How much rebar do I need?
Type the slab size, the bar spacing and the edge cover, and pick the bar size. The rebar calculator lays out the grid, adds laps and waste, and tells you how many stock bars to buy and what they weigh.
- Stock bars to buy
- 18
Buy 18 stock bars (352.7 ft of steel, about 236 lb).
- Bars along the length
- 8
- Bars along the width
- 14
- Total length (ft)
- 352.7
- Total length (m)
- 107.5
- Weight (lb)
- 236
- Weight (kg)
- 107
- Lap splices
- 0
- Lap length (in)
- 20
Stock bars to buy: 18. Buy 18 stock bars (352.7 ft of steel, about 236 lb).
How to calculate
Works out the rebar grid for a concrete slab: bars each way from the size, spacing and edge cover, laps where a run is longer than a stock bar, waste, the number of stock bars to buy, and their weight.
Example with the default inputs (Slab length 20 ft, Slab width 12 ft, Spacing 18 in, Edge cover 2 in, Bar size #4 (1/2 in, 13 mm), Stock bar length 20 ft, Waste allowance 10%): Buy 18 stock bars (352.7 ft of steel, about 236 lb).
Method: Bars along the length = ⌊(width − 2 × cover) ÷ spacing⌋ + 1, each (length − 2 × cover) long, and the same the other way; a run longer than a stock bar gets ⌈(run − stock) ÷ (stock − lap)⌉ laps of 40 bar diameters; total = (runs + laps) × (1 + waste); stock bars = ⌈total ÷ stock length⌉; weight = feet × lb/ft.
- One layer of bars at the same spacing both ways, as in most slabs on the ground.
- Laps of 40 bar diameters and 10% waste are Hercules Chair’s rules of thumb, not a design. Your plans, engineer and local code set the bar size, spacing, cover and lap length.
- Weights per foot are nominal; the real weight varies slightly by maker.
Worked examples
Each example is checked against the calculator on every build.
- Slab length 20 ft, Slab width 12 ft, Spacing 18 in, Edge cover 2 in, Bar size #4 (1/2 in, 13 mm), Stock bar length 20 ft, Waste allowance 10% gives Bars along the length 8, Bars along the width 14, Lap splices 0, Total length (ft) 352.733333, Total length (m) 107.51312, Stock bars to buy 18, Weight (lb) 235.625867, Weight (kg) 106.878095, Lap length (in) 20.Source: Hercules Chair, How to Calculate Rebar for a Slab (lap at least 40 bar diameters; 10% waste; 18–24 in spacing for flat slabs; 2 in edge cover), https://herculeschair.com/how-to-calculate-rebar-for-slab-the-right-way/; Harris Supply Solutions, Steel Rebar Sizes (#3 0.376 lb/ft … #8 2.67 lb/ft; standard 20 and 30 ft lengths), https://www.harrissupplysolutions.com/steel-rebar-sizes-stock.html
- Slab length 40 ft, Slab width 10 ft, Spacing 12 in, Edge cover 3 in, Bar size #4 (1/2 in, 13 mm), Stock bar length 20 ft, Waste allowance 10% gives Bars along the length 10, Bars along the width 40, Lap splices 20, Total length (ft) 889.166667, Stock bars to buy 45, Weight (lb) 593.963333.Source: Hercules Chair, How to Calculate Rebar for a Slab (lap at least 40 bar diameters; 10% waste; 18–24 in spacing for flat slabs; 2 in edge cover), https://herculeschair.com/how-to-calculate-rebar-for-slab-the-right-way/; Harris Supply Solutions, Steel Rebar Sizes (#3 0.376 lb/ft … #8 2.67 lb/ft; standard 20 and 30 ft lengths), https://www.harrissupplysolutions.com/steel-rebar-sizes-stock.html
- Slab length 19.69 ft, Slab width 13.12 ft, Spacing 11.81 in, Edge cover 1.969 in, Bar size #5 (5/8 in, 16 mm), Stock bar length 19.69 ft, Waste allowance 5% gives Bars along the length 14, Bars along the width 20, Total length (m) 168.63, Stock bars to buy 29, Weight (kg) 261.739896.Source: Harris Supply Solutions, Steel Rebar Sizes (#3 0.376 lb/ft … #8 2.67 lb/ft; standard 20 and 30 ft lengths), https://www.harrissupplysolutions.com/steel-rebar-sizes-stock.html (#5 1.043 lb/ft)
How it works
The slab gets one layer of bars in a square grid: the same spacing s both ways, with every bar end kept a cover c in from the edge. Let L and W be the slab length and width.
- Runs. A bar along the length is L − 2c long; a bar along the width is W − 2c long. If either is 0 or less there is no answer.
- Bars each way. Bars along the length = ⌊(W − 2c) ÷ s⌋ + 1 (they are spread across the width). Bars along the width = ⌊(L − 2c) ÷ s⌋ + 1.
