CBM calculation: formula, loading metres and volume traps
Updated
CBM — cubic metres — is the universal currency of freight: it's what forwarders quote LCL groupage on, and what decides between a 20ft and a 40ft container. The formula fits on one line. The mistakes it hides fill half-empty containers — or leave pallets on the dock.
This guide covers the formula, how CBM relates to the loading metre (LDM) used in road freight, real container volumes computed from interior dimensions, and — most importantly — the three cases where CBM lies.
The CBM formula
For a rectangular package, in metres:
CBM = length × width × height × quantity
Example: 120 cartons of 600 × 400 × 400 mm. Convert to metres (0.6 × 0.4 × 0.4 = 0.096 m³), then 0.096 × 120 = 11.52 m³. Two habits prevent most errors: convert before multiplying (mixing centimetres and metres is the number-one bug in inherited spreadsheets), and always use the package's outer dimensions, packaging included — double-wall corrugated adds roughly 14 mm per dimension between inner and outer measurements.
For a loaded pallet, use the pallet footprint (it almost always sticks out beyond the cartons) and the total height, deck included.
CBM vs loading metres (LDM): two different logics
CBM thinks in volume; the loading metre thinks in trailer floor length occupied, over the standard usable width of 2.4 m. The formula: LDM = (cargo length × cargo width) / 2.4, in metres.
A Euro pallet (1200 × 800 mm) on the floor is 0.96 / 2.4 = 0.4 LDM. A 13.6 m trailer offers 13.6 LDM — which is where the 34-Euro-pallet maximum comes from (13.6 / 0.4), when the geometry cooperates (see how many pallets fit in a truck).
Rule of thumb: ocean and air freight quote on volume (CBM); European road part-loads quote on floor space (LDM) — because in a trailer, non-stackable cargo reserves its rectangle of floor regardless of its height.
Container volumes: real vs brochure
The volumes below are computed from the interior dimensions of ALPHA LOADING's presets (sourced from Maersk / Hapag-Lloyd). Note that interior dimensions vary by ±10 to ±30 mm between manufacturers and series, even for the same ISO type.
| Container | Int. length | Int. width | Int. height | Computed volume | Door (W × H) | Payload |
|---|---|---|---|---|---|---|
| 20ft Dry | 5,896 mm | 2,350 mm | 2,393 mm | ≈ 33.2 m³ | 2,350 × 2,274 mm | 28,200 kg |
| 40ft Dry | 12,032 mm | 2,350 mm | 2,393 mm | ≈ 67.7 m³ | 2,340 × 2,274 mm | 28,800 kg |
| 40ft High Cube | 12,032 mm | 2,350 mm | 2,697 mm | ≈ 76.3 m³ | 2,340 × 2,577 mm | 28,620 kg |
| 45ft High Cube | 13,556 mm | 2,352 mm | 2,698 mm | ≈ 86.0 m³ | 2,340 × 2,585 mm | 27,600 kg |
The three cases where CBM lies
1. Usable volume is not geometric volume
A 33.2 m³ capacity does not mean 33.2 m³ of loadable space. Real packages leave voids (you can't slice cartons), the top layer stops short of the ceiling, and the door is lower than the interior: 2,274 mm of opening versus 2,393 mm of ceiling height on a 20ft Dry. A 2,350 mm-tall crate “fits” on paper… and hits the door header. Depending on cargo uniformity, loading 80–90% of the geometric volume is typical — far less for irregular freight.
2. Non-stackable cargo: volume counts double
A non-stackable item 1 m tall under a 2.4 m ceiling sterilises the 1.4 m above it: its real footprint is its floor area × the full container height. This is the classic CBM quoting trap: 20 m³ of non-stackable goods will not fit in a 33 m³ 20ft box — you must think in floor area, i.e. in LDM. Stackability is the variable that decides whether your CBM is fact or fiction.
3. Weight maxes out before volume — especially in a 20ft
A 20ft Dry carries 28,200 kg for 33.2 m³: beyond roughly 850 kg/m³ of density (tiles, paper, beverages, metal), weight saturates before volume does. A counter-intuitive detail worth memorising: a 40ft does not carry twice a 20ft — 28,800 kg versus 28,200 kg, nearly identical. For dense freight, two 20ft boxes carry almost double what one 40ft does. On a 20ft the binding constraint is almost always weight; on a 40ft, almost always volume.
From CBM to a load plan
CBM sizes a shipment; it doesn't prove it fits. The proof is a load plan: the actual geometry, item by item, with stacking, orientations and door clearance accounted for — plus the list of what doesn't fit. That is exactly what ALPHA LOADING computes in seconds, with a free account (3 simulations per month): paste your dimensions, it tells you whether you need a 20ft, a 40ft or a 40ft HC, and shows you the load in 3D.
Frequently asked questions
- How do I calculate CBM from centimetres?
- Convert to metres first, then multiply: a 60 × 40 × 40 cm carton is 0.6 × 0.4 × 0.4 = 0.096 m³. Alternatively multiply in centimetres and divide by 1,000,000 (96,000 / 1,000,000 = 0.096 m³).
- How many CBM fit in a 20ft container?
- About 33 m³ of geometric volume (interior dimensions of 5,896 × 2,350 × 2,393 mm on our presets). Realistically loadable volume is typically 80–90% for uniform, stackable freight — and much less for irregular or non-stackable cargo.
- How many CBM is a Euro pallet?
- The footprint is 1.2 × 0.8 = 0.96 m². Loaded to the usual European height of 1.8 m, a Euro pallet represents about 1.73 m³ — and 0.4 LDM of trailer floor.
- What is the difference between CBM and a freight ton?
- In LCL ocean freight, rates are often quoted “per w/m” (weight or measure): the carrier bills whichever is greater of the weight in tonnes or the volume in cubic metres. Light cargo pays for its volume; dense cargo pays for its weight.