Stainless steel is priced by weight, and there are two kinds of "weight":theoretical weight(calculated from dimensions) andactual weighed weight(weighed on a scale). The two naturally differ, and the difference is mainly not due to "inaccurate scales" but to geometry and tolerances.
One-sentence judgment
Negative tolerance within the compliant range is the main reason theoretical weight is higher than actual weight.The contract must state whether pricing is "by theoretical weight" or "by actual weighing," and which table the thickness tolerance follows — this one sentence can save all later disputes.
How theoretical weight is calculated
Common formula for steel plate/coil:
Weight (kg) = Length (m) × Width (m) × Thickness (mm) × Density (g/cm³)
Density is taken by grade (common convention):
- 304 / 304L / 316 / 316L: about7.93(316L often uses 7.98, the difference is very small);
- 321 / 316Ti: about 7.9;
- 2205 duplex steel: about7.80;
- 430 ferritic: about 7.70.
Pipe, round bar, and flat bar each have their own formulas (essentially "cross-sectional area × length × density").
Three sources of error
① Thickness tolerance (the largest item).Standards allow negative thickness deviation (the allowable value differs by standard and thickness range). A sheet nominally 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.Density values differ slightly by grade and by standard handbook (7.93 vs 7.98), and on large volumes this is also money. Theoretically the actual grade should be used; in practice most calculate uniformly at 7.93.
③ Cutting loss and edges.Leveling, slitting, and laser cutting all involve edge trim and head/tail loss. Theoretical weight is calculated from 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 about5%higher;
- Nominal 2.0 mm, actual 1.8 mm → theoretical weight is about10%higher;
- Nominal 10 mm, actual 9.7 mm → difference about3%(negative tolerance is relatively smaller for thick plate).
Thin sheet is more likely than thick plate to show an obvious "theoretical heavier than actual."
How to write procurement so you don't lose out
- Fix the pricing method: by theoretical weight / by actual weighing, choose one;
- State the thickness tolerance clearly: which standard and which grade (differences between standards are not small, see the other article "Thickness Tolerance");
- Whether negative tolerance is allowed under a separate agreement: if you want "positive tolerance only," state it clearly; the price will differ;
- Weighing location and method: ex-works weighing 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
- Quote by theoretical weight but collect payment by scale weight on delivery (or the reverse), and neither side accepts the other's calculation;
- 只看“总价更便宜”而不核对公差等级——负公差大的材料单价更低，但实际使用中材料不够；
- 用304的密度计算2205的重量（7.93对7.80，相差1.6%）；
- 忽略加工切割损耗，按成品尺寸倒推采购量，结果总是差一点。
告诉我们牌号、规格和计价方式，我们会按相应标准报理论重量和实际交货重量，并在合同中注明公差等级——重量写清楚了，就没有争议。
