# How much rebar do I need?

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.

- Page: https://www.acalculator.org/construction/rebar-calculator
- JSON spec: https://www.acalculator.org/construction/rebar-calculator.json
- Version: c1232e814a93

## Default answer

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).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| l | Slab length | The long side of the slab. |
| w | Slab width | The short side of the slab. |
| s | Spacing | The distance between bars, centre to centre, the same both ways. |
| c | Edge cover | How far the bar ends stay back from each edge of the slab. |
| bar | Bar size | The US bar size, with its diameter in inches and the metric size it matches. |
| stock | Stock bar length | The length of the bars you buy (20 ft is the most common). |
| waste | Waste allowance | Extra steel for cutting offcuts and mistakes, in percent. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| bars | Stock bars to buy | The total length with waste ÷ the stock bar length, rounded up. |
| alongLength | Bars along the length | Bars that run the long way, across the width. |
| alongWidth | Bars along the width | Bars that run the short way, along the length. |
| feet | Total length (ft) | All the steel with laps and waste, in feet. |
| meters | Total length (m) | All the steel with laps and waste, in metres. |
| pounds | Weight (lb) | Total length in feet × the bar’s weight per foot. |
| kilograms | Weight (kg) | The same weight in kilograms. |
| laps | Lap splices | How many overlaps join bars in runs longer than a stock bar. |
| lapInches | Lap length (in) | 40 bar diameters. |

## 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.

## Assumptions

- 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

1. l = 6.096, w = 3.6576, s = 0.4572, c = 0.0508, bar = 4, stock = 6.096, waste = 10% gives alongLength = 8, alongWidth = 14, laps = 0, feet = 352.733333, meters = 107.51312, bars = 18, pounds = 235.625867, kilograms = 106.878095, lapInches = 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.
2. l = 12.192, w = 3.048, s = 0.3048, c = 0.0762, bar = 4, stock = 6.096, waste = 10% gives alongLength = 10, alongWidth = 40, laps = 20, feet = 889.166667, bars = 45, pounds = 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.
3. l = 6, w = 4, s = 0.3, c = 0.05, bar = 5, stock = 6, waste = 5% gives alongLength = 14, alongWidth = 20, meters = 168.63, bars = 29, kilograms = 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).

## FAQ

### 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.

## Sources

- Harris Supply Solutions, Steel Rebar Sizes: US bar sizes #3 to #8 with nominal diameter and weight per foot; standard lengths of 20 and 30 ft. https://www.harrissupplysolutions.com/steel-rebar-sizes-stock.html (retrieved 2026-10-02)
- Hercules Chair (T. J. Harris Co.), How to Calculate Rebar for a Slab the Right Way: lap at least 40 times the bar diameter; 10% waste; 18 to 24 in spacing for flat slabs, 12 in for footings; 2 in cover for slab bottoms and walls. https://herculeschair.com/how-to-calculate-rebar-for-slab-the-right-way/ (retrieved 2026-10-02)
- NIST Special Publication 811, Appendix B (1 ft = 0.3048 m, 1 in = 2.54 cm, 1 lb = 0.45359237 kg exactly). https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-02)