If you are paying for stainless steel and only remember one thing about weight, make it this: the theoretical weight on the datasheet and the actual weight on the weighbridge are two different numbers, and the difference — usually a small percentage, sometimes more — is real money on a container load. The theoretical weight comes from density times the nominal dimensions; the actual weight comes from what the metal really weighs after rolling tolerance, and the two diverge because the mill rolls to a tolerance band, not to the nominal number you wrote down.
Theoretical vs actual weight: where the gap comes from
Theoretical weight is calculated from the nominal thickness, width and length times the density of the grade. For stainless steel the density is usually taken as 7.93 g/cm³ for the 300 series (304, 316) and 7.80 g/cm³ for duplex 2205. Actual weight is what the coil or plate weighs on a calibrated scale.
The gap has three causes:
- Thickness tolerance. A 3 mm plate is rolled within a tolerance band — typically plus or minus a fraction of a millimeter depending on the standard — not to exactly 3.000 mm. Over a full plate, the average thickness can sit slightly above or below nominal.
- Density variation between heats. The published density is a representative number, not a constant; the actual alloy composition shifts it by a fraction of a percent.
- Cutting and handling loss. A cut-to-length or slit order loses a little material to the kerf and the offcuts, which shows up as a small difference between billed weight and received weight.
Why it matters on the invoice
Stainless steel is bought and sold by weight, not by piece. If the price is $2,850 per tonne and the theoretical weight is 20.0 tonnes but the actual weight is 20.4 tonnes, that 0.4 tonne is a 2% difference — $1,140 on a single container. The buyer who does not know which weight governs the invoice is the buyer who gets a surprise at the end of the month.
The rule to fix on the order: state whether the price is per theoretical weight or per actual weight, and how the difference is settled. A common and fair approach is to invoice on actual weighbridge weight with a tolerance agreed up front, so neither side carries the whole variance.
The two numbers to have before you order
- The grade's density. 304 and 316 use 7.93 g/cm³; 2205 uses about 7.80 g/cm³; 430 uses about 7.70 g/cm³. Use the right one — a duplex order calculated at the austenitic density is wrong from the start.
- The tolerance standard. Thickness tolerance for stainless sheet is set by A240, and it varies with width and thickness. The certificate for your heat will state the measured thickness, and that measured number is what the actual weight should be checked against.
How to keep the two weights from becoming a dispute
Write the weight basis into the order — "invoice on actual weighbridge weight, tolerance ±1%" is a complete clause, "price per tonne" is not. Ask for the weighbridge ticket and the mill certificate together, and check the measured thickness on the certificate against the theoretical calculation. When the two weights are reconciled before the goods leave, the number on the invoice stops being a surprise and becomes a number you already agreed to.