Steel weight:
from volume and density to kilograms

Every steel weight starts with volume

Steel is sold by weight but ordered by size. A fabricator quotes a rate per kilogram, a drawing gives millimetres, and the truck has a legal load limit. Converting between them takes only one multiplication once you know the volume.

Structural and mild steel has a density close to 7,850 kg/m³, which is the same as 7.85 g/cm³. Stainless grades are a little heavier, around 7,900 to 8,000 kg/m³, and cast iron a little lighter. For purchase planning, 7,850 is the number most people use.

When the volume comes from millimetres, convert first: 10 mm is 0.01 m, and 1,000 mm is 1 m. Skipping that step is the usual cause of answers that are 1,000 times off.

Metric habits help here. Many drawings use millimetres for cross-sections and metres for lengths, so write the unit beside every figure while you work and strike it off as you convert. Anyone who checks your sheet later can then see at a glance that the thickness was changed to metres before multiplying.

A sanity check takes seconds: a cubic metre of steel is nearly eight tonnes. If a small bracket comes out in tonnes, or a full plate comes out in grams, a unit slipped somewhere.

A plate worked out step by step

The same numbers give a handy rule: a 1 mm sheet of steel weighs 7.85 kg per square metre, so a plate that is t millimetres thick weighs 7.85 × t kg per square metre. A 10 mm plate is 78.5 kg/m², and the 2 m² plate above is exactly twice that.

Cut shapes follow from the same idea. A plate with a circular hole, for instance, is the full rectangle minus the area of the hole, multiplied by thickness and density. For a lot of small holes, estimate the removed area as a share of the plate, such as 10 per cent, and scale the weight down by the same share.

Round bars and the d²/162 shortcut

Reinforcement bars are cut from a circular cross-section, so the area is π/4 times the diameter squared. Multiply by 7,850 kg/m³ and express the diameter in millimetres, and the numbers collapse into a tidy fraction.

The shortcut d² ÷ 162 gives 0.889 kg/m for a 12 mm bar, which differs from the exact 0.888 by far less than the manufacturing tolerance on the bar itself. That is why site engineers use it on paper.

Pipes and hollow sections: subtract the void

A pipe or box section is easier if you think of a solid bar with a hole taken out. Work out the external volume, work out the inside volume, and take the difference. Multiply the result by density.

A pipe with a 60 mm outside diameter and 3 mm wall has a mean diameter of 57 mm, so the cross-section is about π × 57 × 3 = 537 mm². At 7,850 kg/m³ that is roughly 4.2 kg per metre. Manufacturers' tables list slightly different figures because of standard sizes and tolerances, so use the table when you are buying and the formula when you are estimating.

Box sections follow the same pattern. A 50 × 50 mm tube with 3 mm walls has an inner square of 44 × 44 mm, so the steel area is 2,500 − 1,936 = 564 mm², about 4.43 kg per metre at 7,850 kg/m³.

Adding up a whole job

A single bar is easy. A slab, beam or foundation has dozens of bar marks, and the total weight drives both the purchase order and the delivery plan. The routine is the same: work out the length of each bar, count them, multiply by weight per metre.

At a hypothetical purchase rate of ₹60 per kilogram, the net steel for those bars is about ₹12,800 and the ordered quantity about ₹13,400. The rate is a placeholder; supplier prices move with the market, so get a fresh quote before budgeting.

Bundles are usually sold in tonnes, where 1 tonne is 1,000 kg. The 224 kg above is 0.224 tonne, a fraction of a bundle, so check the supplier's minimum order and cut-length options before assuming the arithmetic is the whole answer.

The same method for other metals

Nothing in the formula is special to steel. Change the density and you can estimate any solid metal part, which helps when you are comparing materials or checking a quotation.

Densities are representative values and differ by alloy and temper. Use the datasheet for the exact grade when weight is critical, such as in lifting plans or shipping documents.

When the estimate and the weighbridge disagree

• Rolled sections have rounded corners and fillets that a simple rectangle does not capture.
• Rust, mill scale, galvanising or paint add weight that a bare-metal formula omits.
• Thickness tolerance means a sheet sold as 10 mm can be a fraction above or below that.
• Grades differ: stainless steel and tool steel are slightly heavier than mild steel.
• Offcuts and cutting loss belong in the purchase quantity, not in the weight of the finished part.

For structural design loads, take the figures from the relevant standard or the supplier's section tables rather than from a rule of thumb.