Rebar Weight Per Metre: A UK Guide

Reinforcing bar in the UK is manufactured to BS 4449:2005+A3:2016, and the standard sizes are 6, 8, 10, 12, 16, 20, 25, 32 and 40 mm.

Two sizes cover most domestic groundwork. 12 mm is the default for ground-bearing slabs and light strip foundations. 16 mm appears in raft edges, heavier footings and anywhere a structural engineer has been near the drawing.

Takeaway: for a quick mental figure, 12 mm bar is a shade under 0.9 kg per metre and 16 mm is a shade over 1.5 kg.

The weight of a steel bar is its cross-sectional area times its length times the density of steel. Structural steel is taken as 7,850 kg per cubic metre, and that figure is fixed regardless of grade, so a bar's weight depends only on its diameter.

Written out with the diameter in millimetres:

Weight (kg/m) = 0.006165 × d²

There's a shortcut that every steel fixer knows and almost no calculator explains:

Weight (kg/m) = d² / 162

Both give the same answer. A 12 mm bar: 144 divided by 162 is 0.888 kg per metre. A 20 mm bar: 400 divided by 162 is 2.47 kg per metre. Close enough to the tabulated figure for ordering, and you can do it in your head on site.

Notice what the squared term does. Going from 12 mm to 16 mm is a 33% increase in diameter and a 78% increase in weight. Going from 16 mm to 20 mm adds another 56%. Diameter increases look modest on a drawing and land hard on an invoice.

Takeaway: divide the diameter squared by 162 for kilograms per metre, and remember that weight rises with the square of the size, not in step with it.

Here's where most guidance points people in the wrong direction. When a slab feels under-reinforced, the reflex is to step the bar up a size. Go from 12 mm to 16 mm and it feels like a safety margin.

For a given area of steel, it's often the worse choice.

Reinforcement resists cracking by bonding to the concrete along its surface. Spread the same steel area across more bars at closer centres and you get more bonded surface, more crack-control points, and finer cracks distributed through the slab. Concentrate it into fewer fat bars and you get the same tensile capacity with wider cracks between them. Eurocode 2, published in the UK as BS EN 1992-1-1, handles this explicitly: its crack-width provisions cap bar diameter and bar spacing precisely because the two interact.

There's a practical dimension too. 16 mm bar bends harder, needs a bigger former radius under BS 8666, is heavier to lift into position, and is more likely to end up sitting on the ground rather than on chairs at the right depth. Steel in the wrong place does nothing at any diameter.

The honest answer is that bar size belongs to whoever did the design. If your slab genuinely needs more steel, more bars at tighter centres is usually the better route than fewer, thicker ones.

Takeaway: don't uprate bar diameter to buy confidence. Closer centres at the same size generally control cracking better and cost less.

A ground-bearing slab, 6.0 m by 3.5 m, reinforced with a single mat of 12 mm bar at 200 mm centres both ways, 50 mm cover all round.

Bars running the long way (6.0 m direction): Spaced across the 3.5 m width, less cover both sides: (3,500 - 100) / 200 = 17 spaces, so 18 bars. Each one is 6.0 m less 50 mm cover at each end, so 5.9 m. 18 × 5.9 = 106.2 m

Bars running the short way (3.5 m direction): Spaced along the 6.0 m length: (6,000 - 100) / 200 = 29.5, so 31 bars. Each 3.4 m long. 31 × 3.4 = 105.4 m

Total bar length: 106.2 + 105.4 = 211.6 m Net steel weight: 211.6 × 0.888 = 187.9 kg

Call it 188 kg, and that's the figure a weight calculator gives you. Now order it.

Bar comes in stock lengths, commonly 6 m for merchant supply. The 18 long bars each take one 6 m length, with 100 mm trimmed off. The 31 short bars each need 3.4 m, and a 6 m length won't yield two of those, so each short bar also consumes a full stock length and leaves a 2.6 m offcut.

Stock lengths required: 18 + 31 = 49 lengths Weight purchased: 49 × 6 × 0.888 = 261 kg

That's 261 kg bought against 188 kg fixed, a 39% overage before anyone mentions laps. At roughly £850 per tonne in 2026 the calculated figure prices at about £160 and the delivered figure at about £222.

The fix is cut-to-length. Order the short bars cut at 3.4 m from a supplier like ParkerSteel or Express Reinforcements and you pay for close to the 188 kg, plus a cutting charge. On a garage base the saving is modest. On anything larger it stops being modest quickly.

