How to use this calculator
- 1Enter the slab length and width — the outside size of the concrete.
- 2Enter the edge cover and the number of mats shown on your drawings.
- 3Choose the bar size. The weight per length and a cautious lap fill in; change them if your drawings say otherwise.
- 4Enter the bar spacing and the length of the bars your supplier sells.
- 5Read the bars to buy and their weight, and the cut list: how many bars run each way and how long each one is.
The formula
- 01Bar length each way = slab size − 2 × edge cover
- 02Bars along the length = (bar length across ÷ spacing, rounded up) + 1 — and the same the other way
- 03Short bars: as many as fit are cut from each stock bar
- 04Long bars: stock bars joined with a lap; pieces = (bar length − lap) ÷ (stock length − lap), rounded up
- 05Weight = bars to buy × stock length × weight per length (other diameters: d² ÷ 162 kg/m)
- 06Binding wire = weight × wire % (1.3 % = 13 kg per tonne)
The number of spaces is rounded up, so the real spacing is never wider than the spacing you enter, and one extra bar closes the last space. Each direction is counted on its own, and the count is multiplied by the number of mats.
Bars that fit inside one stock length are cut from it, as many per stock bar as will fit. Longer bars are made of stock bars joined with an overlap called a lap. Offcuts are treated as waste rather than joined together: that is the simplest way to build and never leaves you short. The "offcuts left over" line shows how much steel that is — if it is a lot, a different stock length or cut-to-length bars may save money.
Weights are the nominal masses from ASTM A615 for US bar sizes (#4 = 0.668 lb/ft, #8 = 2.670 lb/ft) and BS 4449 / EN 10080 for metric bars (12 mm = 0.888 kg/m, 25 mm = 3.85 kg/m). They are the weight of a round bar of steel at 7,850 kg/m³, which is the site rule d² ÷ 162 kg per metre with d in millimetres. Pakistani mills roll both the ASTM inch sizes (#3 = 3/8 in = 9.5 mm up to #8 = 1 in = 25.4 mm) and metric bars; they are close but not the same — a #4 bar (12.7 mm) weighs 0.994 kg/m, a 12 mm bar 0.888 — so pick the size on your drawings or the mill's label. The lap is filled in as 60 bar diameters, which is on the cautious side: ACI 318 Class B tension laps for Grade 60 bars work out at about 49 bar diameters in 4,000 psi concrete and 57 in 3,000 psi, and Eurocode 2 laps are typically 40–60 diameters. Your drawings always take priority.
Worked example
A 24 ft × 12 ft slab with 3 in edge cover and bars 12 in apart needs 13 bars of 23.5 ft one way and 25 bars of 11.5 ft the other way.
The long bars don't fit in one 20 ft stock bar, so they are joined with a 30 in lap — 13 laps in all.
That comes to 43 bars to buy, weighing 574.5 lb at 0.668 lb/ft.
Tips before you buy
- Support the grid on bar chairs or concrete spacer blocks so it stays at the depth shown on your drawings while the concrete is poured.
- Tie the bars together where they cross with tie wire so the grid doesn't shift when people walk on it during the pour.
- Many rebar suppliers cut and bend to a bar schedule. Give them the counts and lengths from the results to cut waste.
- Check whether your slab needs loose bars or welded wire mesh (fabric sheets) — your drawings or local code will say which.
- Buy extra tie wire and chairs; they are cheap, and running out halfway through tying a grid is frustrating.
Frequently asked questions
How do I calculate how much rebar I need for a slab?
Take the edge cover off each side, divide each direction by the spacing, round up and add one bar. Multiply by the bar length each way, work out how many stock bars that takes, and multiply by the weight per foot or metre. This calculator does all of that, including laps.
What spacing should rebar be in a slab?
It depends on what the slab carries, how thick it is and the ground under it, so it must come from your drawings, an engineer or your local code. Spacings such as 12 in or 300 mm are common, but this calculator doesn't choose one for you.
How much should rebar overlap?
The lap depends on the bar size, steel grade, concrete strength and where the joint is. Crews often quote 40 bar diameters, but ACI 318 Class B tension laps for Grade 60 bars in ordinary concrete work out at roughly 49–57 diameters. We fill in 60 diameters to be safe; always use the lap on your drawings.
How far from the edge should rebar stop?
Far enough to leave the concrete cover your drawings ask for. ACI 318 requires 3 in of cover where concrete is cast against and permanently exposed to earth. Eurocode 2 recommends at least 75 mm where concrete is cast directly against soil, or 40 mm on prepared ground or blinding.
How much does rebar weigh?
#3 weighs 0.376 lb/ft, #4 0.668, #5 1.043, #6 1.502, #7 2.044 and #8 2.670 lb/ft (ASTM A615). Metric bars weigh 0.222 kg/m for 6 mm, 0.395 for 8 mm, 0.617 for 10 mm, 0.888 for 12 mm, 1.58 for 16 mm, 2.47 for 20 mm, 3.85 for 25 mm and 6.31 for 32 mm (BS 4449). Any other size: d² ÷ 162 kg per metre. So a 20 ft #4 bar is about 13.4 lb and a 12 m 12 mm bar about 10.7 kg.
What do rebar sizes like #4 mean?
US bar numbers give the nominal diameter in eighths of an inch: #4 is 4/8 = 1/2 in and #5 is 5/8 in. Metric bars are named by their diameter in millimetres; in the UK a T12 or H12 bar is a 12 mm high-strength bar. In Pakistan both kinds are sold: ASTM #3–#8 bars (in inches) and metric bars, so check which one your drawings and your supplier mean.
How much binding wire do I need for rebar?
There is no standard figure; the site rule of thumb is 9–13 kg of binding (tie) wire per tonne of bars — more for thin bars with many crossings (about 12 kg for 8 mm), less for thick bars (about 7 kg for 28–32 mm). The calculator fills in 1.3 %, the top of that range, so you don't run short.
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Sources
- Caltrans BCM 52-1: ASTM standard reinforcing bar sizes and weights (from the CRSI Manual of Standard Practice)
- Next Day Steel: UK rebar sizes and mass per metre to BS 4449, 6 m lengths
- ACI 318-19 Building Code for Structural Concrete: cover (Table 20.5.1.3.1) and lap splices (25.4, 25.5)
- Eurocode 2 (EN 1992-1-1) 4.4.1.3(4): cover for concrete cast against prepared ground (40 mm) or soil (75 mm)
- IS 1786:2008, Table 1: nominal mass of 6–40 mm bars (0.00785 kg per mm² per metre, the source of d² ÷ 162)
- Nucor – Rebar: #3–#18 weights; cut lengths of 20, 40 and 60 ft
- Agha Steel (Pakistan) – ASTM A615 bars #3–#8 with diameters and kg per foot
- Civil Sir – binding wire rule of thumb: 9–13 kg per tonne of steel
Formulas last reviewed: September 30, 2026 ·How we check our formulas