How is CBM calculated for a shipment?
Measure each carton on the outside, after it is taped and stacked ready to ship, in centimeters (or convert from inches). Multiply length by width by height to get the volume of one carton in cubic centimeters, multiply that by how many cartons share that size, then repeat for every other size in the load and add the results. Divide the running total by 1,000,000 to convert cubic centimeters into cubic meters, which is what CBM means. A cubic foot is smaller: multiply CBM by 35.31 to get cubic feet, which some US-side warehouses and Amazon FBA paperwork ask for instead.
Worked example: a mixed carton shipment
Take the default numbers already loaded into the calculator above: five cartons at 60 x 40 x 40 cm holding 8 kg each, plus three cartons at 50 x 40 x 40 cm holding 10 kg each.
- First size: 60 x 40 x 40 = 96,000 cm³ per carton, times 5 cartons = 480,000 cm³.
- Second size: 50 x 40 x 40 = 80,000 cm³ per carton, times 3 cartons = 240,000 cm³.
- Total volume: 720,000 cm³, which is 0.72 CBM (25.4 cubic feet).
- Total actual weight: (5 x 8 kg) + (3 x 10 kg) = 70 kg.
- Volumetric weight at a 6,000 divisor: 720,000 / 6,000 = 120 kg. Since 120 kg is more than the 70 kg actual weight, this shipment bills as 120 kg by air, not 70 kg, which is the part a hand calculation most often gets wrong.
Why do air and sea shipments bill differently?
Air and express space is scarce and the aircraft cares about both weight and how much cabin volume a shipment occupies, so the airline (or the forwarder pricing the space) converts volume into an equivalent weight and bills whichever number is larger. Sea freight works on the opposite logic for a small load: a ship has far more spare volume than spare weight capacity, so LCL (less than container load) is priced directly in CBM, or per 1,000 kg if that number happens to be larger, without the extra volumetric conversion step air freight uses. That is why a light, bulky shipment (packaging foam, lampshades, inflated toys) often looks expensive by air and cheap by sea, while a dense, heavy shipment (tools, hardware, ceramics) shows the opposite pattern.
How do you measure a carton for shipping?
- Measure the outside of the sealed carton, including any tape, corner protectors or overbox, not the product alone.
- Round dimensions up to the nearest whole centimeter (or inch): warehouses and carriers round up, never down.
- Measure every carton size separately if a shipment is mixed. Averaging carton sizes across a load produces the wrong volume.
- Weigh cartons after packing, not the product spec sheet from the supplier, which usually omits the box, void fill and any pallet.
- If cartons are palletized, measure the pallet's full footprint and height, not just the cartons stacked on it, since that is what actually occupies space in the aircraft or container.
What mistakes make a CBM calculation wrong?
- Entering per-carton weight where the field expects total weight, or the reverse. This is the single most common quote-request error we see. The calculator above keeps every carton's quantity and per-carton weight separate, then hands the quote form the already-summed total, so this mistake cannot happen once you use the button below the results.
- Mixing units mid-calculation. A shipment measured in inches with a weight in pounds, dropped into a formula built for centimeters and kilograms, produces a volumetric weight that is off by a large, unpredictable factor. Convert everything to one unit system before doing the arithmetic by hand, or use the unit toggles above so the conversion happens once, correctly.
- Rounding too early.Rounding each carton's volume before summing, rather than summing first and rounding the total once, compounds a small error across every carton in the shipment.
- Using the product's dimensions instead of the packed carton's. Void fill, packaging inserts and the box itself all add volume the product spec sheet never counted.
- Comparing a sea CBM quote against an air chargeable-weight quote as if they were the same unit. They price the same shipment on different bases; convert one to the other, or compare the final all-in dollar quote, before deciding.
FAQ
What is CBM in shipping?
CBM stands for cubic meter, the standard unit ocean freight is priced by. It is the volume a shipment takes up: length times width times height of every carton, in meters, added together. A shipment does not need to be heavy to have a large CBM; a load of lampshades or foam packaging can be light and still fill a lot of space.
How do I calculate CBM for multiple cartons of different sizes?
Work out the volume of each carton size separately (length x width x height in meters, multiplied by how many cartons of that size you have), then add every size together. That is exactly what the calculator's rows above do: one row per carton size, summed at the bottom, so a mixed shipment does not need a spreadsheet.
What is the difference between CBM and chargeable weight?
CBM is a volume. Chargeable weight is what a shipment actually bills at, which is the greater of its actual weight and its volumetric weight (volume converted to an equivalent kilogram figure using a divisor). Sea freight is usually quoted directly in CBM (or a freight ton, whichever is bigger); air and express freight are quoted in chargeable weight.
Why does air freight use a different divisor than express couriers?
There is no single industry-wide divisor. Common ones in general air cargo cluster around 6,000 cm3 per kg, and express/courier networks commonly use a lower figure such as 5,000, which produces a heavier volumetric weight for the same box. A lower divisor is worse for a light, bulky shipment. We publish our own divisor on the chargeable weight reference rather than guessing at a courier's, since that number can change by service and by contract.
Does this calculator send my shipment details anywhere?
No. Every number is computed in your browser as you type. Nothing is saved or transmitted unless you press the button to carry the totals into the quote form, which only fills in the form fields on this site for you to review before sending.