Our guide to how air freight pricing works covers chargeable weight: a volumetric (dimensional) weight worked out from cubic dimensions and compared against actual weight, with the higher of the two billed. That convention, and the closely related CBM-based dimensional weight used in parcel and courier pricing, is built for dense boxes moving by air. Ocean breakbulk, bulk and less-than-container-load (LCL) cargo runs on a different, older convention with its own vocabulary: stowage factor and the revenue ton. This post covers that mechanics, which the chargeable-weight and general rate-calculation guides do not go into.
What "stowage factor" actually means
The IMO's 2011 Code of Safe Practice for Ships Carrying Timber Deck Cargoes defines Stowage Factor (SF) plainly: "the volume occupied by one tonne of a cargo when stowed and separated in the accepted manner." In practice that means cubic metres per tonne (m³/t), or cubic feet per tonne in older tariffs, and it is an empirical figure built from how a commodity has actually stowed in previous shipments, voids, dunnage and broken stowage included, not a theoretical density calculation.
Stowage factor varies enormously by commodity, and the spread is the whole point of the concept. A published stowage-factor reference used by cargo lines for general tariff guidance puts iron and steel products such as pig iron or iron ore pellets at around 0.4 m³ per tonne: dense cargo that takes up very little space for its weight. Bulk wheat sits around 1.25 to 1.4 m³ per tonne, and baled cotton spans roughly 1.7 to 3.7 m³ per tonne depending on how tightly the bales are compressed. The same reference carries the caveat that its figures are "merely a guide and no substitute for checking accurate stowage factors from local agents and/or shippers," which is the right way to treat any published stowage-factor table, including the ranges above: a starting point for planning, not a number to quote to a customer without checking the actual commodity and packing.
Why stowage factor decides weight or volume pricing
A ship, a container or a consolidator's LCL load has two limits at once: a weight limit and a volume limit. Which one binds first for a given cargo depends entirely on its stowage factor.
- Dead weight cargo is heavy for its bulk. Low stowage factor commodities such as steel, machinery or ores reach the weight limit long before the available space is full, so the carrier is leaving volume unused and prices on weight.
- Measurement cargo is light for its bulk. High stowage factor commodities such as baled cotton, empty drums or bulky manufactured goods fill the available space long before the weight limit is reached, so the carrier prices on volume.
The commercial reality is blunt: the carrier charges on whichever basis, weight or volume, produces the higher freight for that specific shipment. That is not a hidden practice; it is how ocean tariffs are built, and it is the entire reason the next concept, the revenue ton, exists.
The revenue ton: one billing unit for either basis
A tariff cannot usefully quote two separate rates, one per tonne and one per cubic metre, and then make a shipper work out which applies. Instead it quotes a single rate per revenue ton (also called a freight ton), a billing unit defined, as a trade-law reference puts it, by taking "the greater of" the shipment's weight-based tonnage or its volume-based tonnage. Historically that comparison was 1 cubic metre against 1 metric tonne: whichever of those two produces more revenue tons for the carrier is the one used, and the freight rate is then applied per revenue ton regardless of whether weight or volume actually governed. This weight-or-measure principle is often abbreviated W/M on a tariff or rate sheet, and it is why a single quoted rate can cover both a steel shipment and a cotton shipment without the carrier ever pricing itself out on either one.
Why "1 CBM = 1,000 kg" is a convention, not a law
Because the classic revenue-ton comparison is 1 cubic metre against 1 metric tonne (1,000 kg), many LCL consolidators and freight forwarders quote "1 CBM = 1,000 kg" as a rule of thumb when deciding whether a given shipment will be charged by weight or by volume. Treated as a shorthand for their own general cargo mix, that is a reasonable approximation. Treated as a physical law or an industry-wide regulation, it is a common misconception, and a costly one to assume without checking.
There is no regulator or standards body that fixes the weight-to-volume breakpoint for ocean freight at 1,000 kg per cubic metre. The actual ratio used for any given shipment is whatever a specific carrier's or consolidator's own tariff sets, and that can and does vary by trade lane, by service and by the general cargo mix that operator is pricing around. Our guide to how freight rates are calculated across ocean, air and road covers the broader rate-building picture this sits inside. The only way to know the breakpoint that actually applies to your shipment is to ask the carrier or forwarder issuing the quote, in writing, before you commit cargo to it.
A worked example (illustrative numbers only)
The figures below are hypothetical, built only to show the mechanics, not quoted rates from any carrier.
Suppose a shipment weighs 3,000 kg (3 tonnes) and measures 6 cubic metres, and the carrier's tariff uses the classic 1 CBM-to-1-tonne comparison.
- Weight basis: 3,000 kg = 3 revenue tons.
- Volume basis: 6 CBM = 6 revenue tons.
- Result: the volume basis is higher, so the carrier bills 6 revenue tons. This is a measurement cargo on this carrier's tariff: light for its bulk, so volume governs.
Now suppose a second, hypothetical shipment of steel fittings weighs 8,000 kg and measures 4 cubic metres on the same tariff.
- Weight basis: 8,000 kg = 8 revenue tons.
- Volume basis: 4 CBM = 4 revenue tons.
- Result: the weight basis is higher, so the carrier bills 8 revenue tons. This is a dead weight cargo: heavy for its bulk, so weight governs.
The arithmetic is simple once the convention is understood. What varies, and what cannot be assumed from one carrier to the next, is the ratio used to compare weight against volume in the first place.
What to check before you book breakbulk, bulk or LCL freight
- Ask for the weight-to-volume ratio in writing. Do not assume 1 CBM equals 1,000 kg; confirm the figure the carrier or consolidator is actually applying to your shipment.
- Get the stowage factor for your specific commodity and packing, not a generic category. Stowage factor can vary within one commodity depending on grade, packaging and how tightly it is baled or palletised, and a published reference table is a starting point, not a substitute for checking with the shipper or carrier.
- Weigh and measure accurately before quoting. A revenue-ton calculation is only as good as the weight and volume figures that go into it, and an error in either one changes which basis governs.
- Decide between FCL and LCL with stowage factor in mind. A high stowage factor (bulky, lightweight) shipment often makes more sense as a full container, since LCL consolidators price measurement cargo by volume and that cost adds up quickly.
Freight pricing has more conventions than most people expect, and getting the vocabulary right is most of the battle when a quote looks higher than the weight alone would suggest. You can browse more than 29,300 logistics companies by country and service in the CargoLinked directory, or post your shipment on the public requests board and let forwarders who handle your cargo type and lane quote directly.
Stowage factor figures in this article are general reference ranges from a published cargo-handling guide, described by its own publisher as general guidance rather than a guarantee, and will vary by exact commodity, grade and packing. Confirm the actual stowage factor and weight-to-volume ratio for your shipment with your carrier or forwarder before relying on any number here.