Takeaway: calculate net weight to check the design, then work out stock lengths separately to price the order, because the two figures are not the same number.

Bars in tension have to overlap where they meet, and the lap transfers force from one bar to the next through the surrounding concrete. Eurocode 2 gives a design lap length that depends on bond conditions, concrete strength and how much of the steel is lapped at one point. The site rule of thumb sits around 40 to 50 times the bar diameter, so a 12 mm bar laps at roughly 480 to 600 mm.

Every lap is bar you paid for and that carries no additional load. On a slab small enough that bars run full width there are no laps at all, which is the case in the Wakefield example above. Go beyond about 6 m in either direction and laps arrive, and on a large raft they can add 10 to 15% to the total tonnage.

Takeaway: on anything wider than a stock bar length, add 10 to 15% for laps before you order, and confirm the actual lap length against the engineer's detail rather than the rule of thumb.

For most domestic slabs the reinforcement isn't loose bar at all. It's prefabricated fabric to BS 4483, supplied in sheets of 4.8 m by 2.4 m.

A142 is the light fabric used in domestic floor slabs. A393 is the heavy one, and at 71 kg a sheet it takes two people to place.

Mesh is faster. One sheet goes down in the time it takes to tie a couple of square metres of loose bar, and the spacing is right by definition rather than by whoever was holding the tape. The trade-off is waste at edges and around openings, and the laps: fabric normally laps two full squares, roughly 400 mm, which eats a surprising amount of a small slab. On the 21 m² Wakefield base you'd get through three or four sheets once laps are allowed for, against a theoretical coverage of under two.

Worth noting the two options in that example aren't like for like. 12 mm bar at 200 centres provides about 565 mm² of steel per metre width. A393 provides 393 mm². The bar mat is a heavier specification, which is part of why it weighs more.

Takeaway: for a simple rectangular domestic slab, mesh is normally faster and cheaper fixed. Loose bar earns its place where the shape is awkward or the design calls for more steel than fabric provides.

UK reinforcement is grade B500 to BS 4449, where 500 is the characteristic yield strength in newtons per square millimetre. The letter after it is the ductility class.

B500A is the lowest ductility class, typically cold-worked, and turns up mostly in mesh and small-diameter wire. B500B is the standard bar grade, and unless a drawing says otherwise, it's what a UK merchant means by rebar. B500C has the highest ductility and appears where a design specifically requires it.

Grade doesn't change the weight. All three are steel at 7,850 kg per cubic metre, so a 12 mm B500A bar and a 12 mm B500C bar weigh exactly the same per metre. What changes is how the steel behaves as it yields.

Most UK rebar is produced domestically, with Celsa Steel UK in Cardiff supplying a large share of it, and merchants including Travis Perkins and Jewson stocking cut lengths and mesh for smaller orders.

Takeaway: specify B500B unless the drawing says otherwise, and don't expect grade to affect the tonnage.

I asked a steel fixer working on housing sites around West Yorkshire what actually goes wrong with reinforcement quantities on domestic jobs. He barely mentioned the arithmetic.

His answer was chairs and spacers. He said people order the steel correctly, then buy nothing to hold it up, and the mat ends up lying in the bottom of the pour where it does very little. He reckoned the cover being wrong is more common than the tonnage being wrong, and it's harder to spot afterwards because you can't see it once the concrete goes in.

The second thing he mentioned was cut lengths. On anything with a repeated bar length he schedules it cut, because the offcuts on 6 m stock are unusable often enough that the cutting charge pays for itself. He put it plainly: the stack of 2.6 m ends behind every site cabin is somebody's money.

Takeaway: budget for chairs and spacers at the same time as the steel, because reinforcement at the wrong depth is a more expensive mistake than reinforcement in the wrong quantity.

The formula is one line and the table above covers every UK size, but a real slab means two directions, cover deductions, spacing arithmetic and a lap allowance, which is four chances to slip a decimal. The rebar weight calculator works it through for you.

If you're pricing the concrete alongside the steel, the concrete volume calculator and the concrete footing calculator cover the other half of the order.

This guide is for estimating and planning. Reinforcement design is structural work and belongs to a qualified structural engineer. Bar size, spacing, cover, lap lengths and grade must come from a design to BS EN 1992-1-1 and a schedule to BS 8666 for your specific project. Building work in England and Wales is subject to the Building Regulations, and structural elements need Building Control approval. Confirm every figure here against the current standards and your engineer's drawings before ordering or fixing anything.