Rebar Calculator
Estimate how many reinforcing bars, how many metres and how many kilos of rebar a concrete slab needs.
An estimate of material quantities only. Bar size, spacing, cover and laps must follow the structural design (in Europe, Eurocode 2) – this calculator does not size reinforcement.
About this tool
Enter the size of the slab, the bar spacing and the edge cover, and pick the bar diameter. The calculator lays out a square grid and tells you how many bars run each way, the total length including an allowance for laps and offcuts, how many 6 m or 12 m stock bars to buy and the total weight of steel. Choose two layers for a top and bottom mesh.
Frequently asked questions
Subtract the edge cover from both sides, divide by the spacing and add one. A 6 × 4 m slab with 5 cm cover and bars every 20 cm needs floor(3.9 ÷ 0.2) + 1 = 20 bars of 5.9 m one way and floor(5.9 ÷ 0.2) + 1 = 30 bars of 3.9 m the other way – 235 m per layer before laps.
Steel weighs about 7,850 kg/m³, so a round bar weighs π/4 × d² × 7,850 kg per metre, or roughly d² ÷ 162 with d in millimetres. That gives 0.395 kg/m for 8 mm, 0.617 kg/m for 10 mm, 0.888 kg/m for 12 mm, 1.58 kg/m for 16 mm and 2.47 kg/m for 20 mm.
Where one bar ends and the next continues, the two must overlap (a lap splice) so the force can pass between them, and bars cut from stock lengths leave offcuts. The required lap length depends on bar size and concrete and is set by the design; 10–15 % extra is a common starting point for an estimate.
Cover is the concrete between the steel and the surface. It protects the bars from rust and fire and lets them bond to the concrete. The minimum cover depends on exposure and is given in the design code (Eurocode 2 in Europe); use bar chairs or spacers to hold the bars at the right height.
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