Why does container loading need an allowance, and how much?
Stone is heavy cargo, so a 20ft container usually hits the weight limit long before it runs out of space. Aiming at a theoretical square-metre figure puts you on the edge of overweight. The allowance is threefold, not one.
Why does weight come before volume?
Because granite density is about 2700 kg/m³, so 30mm slabs weigh roughly 81 kg per square metre. A 20ft container's payload limit is in the region of the high twenties of tonnes, so weight usually tops out before space does. The first question is therefore always how heavy is this container, not how many more square metres will fit.
What are the three allowances?
One for thickness tolerance: a nominal 30mm cut at 32mm is about six percent more weight. One for packing: pallets, crates and separators all weigh something and are not part of the slab area. One for weighing error and box condition: verified gross mass rules differ between carriers, weighbridges differ between ports, and container condition varies. Each is small; together they are not negligible.
How do you calculate so reality does not embarrass you?
Work backwards. Take the carrier's payload limit, subtract the estimated packing weight, apply a discount for tolerance and weighing error, and what remains is usable payload. Divide usable payload by the weight per square metre to get the loadable area. That figure is usually smaller than what feels like it should fit — and that is correct.
What happens if you overload?
Three outcomes, all unpleasant: the port weighbridge stops it and demands devanning and splitting, with the cost and delay on you; the carrier refuses it against verified gross mass and the cargo sits in the yard accruing storage; or worst, it sails and is caught at destination, with a fine and detention. All cost far more than the freight on a few square metres.
Is a part-filled container a loss?
Not necessarily. Stone rarely fills by volume — heavy cargo reaching its weight limit first is normal. The real waste is not empty space, it is overloading in order to fill it. Space can be packed with separators and bracing; weight cannot. So the test is: weight compliant, voids filled, cargo not shifting — that is a full container.
How do you put this on the supplier at order stage?
One question is enough: what is the total gross weight of this container, and how was it calculated? The answer should break down slab weight, packing weight and the total. Anyone who cannot is usually guessing. Put the gross weight in the confirmation email before loading, which is worth more than arguing about responsibility afterwards.