Rebar Calculator for Concrete Slabs
Enter the slab and the rebar grid. The calculator counts bars in both directions, adds laps where bars exceed stock length, and shows how many 20 ft bars (6 m in metric) to buy. After switching units, re-enter your measurements.
Updated
Report a problem
Formula
- Clear size:
slab size − 2 × coverin each direction - Bars in a direction:
floor(clear span ÷ spacing) + 1 - Lap length:
lap factor × bar diameter(default 40 diameters) - A bar longer than the 20 ft stock length (6 m in metric) is built from several pieces, each joint adding one lap
- Stock bars:
total length × (1 + waste % ÷ 100) ÷ stock length, rounded up - Crossing points:
lengthwise bars × crosswise bars
Worked example
A 20 ft by 10 ft slab, 12 in grid both ways, 3 in cover, #4 bars, 10% waste. The clear size is 19.5 × 9.5 ft. There are floor(9.5 ÷ 1) + 1 = 10 lengthwise bars of 19.5 ft and floor(19.5 ÷ 1) + 1 = 20 crosswise bars of 9.5 ft. Total = 195 + 190 = 385 ft. With 10% waste, 423.5 ÷ 20 = 21.2, so 22 bars. There are 10 × 20 = 200 crossings, and the lap is 40 × 0.5 in = 20 in (508 mm).
Tie wire
Tie wire is not an output, but the crossing count gives it. Allow about 12 in (30 cm) of wire per tie. Tying every crossing in the example uses about 200 ft, and tying alternate crossings about half.
Assumptions
- Spacing is measured between bar centres, and one bar sits at each clear edge.
- Bar diameters follow the US #n = n/8 in designation.
- Lap length is a rule of thumb. The design code and engineer decide the real value.
- Chairs, spacers, hooks and bends are not counted.
Frequently asked questions
How much rebar do I need for a slab?
Subtract the edge cover from each side, count bars as floor(span / spacing) + 1 in each direction, and add up their lengths. A 20 ft by 10 ft slab with a 12 in grid and 3 in cover has 10 bars of 19.5 ft and 20 bars of 9.5 ft, which is 385 ft. With 10% waste that is 423.5 ft, or 22 bars of 20 ft.
How long should the lap be?
A common rule of thumb is 40 bar diameters. For a #4 bar (0.5 in) that is 20 in, or 508 mm. Required laps depend on the design code, steel grade and concrete, so use the engineer's drawings when there are any.
How much tie wire do I need?
Tie every crossing point or every second one, using about 12 in (30 cm) of wire per tie. The output shows the crossing points, so the 20 by 10 ft example has 200 crossings. Tying all of them takes roughly 200 ft of tie wire, and tying alternate ones about half that.
Is this a structural design?
No. It is a quantity estimate from the layout you enter. Bar size, spacing and cover must come from a structural design or the code that applies to your project. Chairs or spacers that hold the grid off the ground are not counted.