How to Calculate CBM for Ocean Freight — Formula & Worked Examples
CBM (cubic metre) is the volume unit that ocean freight pricing is built on. Freight forwarders quote LCL rates per CBM, container capacity is expressed in CBM, and your freight cost per unit depends directly on how many CBM your order occupies. The formula itself is simple — the mistakes usually happen in unit conversion and in forgetting that quantity multiplies everything.
The formula
CBM = (Length m × Width m × Height m) × Quantity
Measure the outer dimensions of the export carton, not the product or the inner box. Convert centimetres to metres by dividing by 100 before multiplying.
Worked example
You are importing 500 cartons, each 40 × 30 × 30 cm. In metres: 0.40 × 0.30 × 0.30 = 0.036 CBM per carton. Total: 0.036 × 500 = 18.0 CBM. That is roughly 63% of a 20ft container's usable space — and right in the zone where you should compare LCL and FCL quotes (see our LCL vs FCL guide).
CBM and cubic feet
US-facing suppliers and some forwarders quote in cubic feet (CFT). The conversion is fixed: 1 CBM = 35.315 CFT. If a supplier tells you a carton is 1.27 CFT, that is 1.27 ÷ 35.315 ≈ 0.036 CBM.
Chargeable weight: when volume is priced as weight
For LCL sea freight, most forwarders apply a weight/measure (W/M) rule of 1 CBM = 1,000 kg: you pay for whichever is greater, the actual weight in tonnes or the volume in CBM. Dense cargo (tiles, hardware) is usually weight-rated; light voluminous cargo (furniture, textiles) is volume-rated. Air freight uses a different divisor (typically length × width × height in cm ÷ 6,000 per piece), which is why the same carton can be “small” by sea and “huge” by air.
Common mistakes
- Using inner product dimensions instead of the packed export carton.
- Ignoring the master carton: 10 retail boxes shrink-wrapped together occupy more space than 10 × the single-box volume.
- Rounding down: shipping lines round volumes up, so calculate to three decimals like the calculator does.
- Forgetting pallets: palletised cargo adds the pallet footprint and height — check our pallet loading guide.
CBM in real freight quotes: how forwarders actually bill you
When you email a freight forwarder for a quote, the number they price against is almost never your raw geometric CBM. Ocean LCL is billed on the weight-or-measure (W/M) revenue tonne: the forwarder compares your total CBM against your total weight in tonnes and charges whichever is greater. A pallet of ceramic tiles at 0.8 CBM but 1.1 tonnes is billed as 1.1 revenue tonnes; a pallet of pillows at 1.5 CBM but 0.2 tonnes is billed as 1.5. Understanding which side of that line your cargo sits on is the difference between an accurate budget and a nasty surprise at invoicing.
Air freight flips the maths entirely. Airlines use a volumetric divisor of 6,000 (cm³ per kg), which means one CBM of air cargo is treated as 167 kg of chargeable weight. That is why the same carton that is trivially cheap by sea can be prohibitively expensive by air — a lesson e-commerce sellers learn the hard way when they air-freight a first batch of a bulky product to hit a launch date.
Worked example: a mixed pallet
Suppose you are importing three carton types on one order: 200 cartons of 40×30×30 cm, 150 of 60×40×40 cm, and 80 of 50×50×50 cm. Convert and sum:
- 200 × (0.40×0.30×0.30) = 200 × 0.036 = 7.20 CBM
- 150 × (0.60×0.40×0.40) = 150 × 0.096 = 14.40 CBM
- 80 × (0.50×0.50×0.50) = 80 × 0.125 = 10.00 CBM
Total: 31.6 CBM. That figure sits just above the 28-CBM threshold where a 20ft FCL usually becomes cheaper than LCL, and it is more than half a 40ft container — so the practical decision is whether to consolidate into a single 40ft or split across shipments. The calculator runs this comparison automatically once you enter your carton mix.
Why three decimal places matter
Shipping lines round volume up, not to the nearest value. A shipment measured at 15.02 CBM is billed as 16 CBM by many LCL consolidators that round to the whole cubic metre. On a lane priced at $40/CBM that rounding costs you nearly $40 for two hundredths of a cubic metre. Measuring your cartons precisely — and knowing your true total to three decimals — is the cheapest cost-control measure in the entire shipping process. It costs nothing but a tape measure and five minutes.
Frequently confused terms
CBM vs CBF: CBM is cubic metres; CBF (or CFT) is cubic feet. 1 CBM = 35.315 CFT. US-facing suppliers quote CFT; European forwarders quote CBM. Always confirm the unit before comparing quotes.
Gross vs net CBM: gross CBM includes the packaging and pallet; net CBM is product only. Freight is always billed on gross. When a supplier quotes you a “CBM per unit” figure, confirm whether it includes the export carton and any master carton, because that is what fills the container.