- Laps. Lap length = 40 × the bar diameter. A run no longer than one stock bar needs no lap. A longer run needs k = ⌈(run − stock) ÷ (stock − lap)⌉ laps, and its steel is run + k × lap. The stock bar must be longer than two laps.
- Total = (bars along the length × their steel + bars along the width × their steel) × (1 + waste ÷ 100).
- Stock bars = ⌈total ÷ stock length⌉.
- Weight = total in feet × the bar’s weight per foot; kilograms = pounds × 0.45359237.
Bar sizes:
| Size | Diameter | Weight |
|---|---|---|
| #3 | 0.375 in | 0.376 lb/ft |
| #4 | 0.5 in | 0.668 lb/ft |
| #5 | 0.625 in | 1.043 lb/ft |
| #6 | 0.75 in | 1.502 lb/ft |
| #7 | 0.875 in | 2.044 lb/ft |
| #8 | 1 in | 2.67 lb/ft |
Rules. Slab sides from 0.1 m to 1,000 m; spacing from 2.5 cm to 3 m; cover from 0 to 1 m; stock bar from 1 m to 30 m; waste from 0% to 50%.
Exact arithmetic. Every length is read as the exact decimal you typed in its unit (1 ft = 0.3048 m, 1 in = 0.0254 m), so the floors and ceilings above are exact: a spacing that fits a whole number of times counts that last bar.
Output format. Bar counts and laps are whole numbers; lengths show 1 decimal and weights 0 decimals, rounded half up.
Assumptions
- One layer, a square grid, bars ending at the cover line. Chairs, ties, bends, dowels and edge bars are not counted.
- The 40-diameter lap and the 10% waste are the Hercules Chair rules of thumb, not a structural design; your plans, engineer and local code come first.
- Weights are nominal per-foot weights.
Worked examples by hand
20 × 12 ft slab, #4 at 18 in, 2 in cover, 20 ft stock, 10% waste. Runs: 240 − 4 = 236 in and 144 − 4 = 140 in. Bars along the length ⌊140 ÷ 18⌋ + 1 = 8; along the width ⌊236 ÷ 18⌋ + 1 = 14. Both runs fit in 240 in, so 0 laps. Steel 8 × 236 + 14 × 140 = 3,848 in = 320.667 ft; × 1.1 = 352.7 ft (107.5 m). Stock bars ⌈352.733 ÷ 20⌉ = 18. Weight 352.733 × 0.668 = 236 lb (107 kg). Lap length 40 × 0.5 = 20 in.
40 × 10 ft slab, #4 at 12 in, 3 in cover, 20 ft stock, 10% waste. Runs 474 in and 114 in. Bars along the length ⌊114 ÷ 12⌋ + 1 = 10; along the width ⌊474 ÷ 12⌋ + 1 = 40. Each long run needs ⌈(474 − 240) ÷ (240 − 20)⌉ = 2 laps, so 20 laps and 474 + 40 = 514 in of steel per run. Total (10 × 514 + 40 × 114) × 1.1 = 10,670 in = 889.2 ft; 45 stock bars; 594 lb.
6 × 4 m slab, #5 at 30 cm, 5 cm cover, 6 m stock, 5% waste. Runs 5.9 m and 3.9 m. Bars along the length ⌊3.9 ÷ 0.3⌋ + 1 = 14; along the width ⌊5.9 ÷ 0.3⌋ + 1 = 20. Steel 14 × 5.9 + 20 × 3.9 = 160.6 m; × 1.05 = 168.63 m; 29 stock bars; 553.25 ft × 1.043 = 577.0 lb = 262 kg.
Other questions people ask
How much rebar do I need for a slab?
Count the bars each way and add up their lengths. A 20 × 12 ft slab with #4 bars at 18 in and 2 in edge cover has 8 bars running the long way and 14 the short way: 320.7 ft of bar, or 352.7 ft with 10% waste, which is 18 bars of 20 ft.
How do I work out the number of bars?
Take the side, subtract the cover at both edges, divide by the spacing, round down and add one bar for the start. Across 12 ft (144 in) with 2 in cover at 18 in spacing: (144 − 4) ÷ 18 = 7.8, so 7 + 1 = 8 bars.
What is a lap splice?
Where a run is longer than one stock bar, two bars overlap and are tied together. A common rule of thumb is a lap of 40 bar diameters: 20 in for #4 (1/2 in) bar. Your plans or local code may ask for more.
How much does rebar weigh?
Per foot: #3 0.376 lb, #4 0.668 lb, #5 1.043 lb, #6 1.502 lb, #7 2.044 lb and #8 2.67 lb. So 352.7 ft of #4 bar weighs about 236 lb (107 kg).
What spacing and cover should I use?
For flat slabs such as patios and driveways, 18 to 24 in spacing is typical, and 12 in for footings or heavier loads. Keep the bars about 2 in in from the edges. Follow your plans and local code where they differ.
Why add waste?
Offcuts from cutting bars to length, bends and mistakes use extra steel. The page adds 10% by default, a common allowance; set it to 0 if you have already counted every piece.