How do I calculate rebar weight?

Multiply the total bar length you need (including splices and waste) by its linear mass in kg/m, which follows mass ≈ 0.00617 x diameter² (mm). A 12 mm bar weighs about 0.888 kg/m, so 720.72 m of it weighs roughly 640 kg.

What size rebar should I use for a slab?

Residential slabs are commonly reinforced with 10-12 mm bar on a 200-300 mm grid. Driveways, footings and structural loads typically need a larger diameter or tighter spacing, sized from an engineer's spec.

Is rebar more expensive in Gibraltar than in the UK?

Yes, typically — because reinforcing steel is shipped in rather than produced locally, expect a modest import premium over equivalent UK or Spanish mainland trade prices, on top of the usual freight lead times for larger orders.

Rebar Calculator — Bar Count, Weight & Cost for Slab Grids | Gibraltar

Rebar Calculator: How Much Rebar Do I Need for a Slab?

Reinforcing a slab or footing means working out three numbers before you order anything: how many bars the grid needs, how much they weigh (rebar is usually sold by weight, not by the piece), and how much extra length lap splices add when your stock bars are shorter than the run they cover. This rebar calculator works out the reinforcing grid from your slab dimensions and bar spacing, then converts that into bar count, total length, weight and a material cost estimate.

It is a grid-spacing calculator, not a volume tool: enter the slab length and width, the bar spacing, and the edge clearance the bars sit back from the slab edge, and it lays out the grid the way a steel-fixer would on site — adding a lap-splice allowance wherever a run exceeds your stock bar length, plus a waste percentage for cutting losses. Bar-size presets follow the sizing and nominal-mass conventions used across national rebar standards, so the weight matches what a supplier will quote.

How to Use the Rebar Calculator

Enter the slab length and width, choosing your unit (m, cm, ft or in) for each.
Enter the grid spacing — the centre-to-centre distance between adjacent bars in both directions.
Enter the edge clearance — how far the outermost bars sit back from the slab edge, set by your required concrete cover.
Choose a bar-size preset (e.g. 12 mm / #4) or a custom diameter — this sets the linear mass (kg/m) used for weight.
Enter the stock length the bars are sold in (commonly 6 m or 12 m, or 20 ft in the US) — this drives lap splices and piece count.
Adjust the lap-splice multiplier if your engineer specifies something other than the common 40 x diameter rule of thumb.
Set the waste allowance (0-15%) for cutting losses.
Read off the bar count, total length, weight and cost, editing the price per bar to match your supplier.

The Math Behind the Calculation

The calculator shrinks the slab to a working grid by pulling in each edge by the edge clearance: working length = slab length − (2 x edge clearance), same for width. It then finds how many bar positions fit along each direction at your spacing, using floor(dimension ÷ spacing) + 1. Raw total length is each position count multiplied by the working dimension in that direction, added together.

Lap splices are added wherever a run exceeds your stock bar length: each such run needs ceil(run ÷ stock length) − 1 splices, each consuming the lap multiplier (typically 40) times the bar diameter. Waste % is applied on top, and the result divided by stock length and rounded up for bar count. Weight = total length x linear mass, where mass (kg/m) ≈ 0.00617 x diameter² (mm) — the standard nominal-mass formula for reinforcing steel.

Worked example: a 10 m x 6 m slab, 200 mm spacing, 50 mm edge clearance, 12 mm bars, 6 m stock lengths, 40x-diameter splices, 5% waste. Working grid = 9.9 m x 5.9 m, giving 50 positions along the 9.9 m side and 30 along the 5.9 m side: raw length = (50 x 9.9) + (30 x 5.9) = 672 m. The 9.9 m runs exceed the 6 m stock, so each of the 30 runs needs one splice: 30 x (40 x 0.012 m) = 14.4 m extra, for 686.4 m pre-waste. Adding 5% waste gives 720.72 m, or 121 bars (rounded up from 720.72 ÷ 6), weighing about 720.72 x 0.888 ≈ 640 kg.

Common Rebar Sizes and Nominal Weights

10 mm (≈ US #3) — about 0.617 kg/m
12 mm (≈ US #4) — about 0.888 kg/m
16 mm (≈ US #5) — about 1.578 kg/m
20 mm (≈ US #6) — about 2.466 kg/m
25 mm (≈ US #8) — about 3.853 kg/m
Source: standard nominal-mass formula for reinforcing steel, mass (kg/m) ≈ 0.00617 x diameter² (mm), the basis of the weight tables behind national rebar standards (e.g. BS 4449, EN 10080, ASTM A615); confirm grade and rounding against your local standard for structural work.

Gibraltar Rebar Standards

Gibraltar follows UK/EU practice for reinforcement, with BS 4449 (implementing BS EN 10080) the reference standard and grade B500B/B500C the common ductility class for T-bar reinforcement.
Because steel is imported (typically from Spain or the UK), local prices carry a shipping/import premium over mainland UK or Spanish trade prices.

Pro Tips and Common Mistakes

At a typical local price of around £8.50 per 6 m bar (reflecting the import premium on shipped steel), the worked example above (121 bars of 12 mm T-bar, roughly 640 kg total) would cost roughly £1,028.50 in raw material.

Edge clearance is set by your required concrete cover, which varies by exposure — check your local code rather than assuming one number fits every pour.
Always round the bar count up, never down — a shortfall mid-pour is worse than a small surplus.
The 40 x diameter lap-splice rule of thumb is common but not universal — defer to a structural drawing if one exists.
For two mesh layers or different spacing per direction, run the calculator once per layer rather than combining them.
Outside the US, rebar is priced by weight, not per bar — compare supplier quotes on the weight output, not just bar count.

Rebar Calculator