How do you calculate the weight of a steel bar?

Work out its volume from its shape (π x r² x length for a solid round bar), then multiply by the steel's density — 7,850 kg/m³ is the standard reference value for mild/carbon steel. A 12 mm bar, 6 m long, comes out at roughly 5.33 kg. This calculator does that volume and density multiplication for round bar, round tube and rectangular shapes automatically.

What is the density of steel?

It depends on the grade, but 7,850 kg/m³ is the standard figure used for general mild/carbon structural steel. Other common grades range from about 7,715 kg/m³ (tool steel) to roughly 8,030 kg/m³ (stainless steel) — a spread of a few percent that matters once you are ordering by weight rather than by the piece.

How much does a steel pipe weigh?

A pipe (round tube) weighs less than a solid bar of the same outer diameter, because its volume is the outer cylinder minus the hollow bore: π x (outer radius² − inner radius²) x length, multiplied by density. Entering the outer and inner diameter separately, rather than treating it as solid, is essential — leaving the bore at zero will significantly overstate the weight.

What steel grade is standard for structural work in the UK?

S235 or S275 (BS EN 10025) covers most general fabrication, with S355 used for heavier structural loads, and B500B (BS 4449) T-bar is standard for reinforcing bar. Always confirm the exact grade with a structural engineer for anything load-bearing.

Steel Weight Calculator | Bar, Tube & Rebar Weight (kg)

Steel Weight Calculator: How Much Does a Steel Bar, Tube or Plate Weigh?

Steel is bought and shipped by weight, not by length or by the piece, so before you can get a supplier quote, book a freight slot, or check what a crane or truck bed can actually carry, you need to convert a shape and a size into a mass. This steel weight calculator does that conversion directly: pick a stock shape — round bar or rod, round tube or pipe, or rectangular bar or plate — enter its dimensions and length, choose a steel grade, and it returns the weight per piece, the total weight for however many pieces you need, and a material cost estimate.

It is built for anyone who needs a real number rather than a rough guess: fabricators pricing a cutting list, riggers and site managers checking a lift or load rating, and DIYers or engineering students who just want to know what a length of bar stock weighs before they order or carry it. Rather than hiding the density behind one generic "steel" constant, the calculator exposes a full table of steel-grade densities — from tool steel to stainless — since the true figure varies by a few percent depending on alloy, which starts to matter once you are ordering by the tonne.

How to Use the Steel Weight Calculator

Choose the stock shape: round bar/rod (solid), round tube/pipe (hollow), or rectangular bar/plate.
For a round bar, enter its diameter and choose the unit (mm, cm, in).
For a round tube, enter both the outer diameter and the inner diameter (bore) — the gap between them is what makes the tube hollow rather than solid.
For a rectangular bar or plate, enter its width and thickness instead of a diameter.
Enter the length of one piece and choose its unit (mm, cm, m, ft, in).
Set the quantity — how many identical pieces you are weighing or ordering.
Choose a steel grade preset (this sets the density in kg/m³), or enter a custom density if you know your specific alloy.
Choose your output mass unit (kg or lb).
Read off the weight per piece, the total weight, and the estimated cost — edit the price per kg or lb to match your supplier's quote.

The Math Behind the Calculation

Every shape reduces to the same relationship: weight = volume x density x quantity. The volume formula just depends on which shape you picked. A round bar is a solid cylinder, so its volume is π x r² x length, where r is half the diameter. A round tube is a hollow cylinder, so its volume is the solid outer cylinder minus the solid inner one: π x (R² − r²) x length, using the outer and inner radii. A rectangular bar or plate is simply length x width x thickness.

Density is where steel grade comes in. The calculator's default density is 7,850 kg/m³, the standard reference figure used across engineering for mild/carbon structural steel (matching a C1020 hot-rolled preset). Other grades in the table run from about 7,715 kg/m³ for tool steel up to roughly 8,030 kg/m³ for stainless steel — a spread of around 4%. That is negligible on a single offcut, but on a multi-tonne order it is the difference between a quote that matches the invoice and one that does not, so always confirm which grade you are actually buying.

Worked example: a solid round mild-steel bar, 12 mm diameter, 6 m long, at the default density of 7,850 kg/m³. Radius = 6 mm = 0.006 m. Volume = π x r² x length = 3.14159 x 0.006² x 6 = 3.14159 x 0.000036 x 6 ≈ 0.0006786 m³. Weight = volume x density = 0.0006786 x 7,850 ≈ 5.33 kg per bar. For 10 identical bars, total weight = 53.3 kg. (As a sanity check, the classic metric rebar shortcut, weight ≈ diameter² x length ÷ 162, gives 12² x 6 ÷ 162 = 5.33 kg — the same answer, since that shortcut is this exact formula pre-simplified for mild-steel density.)

UK Steel Standards

Structural steel in the UK is specified under BS EN 10025, covering grades such as S235, S275 and S355 for hot-rolled bar, plate and sections.
Reinforcing bar follows BS 4449, typically grade B500B (deformed T-bar) — source: BS EN 10025 and BS 4449:2005+A3:2016.

Pro Tips and Common Mistakes

At a typical retail price of around £1.10 per kg for mild-steel round bar, the 10-bar parcel from the worked example above (53.3 kg of 12 mm bar) would cost roughly £59 in raw material — cutting, delivery and any galvanising are additional.

Never treat a tube as a solid bar — leaving the inner diameter at zero on a pipe massively overstates its weight, since a hollow section can weigh well under half of the equivalent solid one.
Confirm the actual steel grade before you rely on the density figure for a large order — the difference between carbon steel and stainless steel alone is around 2-3%, and that compounds fast once you are pricing by weight rather than by the piece.
Weight is not just a pricing number: freight carriers charge and rate loads by weight, and cranes, hoists, forklifts and vehicle load beds all have a maximum rated capacity that a miscalculated steel weight can quietly exceed.
Structural steel sizing (whether a beam or column can actually carry a given load) is a separate engineering calculation from its weight — this tool tells you what a piece weighs, not whether it is strong enough for your application.
Double-check that width and thickness are not swapped on a rectangular plate, and that every dimension uses the unit you think it does — a mixed mm/cm entry is the single most common source of an order that is 10x too heavy or too light.
Mill stock is sold in fixed standard lengths, not custom-cut to the metre or foot — round your piece count up to whole stock lengths before you place an order.