I spend a lot of my week setting up item masters for OEMup launch factories — pump shops, fabricators, machining units, sheet-metal works. In almost every one of them, somebody is calculating metal weight by hand on a calculator, or carrying a dog-eared “weight per metre” chart taped to the wall, or worst of all, eyeballing it. That number then flows into the customer quote, the steel PO, and the transporter’s LR. One slip and the margin is gone.
The maths is genuinely simple. The mistakes are subtle. This post covers both, and ends with the free tool we built so you never have to do it by hand again.
Why metal weight is the number everything hangs on
In a trading or fabrication business, raw material is bought and sold by weight, but parts are designed and cut by dimension. Weight is the bridge between the two, and it shows up at four points:
- Costing & quoting — steel at ₹65/kg means a 157 kg plate is ₹10,205 of material before you touch labour. A 5% weight error is a 5% margin error.
- Procurement — you order bar and sheet in kg or tonnes; the cutting list is in pieces and millimetres. Someone has to convert.
- Freight — transporters bill on weight (or volumetric weight). Under-declare and you get a debit note; over-declare and you overpay.
- Scrap & yield — offcut and turning scrap is sold back by weight; knowing theoretical weight tells you your real yield.
The one formula behind all of it
There is only one formula, and every “weight per metre” chart you have ever seen is just this formula pre-computed:
Compute the volume from the shape’s dimensions. Convert it to cubic metres (m³). Multiply by the material density in kg/m³. The answer is in kilograms. That’s it.
The only place people trip up is units. Densities are quoted in kg/m³, but you measure parts in millimetres. So the clean workflow is: compute the volume in mm³, divide by 1,000,000,000 to get m³, then multiply by density. Keep everything in millimetres until the final division and you will never mix units.
Volume formulas for each shape
Five shapes cover the overwhelming majority of bar and sheet stock. Compute the volume with these (all dimensions in mm), then apply weight = volume / 1e9 × density.
| Shape | Inputs | Volume (mm³) |
|---|---|---|
| Sheet / Plate | Length, Width, Thickness | L × W × T |
| Round Bar | Diameter, Length | (π × D² / 4) × L |
| Square Bar | Side, Length | Side² × L |
| Rectangular Bar | Length, Width, Height | L × W × H |
| Hex Bar | Across flats (A/F), Length | (√3 / 2) × AF² × L |
The hex-bar one is the one people get wrong — more on that below. The round-bar formula is the workhorse: nearly every shaft, pin and fastener blank starts as round bar.
Density reference table
These are the standard reference densities. Real weight varies slightly with alloy, temper and rolling tolerance, but for quoting and procurement these are accurate enough.
| Material | Common grades | Density (kg/m³) |
|---|---|---|
| Mild Steel | IS2062 E250 / E350 | 7850 |
| Carbon Steel | C20, C45, EN8 | 7850 |
| Stainless Steel | SS304 (7930), SS316 (8000), SS202 (7780) | 7780–8000 |
| Aluminium | 1100, 6061, 6082, 7075 | 2700–2810 |
| Brass | IS319 | 8500 |
| Copper | ETP | 8930 |
| Cast Iron | Gray / Ductile | 7100–7300 |
| Titanium | Grade 2 / Grade 5 | 4430–4500 |
| Bronze | Phosphor / Gunmetal | 8650–8800 |
| Tool Steel | H13, D2, EN31 | 7700–7810 |
| Engineering plastics | PP, PVDF, PTFE, Nylon, HDPE | 910–2140 |
Note that all the steels — mild, carbon, tool, most alloy — sit at roughly 7850 kg/m³. The big jumps are non-ferrous: aluminium is barely a third the weight of steel, which is exactly why it’s chosen for weight-sensitive parts.
Skip the arithmetic
The free OEMup Metal Weight Calculator does every formula above automatically — pick a shape, choose the grade, enter the dimensions, get the weight in any unit, and add several items to a running total.
