along width → floor(235/40) = 5 rows; 35 cm high → floor(239/35) = 6 layers. That is 24 × 5 × 6 = 720 cartons by volume.
- Payload check: 720 cartons × 10 kg = 7,200 kg — well under the ~26,500 kg limit. Volume binds.
- Volume utilisation: 720 × (0.5 × 0.4 × 0.35) = 720 × 0.07 = 50.4 CBM ÷ 67 CBM ≈ 75%.
Leaving 25% of the space empty in this scenario suggests either tighter packing (try a High Cube for taller goods, or add another SKU to fill gaps) or that a smaller 20ft might consolidate the load and reduce freight cost.
Tips for practical use
- Leave 5–10% slack from the calculated maximum for bracing, dunnage, and the reality that real cartons rarely tessellate perfectly.
- For mixed-SKU loads, run the calculation for your largest carton type first to check the primary constraint, then assess remaining capacity.
- The 40ft High Cube is worth the small premium over the 40ft GP whenever goods stack to the standard-height ceiling — the extra 30 cm often adds meaningful units.