A slab grid is two layers of parallel bars at right angles, kept back from the edges and lifted off the ground on chairs. When a run is longer than a bar, two bars overlap by a lap splice and are tied together, which the calculator counts in.
How to use this calculator
- Enter the slab's length and width.
- Enter the spacing your plans or your local code call for, center to center.
- Pick the bar size and the length of bars you can buy and carry.
- Under More options, set how far the bars stop from the edges and the grade stamped on the bars, which sets the lap.
Worked examples
A garage slab
Slab: 24 × 24 ft · Bar spacing, on center: 18 in · Bar size: #4 (1/2 in)
- Bars to buy
- 44 bars, 20 ft
- Crossings to tie
- 256
- Bar supports (chairs)
- 36
- Bars the long way
- 16 runs
- Total length with laps
- 832 linear ft
- Weight
- 556 lb
- Lap splice
- 30 in
A driveway
Slab: 40 × 12 ft · Bar spacing, on center: 12 in · Bar size: #4 (1/2 in)
- Bars to buy
- 67 bars, 20 ft
- Crossings to tie
- 480
- Bar supports (chairs)
- 30
- Bars the long way
- 12 runs
- Bars the short way
- 40 runs
- Total length with laps
- 994 linear ft
- Weight
- 664 lb
- Lap splice
- 30 in
A patio with lighter bars
Slab: 16 × 12 ft · Bar spacing, on center: 18 in · Bar size: #3 (3/8 in)
- Bars to buy
- 19 bars, 20 ft
- Crossings to tie
- 88
- Bar supports (chairs)
- 12
- Bars the long way
- 8 runs
- Bars the short way
- 11 runs
- Total length with laps
- 251 linear ft
- Weight
- 94 lb
- Lap splice
- 22.5 in
A shed floor with short bars
Slab: 12 × 10 ft · Bar spacing, on center: 16 in · Bar size: #4 (1/2 in) · Bar length you buy: 10 ft
- Bars to buy
- 21 bars, 10 ft
- Crossings to tie
- 72
- Bar supports (chairs)
- 9
- Bars the long way
- 8 runs
- Bars the short way
- 9 runs
- Total length with laps
- 198 linear ft
- Weight
- 132 lb
- Lap splice
- 30 in
How the grid is counted
The bars stop short of every edge so concrete covers their ends; the IRC asks for 3 inches of cover where concrete is cast against the earth. Across the width, a bar goes at each spacing, plus one; the same along the length. Each run that is longer than a bar is lapped.
Where two bars overlap, the IRC's lap table asks for 20 inches on grade 40 bars of the #4 (1/2 in) size and 30 inches on grade 60; for the #5 (5/8 in) size, 25 inches and 38 inches. The calculator uses the same number of bar diameters for the other sizes.
Bar sizes and weights
- #3 (3/8 in): 0.375 inch across, 0.376 lb per foot.
- #4 (1/2 in): 0.5 inch across, 0.668 lb per foot.
- #5 (5/8 in): 0.625 inch across, 1.043 lb per foot.
- #6 (3/4 in): 0.75 inch across, 1.502 lb per foot.
Spacing and placing
Your plans or local code set the spacing. City driveway specs show the range: one Texas city asks for #4 (1/2 in) bars 12 inches apart both ways, another 18 inches apart each way for a home driveway.
The IRC puts slab steel in the middle to upper third of the slab, so it has to sit on chairs, not on the ground. Space the chairs no more than 4 ft apart each way, and tie the bars where they cross, at every crossing or every other one.
Common questions
How do I calculate rebar for a slab?
Count the bars each way: the width less the edge cover, divided by the spacing, plus one, and the same for the length. Multiply by the bar lengths, add the laps, and divide by the length of bar you buy. The calculator does it all.
What spacing should rebar be in a slab?
Whatever your plans or local code require. Driveway specs from Texas cities ask for #4 (1/2 in) bars 12 inches to 18 inches apart.
How much does rebar weigh?
A #4 (1/2 in) bar weighs 0.668 lb per foot and a #5 (5/8 in) bar 1.043 lb per foot. The calculator totals the weight of the whole grid.
How much should rebar overlap?
By the lap length in your code: the IRC's table gives 30 inches for grade 60 bars of the #4 (1/2 in) size. Tie the overlapping bars together.
How far should rebar be from the edge of a slab?
Far enough for concrete to cover the ends: the IRC asks for 3 inches of cover where concrete is cast against the earth.