A 40 foot container is twice the length of a 20 foot container. For dimension stone it will not carry twice the weight, and in practice it will not carry more weight at all. That single fact explains why almost every parcel of block and slab in world trade moves in the smaller box, and it explains a good deal more besides.
Most commercial thinking about stone starts with the market: which grade, which buyer, block or finished slab, raw tonnage or processed value. That is the right set of questions in the wrong order. The physics of the cargo settles part of the commercial argument before anyone has opened a negotiation, and an exporter who has not read the numbers is making a strategic choice with a constraint already applied to it that they cannot see.
Weight fills the box, volume never does
The Cargo Handbook, the reference published by BMT, puts the density of granite and marble at 2.65 to 2.75 grams per cubic centimetre, giving a stowage factor of 0.77 cubic metres per tonne. Individual blocks run up to 40 tonnes.
A stowage factor below one is unusual and it has a specific consequence. Stone reaches the container's weight limit while most of the internal volume is still air. Cargo that cubes out, which is to say cargo that fills the space before it reaches the weight limit, is priced sensibly by a per box freight rate, because the shipper is buying the space and using the space. Stone weighs out. The shipper buys a volume, uses a fraction of it, and pays for the rest.
That is the underlying reason containerised block shipping is expensive on a per tonne basis, and it is a structural feature rather than a market condition. No freight cycle fixes it. It is arithmetic about the density of rock.
Line load, and the trap of the larger box
The constraint that actually binds is not the total payload rating. It is line load, the permissible weight per metre of container floor.
The Cargo Handbook gives a maximum of 4.2 tonnes per metre in a 20 foot container and only 2.0 tonnes per metre in a 40 foot container. A box of twice the length with less than half the permitted line load does not carry more concentrated weight. It carries roughly the same or less, while costing meaningfully more to move.
This is why stone ships in 20 foot boxes, and it is not a preference or a convention. An exporter who accepts a 40 foot quotation because the per box rate looks efficient has taken a severe freight penalty per tonne shipped, and will usually discover it only when the box is rejected at the gate or the surveyor refuses the stow.
Vendor sources put the payload ceiling of a 20 foot container at 24,000 to 26,000 kg, with port limits often at 27,000 kg and rail ramps materially lower at 21,500 kg. We flag those as vendor figures rather than published standards, because we have not been able to verify them against an issuing authority, and the range is wide enough to matter. The point that survives the uncertainty is the ranking: the rail ramp is the lowest number in the list. For any move with an inland leg, the binding limit is set well before the ship, by equipment nobody involved in the sale has looked at.
Where blocks stop fitting
Granite density is given at 2.6 to 3.0 tonnes per cubic metre. A block measuring 3 by 2 by 1.5 metres is 9 cubic metres, which comes out at roughly 24 tonnes. That is already at or beyond a normal 20 foot payload, which is why blocks of that size and above move on flat rack, open top or as breakbulk rather than in a closed box.
The Cargo Handbook puts the best commercial raw block size at 6 to 8 cubic metres, and it is worth being clear about where that number comes from. It is not primarily a quarrying preference or a sawing preference. It is the largest parcel that reliably clears the handling and payload constraints described above while still being an economic unit to process. The quarry's block size decision is, in significant part, a shipping decision taken at the face.
For breakbulk parcels the reference describes drop stow or pyramid stow up to five or six tiers, with 16mm as the minimum lashing wire. On any substantial tonnage of large block, this is the honest comparison to run against containerisation, because it is the mode that does not require the exporter to pay for unused volume.
Securing, where the failures actually occur
The securing requirements for stone are unusually specific, and the specificity is a warning.
Hardwood bearers are required, because softwood crushes under the load. A minimum of two lashings per block applies. Lashing eyes rated at 2 to 6 tonnes and drop rings rated at 2 tonnes are explicitly not acceptable for this cargo. Laden containers must never be handled by wire sling.
Each of those is a rule written after something went wrong. A block that shifts in transit does not simply arrive damaged. It arrives having damaged the container, and frequently having damaged the claim, because a stow that does not meet the reference standard transfers liability to the shipper regardless of what the sea did. The cheapest line item in the whole chain is the dunnage, and it is the one most often specified by whoever is loading rather than by whoever is selling.
Why slabs justify the box and blocks often do not
Run the same physics on processed material and the conclusion inverts.
Vendor sources put a 20 foot container at roughly 70 to 80 slabs at 20mm thickness, or 45 to 50 at 30mm. Again, these are vendor figures and we have not verified them independently, so they are useful for planning ranges rather than for contract terms.
The relevant difference is not the count. It is that a tonne of slab carries far more value than a tonne of block, so the same freight bill sits against a much larger cargo value and consumes a much smaller share of it. Slab also requires crate protection and a controlled stow that breakbulk handling does not reliably provide, which means the container is doing real protective work rather than simply enclosing air. Containerisation earns its cost on slab. On large block parcels it frequently does not.
We should be plain about the limit of this analysis. We could find no verified per tonne landed cost comparison for stone moving from Pakistan to China or from Pakistan to Italy. Those numbers circulate, and we are not going to publish one we cannot stand behind. The mechanics above are sound and can be applied to real quotations by anyone holding them. The mechanics do not by themselves produce a price, and a confident landed cost figure offered without a route, a period and a source is an estimate wearing a suit.
The decision is made before it is discussed
The usual framing of the block versus slab question is strategic. Should a producer sell raw and take the certain margin, or process and capture more of the chain. That is a real question and it deserves a real answer.
But the freight economics have already voted. Block is a weigh out cargo that pays for volume it cannot use, and above roughly 8 cubic metres it leaves the container system entirely. Slab is a cargo whose value per tonne absorbs container freight comfortably and whose fragility requires it. The commercial argument for moving up the chain is therefore stronger than the margin comparison alone suggests, because part of the gain arrives as a freight structure that stops working against you.
The practical consequence is that the container specification belongs at the quarry face, not at the port. By the time a block has been cut at 9 cubic metres, the mode, the equipment and the freight per tonne have all been decided by a saw operator who was never shown the stowage table.