Concrete & Masonry
Rebar Calculator
Estimate the rebar for a concrete slab grid. Enter the slab size, bar size and spacing, and this rebar calculator counts the bars each way, the total length including lap splices, the stock bars to buy, tie points, weight and cost. It is a quantity estimate, not a structural design.
Result
Enter your measurements and press Calculate. What to buy, the working and the cost will appear here.
Quantity estimate only. This calculator counts bars for a grid you already know. It does not choose bar sizes, spacing, cover or splice lengths. Those come from your plans, an engineer or your local building code.
How to use the rebar calculator
- Enter the slab length and width.
- Choose the bar size and spacing from your plans. Enter a second spacing if the grid is not square.
- Set the edge cover, the gap between the slab edge and the bar ends, and the number of layers.
- Enter the stock length and lap. Bars longer than the stock are spliced with an overlap.
- Add a price per bar and press Calculate.
How rebar quantity is calculated
bars one way = (slab width − 2 × cover) ÷ spacing, rounded down, + 1
bar length = slab length − 2 × cover (plus laps where it is longer than a stock bar)
Do the same in the other direction, multiply by the number of layers, then fit all the bar lengths into stock bars. Intersections (one way × the other way) give the number of tie points.
Worked example: a 20 × 20 ft slab
A 20 ft × 20 ft slab uses #4 bar at 18 in centers both ways, one mat, 3 in edge cover, 20 ft stock bars at $9 each:
- Bar runs: 20 ft − 2 × 3 in cover = 19.5 ft; 20 ft − 2 × 3 in = 19.5 ft
- Bars along the length: 19.5 ft ÷ 18 in, rounded down, + 1 = 14
- Bars along the width: 19.5 ft ÷ 18 in, rounded down, + 1 = 14
- Stock bars: cut list into 20 ft bars = 28 × 1.1 → 31
- Weight: 546 ft × 0.668 lb/ft = 365 lb
Stock bars to buy: 31 × 20 ft #4 (1/2 in); Total length: 546 ft; Weight: 365 lb; Ties: 196; Estimated cost: $279.00. Open this example in the calculator.
Rebar sizes and weights
| Bar | Diameter | Weight per foot | Per 20 ft bar |
|---|---|---|---|
| #3 (3/8 in) | 0.375 in | 0.376 lb/ft | 7.5 lb |
| #4 (1/2 in) | 0.5 in | 0.668 lb/ft | 13.4 lb |
| #5 (5/8 in) | 0.625 in | 1.043 lb/ft | 20.9 lb |
| #6 (3/4 in) | 0.75 in | 1.502 lb/ft | 30 lb |
US bar numbers are the diameter in eighths of an inch: #4 is 4/8 = 1/2 in. Metric bars are named by their diameter in millimeters.
Placing slab rebar
- Support the bars on chairs or concrete blocks so they sit at the specified depth, usually near the middle of a slab on grade, not on the ground.
- Tie intersections with tie wire. Tying every other intersection is common in slabs; the calculator lets you choose.
- Stagger lap splices so they do not all line up across the slab.
- Keep the cover at the edges and from the ground; bars too close to the surface rust and spall the concrete.
Estimate the concrete with the concrete slab calculator.
Assumptions and limitations
- Bars run straight inside the edge cover in a rectangular grid. Hooks, bends, dowels, chairs and extra bars at openings or edges are not included.
- Lap splices are added only where a bar run is longer than a stock bar.
- Weights are nominal; real bars vary slightly.
Rebar questions
How much rebar do I need for a 20 × 20 slab?
At 18 in spacing both ways with 3 in cover, 14 bars each way, each 19.5 ft long: 28 bars and 546 ft in total, about 365 lb of #4 bar, or 31 stock bars of 20 ft with a 10% allowance.
How many ties do I need for rebar?
One per intersection if every crossing is tied: bars one way × bars the other way. Tying every other intersection halves that.
How much does #4 rebar weigh?
0.668 lb per foot, so a 20 ft bar weighs about 13.4 lb.
Formulas, unit conversions and default values are explained on our methodology page. Results are estimates; check manufacturer data and local codes before you buy or build.