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This rebar calculator works out the reinforcing steel for a concrete slab or patio. Enter the slab size, grid spacing and bar size, and it counts the bars in each direction, adds lap splices where bars must be joined, and gives the total length, weight and number of ties.
How to Use the Rebar Calculator
- Enter the length and width of the slab.
- Enter the grid spacing (12 in or 16 in on center is common for slabs) and the edge clearance.
- Choose the bar size and the length of the bars you can buy (often 20 ft or 6 m).
- Enter the lap splice length in bar diameters.
How the Calculation Works
Bars in each direction = (Slab width − 2 × clearance) ÷ Spacing + 1
Lap length = Lap factor × Bar diameter (40 × ½ in = 20 in for #4)
Each bar runs the full length of the slab minus the clearance at each end. When a run is longer than one bar, the bars overlap by the lap length and are tied together.
Rebar Sizes and Weights
| Bar size | Diameter | Weight per foot | Weight per meter |
|---|---|---|---|
| #3 | 0.375 in (9.5 mm) | 0.376 lb | 0.560 kg |
| #4 | 0.500 in (12.7 mm) | 0.668 lb | 0.994 kg |
| #5 | 0.625 in (15.9 mm) | 1.043 lb | 1.552 kg |
| #6 | 0.750 in (19.1 mm) | 1.502 lb | 2.235 kg |
Example
A 20 × 20 ft slab with #4 bars at 12 in on center and 3 in clearance needs 20 bars of 19.5 ft in each direction: 780 ft of rebar weighing about 521 lb, cut from 40 bars of 20 ft, with 400 intersections to tie.
Rebar Placement Tips
- Support the grid on chairs or concrete bricks so it sits in the middle or upper third of the slab, not on the ground.
- Keep at least 3 in of concrete between bars and soil-facing edges.
- Stagger lap splices so they do not line up across the slab.
- Driveways and slabs carrying vehicles should follow a structural design or local code.
Calculate the concrete for the slab with our concrete calculator.
Sources
Frequently Asked Questions
How much rebar do I need for a 20×20 slab?
With #4 bars at 12 in spacing and 3 in clearance, about 780 linear feet (40 bars of 20 ft), weighing roughly 520 lb.
What size rebar is used in a slab?
Residential slabs and patios commonly use #3 or #4 bars. Driveways and structural slabs often use #4 or #5, as specified by the design.
How far apart should rebar be in a slab?
Slab grids are typically spaced 12 to 18 inches on center in both directions.
How much should rebar overlap?
A common rule of thumb is 40 bar diameters, which is 20 in for #4 rebar, but structural work should follow the engineer’s drawings.
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