Stainless steel is priced by weight, and there are two kinds of "weight": theoretical weight (calculated from dimensions) and actual weighbridge weight (weighed on a weighbridge). The two naturally differ, and the difference is mainly not due to "inaccurate scales", but due to geometry and tolerance.
One-sentence judgment
Negative tolerance within the compliant range is the main reason why theoretical weight is higher than actual weight. The contract must clearly state whether pricing is "by theoretical weight" or "by actual weighbridge weight", and which table the thickness tolerance follows — this sentence can save all subsequent disputes.
How theoretical weight is calculated
Common formula for steel plate/steel coil:
Weight (kg) = Length (m) × Width (m) × Thickness (mm) × Density (g/cm³)
Density is selected according to grade (common practice):
- 304 / 304L / 316 / 316L: about 7.93 (316L often uses 7.98, the difference is very small);
- 321 / 316Ti: about 7.9;
- 2205 duplex steel: about 7.80;
- 430 ferritic: about 7.70.
Pipes, round bars and flat bars each have their corresponding formulas (essentially all are "cross-sectional area × length × density").
Three sources of error
① Thickness tolerance (the largest item). Standards allow negative deviation in thickness (the allowable value differs by standard and by thickness range). A sheet with nominal 2 mm may actually be 1.8 mm — the weight is then about 10% less. This is not "short weight"; it is allowed by the standard. But if the contract only says "by theoretical weight", the buyer pays for 2 mm and receives 1.8 mm material, and the difference becomes a dispute.
② Density value selection. Density values differ slightly among grades and standard handbooks (7.93 vs 7.98), and on large volumes this is also money. Theoretically the value should be selected according to the actual grade; in practice most calculations use 7.93 uniformly.
③ Cutting loss and edges. Leveling, slitting and laser cutting all produce edge trim and head/tail loss. Theoretical weight is calculated according to finished dimensions, while actual material used is more — this part is usually reflected in the processing fee rather than in the weight difference, but it must be explained at quotation.
See the difference in one table
- Nominal 2.0 mm, actual 1.9 mm → theoretical weight is about 5% higher;
- Nominal 2.0 mm, actual 1.8 mm → theoretical weight is about 10% higher;
- Nominal 10 mm, actual 9.7 mm → difference about 3% (negative tolerance for thick plate is relatively smaller).
Thin sheet is more likely than thick plate to show an obvious "theoretical heavier than actual".
How to write procurement terms so you do not lose out
- Fix the pricing method: by theoretical weight / by actual weighbridge, choose one;
- State the thickness tolerance clearly: which grade of which standard (different standards differ considerably, see the other article "Thickness Tolerance");
- Whether negative tolerance is allowed under a separate agreement: if "positive tolerance only" is required, state it clearly; the price will differ;
- Weighing location and method: weighbridge at ex-factory or re-weighing on arrival, and what error range is acceptable;
- Density value: for large volumes, state which density is used for calculation.
Common pitfalls
- Quotation by theoretical weight but delivery collected by weighbridge (or the reverse), and neither party accepts the other's calculation;
- Looking only at "cheaper total price" without looking at the tolerance grade — material with large negative tolerance has a lower unit price, but in actual use there is not enough material;
- Using the density of 304 to calculate the weight of 2205 (7.93 vs 7.80, a difference of 1.6%);
- Ignoring processing cutting loss and back-calculating purchase quantity from finished dimensions, resulting in a shortfall in the end.
Tell us the grade, specification and pricing method, and we will quote the theoretical weight and actual delivery weight according to the corresponding standard, and state the tolerance grade in the contract — when weight is written clearly, there is no dispute.
