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How to Calculate CBM and Choose the Right Container

Ask a freight forwarder for a quote and the first thing they want is your CBM. It is the single number that decides whether you ship in a shared container or book a whole one, and it quietly sets most of your freight bill. Get it wrong and you either pay for space you are not using or discover at the last minute that your order does not fit. This guide covers how to calculate CBM, how much actually fits in each container, and how to choose between LCL, a single container, and a mix.

What CBM means

CBM stands for cubic metre, and it is simply the volume your goods take up. Freight for sea shipments is priced on whichever is greater: the volume in CBM or the weight in tonnes. For most consumer goods, volume is what you pay on, so CBM is the number that matters. It applies to your cartons, not the bare product, because you ship the packaging too.

The CBM formula

Measure one carton in metres, multiply the three dimensions, then multiply by the number of cartons:

1. Take the length, width and height of a single carton in centimetres.
2. Divide each by 100 to convert to metres.
3. Multiply length x width x height to get the CBM of one carton.
4. Multiply by the total number of cartons in the shipment.

For example, a carton measuring 60 x 40 x 30 cm is 0.6 x 0.4 x 0.3 = 0.072 CBM. Ship 300 of them and your total is 21.6 CBM. That one number is what you give the forwarder, and what tells you which container you need.

Always measure the outer carton, not the product. Master cartons include void fill and wall thickness, and that space still costs you freight.

How much fits in a container

Containers have a theoretical internal volume, but you never get all of it because of pallets, carton shapes, and the gaps between them. Realistic loaded capacities are:

• 20ft standard (20GP): around 28 to 33 CBM theoretical, roughly 25 to 28 CBM in practice.
• 40ft standard (40GP): around 58 to 66 CBM theoretical, roughly 54 to 58 CBM in practice.
• 40ft high cube (40HQ): around 68 to 76 CBM theoretical, roughly 60 to 68 CBM in practice.

The high cube is a foot taller than a standard 40ft and gives you the most usable volume, which is why it is the default for lightweight, bulky goods like apparel and homewares.

LCL or FCL: which to book

LCL (less than container load) means your goods share a container with other importers. FCL (full container load) means you book the whole box. The rough rule: below about 15 CBM, LCL is usually cheaper; above it, a full 20ft container often wins on a per-CBM basis, and it moves faster because it skips the consolidation and deconsolidation steps where LCL shipments lose days and pick up handling fees.

• Small or first orders under ~15 CBM: LCL keeps your outlay and risk low.
• Around 25 to 28 CBM: a single 20ft container.
• Around 54 to 68 CBM: a single 40ft or 40HQ.
• Awkward totals in between: this is where a container mix saves money.

When a container mix beats a single size

Say your shipment is 75 CBM. That overflows a single 40HQ, so the obvious answer is two 40HQs, which leaves one of them barely a fifth full and paying for a lot of air. A smarter split might be one 40HQ plus one 20ft, which carries the volume with far less wasted space and a lower combined cost. The best mix is the combination of containers that holds your CBM with the least empty space, and it is rarely the first option that comes to mind.

Skip the trial and error. Our free CBM and container calculator works out your total volume and the cheapest container mix in seconds. Open the free CBM calculator

Chargeable weight: when CBM is not what you pay on

CBM tells you how much space your shipment takes. It does not always tell you what you are charged for, because freight is priced on whichever is greater, volume or weight. Light bulky cargo is charged on volume, dense heavy cargo on weight.

For LCL sea freight the usual convention is that one cubic metre is treated as one tonne, so the rate applies to whichever number is higher, your CBM or your weight in tonnes. Two cubic metres weighing 500 kilos is charged as two revenue tons. Two cubic metres weighing three tonnes is charged as three.

Air freight uses volumetric weight, calculated by dividing the volume in cubic centimetres by 6000, or equivalently multiplying CBM by roughly 167 to get kilos. A one cubic metre carton weighing 40 kilos is charged as about 167 kilos, which is why bulky air freight gets expensive so quickly.

Work out both numbers before you ask for a quote. If your cargo is volume-heavy, reducing carton size does more for the freight bill than reducing weight, and the reverse is true for dense goods.

Measure the carton, not the product

CBM is calculated on the outside of the shipping carton, including the carton walls, and freight is charged on the space the carton occupies rather than the product inside it. A few practical consequences follow.

• Use external carton dimensions from your supplier's packing list, not product dimensions.
• Allow for bulge. Cartons packed with soft goods swell, and a carton measured flat at 40 centimetres can present as 43 once packed.
• Round up rather than down. Forwarders measure the cargo they receive, and a shipment that measures larger than quoted gets re-rated at their number.
• Ask whether the carton size can be changed. Redesigning a carton to fit the pallet or container better is one of the cheapest freight savings available, and suppliers rarely suggest it.

Pallets change the arithmetic

If your cargo is palletised, the pallet footprint decides how much of the container you actually use, not the CBM of the cartons. A standard 20ft container holds around 10 standard pallets on the floor, and a 40ft around 20 to 21, depending on the pallet size and whether they can be double stacked.

Double stacking is where the money is. If your cartons cannot take the weight of a second pallet on top, you lose roughly half the container's cubic capacity even though your CBM calculation said it would fit. Ask your supplier whether the cartons are stackable, and at what height, before you assume the theoretical capacity.

Common mistakes

• Calculating volume from product dimensions instead of the packed carton.
• Assuming a container holds its full theoretical CBM. Loaded capacity is always lower.
• Booking two large containers when a mix of sizes would carry the same goods for less.
• Ignoring weight limits on dense goods, where you can hit the payload cap before you fill the space.

The bottom line

CBM is easy to calculate and expensive to get wrong. Measure your cartons, total the volume, and match it to realistic loaded capacities rather than the numbers on a spec sheet. When your total falls between container sizes, check a mix before you default to another full box. The space you are not paying for is margin you keep.

Related guides

• Sea vs Air Freight: Which to Use and When (FCL vs LCL)
• LCL vs FCL: When to Ship Less Than a Full Container
• Ex-Works, FOB, CIF: Incoterms Explained for Importers

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