What this calculator does
It answers the question every shipper asks before a booking: how many of these cartons go in one box, and how many boxes do I need? You give it the carton’s three dimensions, its packed weight, how many you are shipping, how high the carton can be stacked, and the container you have in mind. It works out the best axis-aligned stow, applies your stacking limit and the container’s payload limit, and tells you which of the two actually decided the answer.
That last part is the useful bit. Half the arguments in freight are between someone who thinks the shipment cubes out and someone who thinks it weighs out; the tool names the constraint instead of leaving you to infer it.
The formula
For one chosen orientation of the carton against the container’s internal length L, width W and height H:
n_L = ⌊L / a⌋ cartons along the container
n_W = ⌊W / b⌋ cartons across the container
n_H = min(⌊H / c⌋, S) cartons high, capped by the stacking limit
N = n_L × n_W × n_H
where a, b, c are the carton’s length, width and height as placed, and S is the maximum number of cartons the bottom one can carry. A rectangular box has six axis-aligned orientations, so the tool evaluates all six and keeps the largest N. The floor divisions are the whole point: three cartons of 400 mm fit in 1200 mm, but so do three cartons of 390 mm — the remaining 30 mm is dead air, and no amount of pushing recovers it.
The stacking limit is applied inside the orientation search, not after it. That matters. With S = 4 a carton stood on its 60 cm side may beat the same carton laid flat, because it gets a tighter footprint and still reaches the permitted four tiers; with S = 2 the ranking can flip. Optimising the geometry first and imposing the limit afterwards gives the wrong stow.
Two reductions are then applied to that geometric block:
N_vol = ⌊N × f⌋ f = usable volume factor
N_mass = ⌊P / m⌋ P = rated payload, m = carton gross mass
N_cont = min(N_vol, N_mass)
The usable volume factor is where you buy back realism. A perfect block stow needs no dunnage, no lashing, no airbags between tiers, no space to swing the doors, and cartons that are exactly the size printed on them. None of that is true. Between 85% and 95% covers most palletless, hand-loaded stows.
Reading the result
Cartons per container is the stow figure — use it to size the order, not to plan the load. Cartons in the last container is what the part container carries; if it comes back very low, compare it against an LCL booking or against trimming the order to a round container quantity.
Volume utilisation is measured against the container’s nominal internal cube, and it is almost never near 100%. A figure in the high sixties is normal for a carton that does not divide the box neatly. It is a diagnostic, not a target: at 45% volume against 99% payload, the cargo is dense and the cube is irrelevant.
The governing constraint is reported as a code:
- 1 — cube. The container’s internal dimensions ran out. More cartons only fit if you change the carton, not the box.
- 2 — floor area. The carton is too tall to double-stack in this container in any orientation, so you are paying for one tier of floor. High-cube boxes sometimes rescue this; a 269.8 cm internal height double-stacks a 130 cm carton where a 239.3 cm standard box does not.
- 3 — stacking limit. The box would take more tiers; your carton’s crush strength (or the consignee’s rule) will not. This is the constraint most often relaxed by better packaging rather than better planning.
- 4 — payload mass. The cargo weighs out. Adding cube-efficiency achieves nothing; the answer is a lighter pack, a different container split, or accepting the part-loaded stow.
Typical values and limits
Nominal internal dimensions for the dry-box series, and the payloads used here, are:
| Box | Internal L × W × H (cm) | Nominal payload |
|---|---|---|
| 20’ GP | 589.8 × 235.2 × 239.3 | 28,200 kg |
| 40’ GP | 1203.2 × 235.2 × 239.3 | 26,700 kg |
| 40’ HC | 1203.2 × 235.2 × 269.8 | 26,460 kg |
| 45’ HC | 1355.6 × 235.2 × 269.8 | 25,600 kg |
Treat every one of those numbers as indicative. ISO 668 fixes the external envelope and the maximum gross mass of the series; it does not fix internal dimensions, which vary by builder and build year, typically by a few centimetres. Tare varies more, and tare is what sets payload — two 40’ HC boxes from different fleets can differ by several hundred kilograms of allowed cargo. The CSC plate on the actual container is the only authority. If the shipment is close to the limit, get the box number and read the plate.
The other limit that catches people is the road one. A container can frequently carry more than the truck at either end is allowed to move: gross combination mass limits, axle limits and per-container road caps are national or sub-national, and they routinely bind at 20–26 t of cargo where the box itself is rated for 28 t. Check the limit that applies at both ends before you fill a 20’ with dense cargo.
Worked example
900 cartons of 60 × 40 × 40 cm at 18 kg each, stackable four high, into a 40’ high cube, with a 90% usable volume factor.
The internal dimensions are 1203.2 × 235.2 × 269.8 cm. Three distinct orientations exist for this carton:
60 × 40 × 40 → ⌊1203.2/60⌋=20 × ⌊235.2/40⌋=5 × min(⌊269.8/40⌋=6, 4)=4 = 400
40 × 60 × 40 → ⌊1203.2/40⌋=30 × ⌊235.2/60⌋=3 × min(6, 4)=4 = 360
40 × 40 × 60 → ⌊1203.2/40⌋=30 × ⌊235.2/40⌋=5 × min(⌊269.8/60⌋=4, 4)=4 = 600
The winner stands the carton on its 40 × 40 face, 60 cm tall: 600 cartons geometrically. Note the 200 carton spread between best and worst orientation — a 50% penalty for loading the same carton the “obvious” way round.
Apply the 90% factor: 600 × 0.90 = 540 cartons per container. The mass check is ⌊26,460 / 18⌋ = 1,470 cartons, so mass is nowhere near binding. The cube governs.
For 900 cartons that is ⌈900 / 540⌉ = 2 containers, the second carrying 360 cartons. Volume utilisation is 540 × 0.096 m³ = 51.84 m³ against a 76.35 m³ nominal internal cube, or 67.9%. Payload utilisation is 540 × 18 = 9,720 kg against 26,460 kg, or 36.7% — a classic light-and-bulky shipment. Total shipment volume is 900 × 0.096 = 86.4 m³ and total gross mass 16.2 t.
FAQ
Why is my volume utilisation only 68% when the calculator says the cube governs? Because cube-out and full are different things. The 32% shortfall is the floor-division waste (each of the three dimensions leaves a remainder) plus the 10% usable factor you asked for. A carton that divides the container cleanly in all three axes would show far higher; almost none do.
Will a real load planner beat this number? Usually, by a few per cent. Commercial load-planning software mixes orientations within a stow, treats the door end separately, and fills voids with a second carton size. Single-orientation block stow is deliberately a floor — a number you can quote without being embarrassed when the container is actually packed.
Should I use nominal internal dimensions or the ones on the container? The ones on the container, whenever the shipment is tight. The values here are the commonly published nominals for the series and are fine for quoting and planning, but they are not a specification. Builders differ, and a container with a repaired floor or an interior liner differs again.
What about weight distribution? Not modelled, and it can override everything above. Dense cargo stowed only in the door end overloads an axle even with the container well inside its payload. Spread the mass, keep the centre of gravity central and low, and check the axle limits for the road move.
Does this work for pallets rather than loose cartons? Run the pallet space calculator first, then treat the loaded pallet as the “carton” here — its footprint, its height including the deck, and its gross weight.
Indicative figures for quoting and planning. Confirm payload against the container’s CSC plate, and confirm road gross and axle limits in both the origin and destination jurisdictions before loading.