Open the Metal Weight Calculator →Three worked examples
1. An MS plate
A 2 m × 1 m × 10 mm mild-steel plate. Volume = 2000 × 1000 × 10 = 20,000,000 mm³ = 0.02 m³. Weight = 0.02 × 7850 = 157 kg. At ₹65/kg that plate is ₹10,205 of raw material.
2. A round bar shaft
A 25 mm diameter EN8 round bar, 1 m long. Volume = π × 25² / 4 × 1000 = 490,874 mm³ = 0.000491 m³. Weight = 0.000491 × 7850 = 3.85 kg per metre. This is exactly the “3.85 kg/m” figure on every steel chart — now you know where it comes from.
3. A hex bar
A 20 mm across-flats brass hex bar, 1 m long. Area = √3/2 × 20² = 346.4 mm². Volume = 346.4 × 1000 = 346,410 mm³ = 0.000346 m³. Weight = 0.000346 × 8500 = 2.94 kg per metre. (In mild steel the same bar is 2.72 kg/m.)
The mistakes that cost you money
Hex bar is always specified by the distance across flats (A/F), which is the size you measure with a vernier on the stock. The correct cross-section area is (√3/2) × A/F². If you accidentally treat the A/F as the side length and use the (3√3/2) × side² formula, you overstate the weight by about three times.
- Mixed units. A diameter in mm and a length in metres is the single most common slip. Convert everything to one unit first.
- Diameter vs radius. The round-bar formula uses diameter (D²/4). Plugging in the radius quietly quarters your answer.
- Thickness in the wrong place. For sheet, the “thickness” is the small dimension — gauge, not length. Easy to swap on a wide thin plate.
- Theoretical vs actual weight. These formulas give theoretical weight. Rolling tolerance, mill scale and rust mean the weighbridge figure can differ by 1–3%. For costing, theoretical is the right basis; for a weighbridge reconciliation, expect a small gap.
- Forgetting quantity. Per-piece weight is only half the job — multiply by the lot quantity before it hits the PO.
From a calculator to your costing
A standalone calculator answers one question at a time. Running a shop means answering it on every item, every day, and then carrying that number into costing, stock and invoices without re-typing it. That’s the difference between a free web tool and an ERP.
Inside OEMup, the exact same weight engine is built into the item master. When you create a steel item, the calculated weight feeds straight into per-kg costing, stock valuation, multi-level BOM rollups, and GST invoices — no spreadsheet, no re-keying, no drift between the quote and the actual. The web calculator on this site is that engine, opened up for anyone to use for free.
Use the free calculator for a one-off number today. When you’re tired of re-typing weights into spreadsheets, book a demo and see the same number flow into a live quote and stock ledger.
FAQ
What is the formula to calculate metal weight?
Weight = volume × density. Compute the shape’s volume, convert to cubic metres, and multiply by the density in kg/m³ (mild steel = 7850). A 1 m × 1 m × 1 mm MS sheet weighs 0.001 × 7850 = 7.85 kg.
What is the density of mild steel, stainless steel and aluminium?
Mild steel about 7850 kg/m³; SS304 about 7930, SS316 about 8000, SS202 about 7780; aluminium about 2700; brass about 8500; copper about 8930.
How do you calculate the weight of an MS plate?
Weight (kg) = length (m) × width (m) × thickness (m) × 7850. A 2 m × 1 m × 10 mm plate = 2 × 1 × 0.01 × 7850 = 157 kg.
How is hex bar weight calculated?
Hex bar is specified across flats (A/F). Area = (√3/2) × A/F²; multiply by length and density. A 20 mm A/F mild-steel hex bar is about 2.72 kg/m.
Is there a free metal weight calculator?
Yes — the OEMup Metal Weight Calculator is free with no sign-up. Pick a shape, choose the grade, enter the dimensions, and read the weight in any unit, with a multi-item total.