How Much Rebar for a 24x24 Garage Slab?
Rebar to buy
40 sticks
576 sq ft, #4 bar at 18 in both ways, 3 in cover, 20 ft sticks, 40 diameter lap splices.
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How that number is worked out
These are the figures this page was built with and every line the calculator produces from them. Change anything above and all of it moves.
- Slab length24 ft
- Slab width24 ft
- Bar spacing, both ways18 in
- Bar size#4, 1/2 in
- Cover from the edge3 in
- Bar stock length20 ft
- Lap splice40 bar diameters
Rebar to buy40 sticks
| Slab area | 576 sq ft |
|---|---|
| Bars running the length | 1623.5 ft each, spaced across the width |
| Bars running the width | 1623.5 ft each, spaced along the length |
| Linear feet, no laps | 752 ft |
| Lap allowance | 13.3 ft8 splices at 1.67 ft each |
| Total linear feet | 765.3 ft |
| Sticks at 20 ft | 40800 ft bought, 8 splices, offcuts carried into the next run |
| Weight | 511 lb0.26 tons |
| Intersections to tie | 256 |
| Chairs | 144one per 4 sq ft |
The material list this produces
| #4 rebar, 20 ft sticks | 40 |
|---|---|
| Tie wire, lb | 2 |
| Chairs | 144 |
| Poly vapor barrier, sq ft | 634 |
Bar spacing compared
Spacing swept at 24 by 24 ft. The middle column is the lap allowance and it barely moves, because the splices come from the slab being wider than a stick and not from how close together the bars sit. The right hand column is intersections to tie, and that one quadruples across the table.
| Bar spacing | Rebar to buy | Lap allowance | Intersections to tie |
|---|---|---|---|
| 12 in both ways | 58 sticks | 16.7 ft | 576 |
| 16 in both ways | 44 sticks | 13.3 ft | 324 |
| 18 in both ways | 40 sticks | 13.3 ft | 256 |
| 24 in both ways | 30 sticks | 10 ft | 144 |
What else matters here
A two car garage pad, 576 sq ft, and the first slab in this set where the steel has to be spliced. The bars come out at 23.5 ft after cover and a stock stick is 20, so the mat is built from chained sticks with a lap at every joint: 13.3 ft of lap across 8 splices, sitting on top of 752 ft of actual bar.
That lap is not waste in the way an offcut is waste. A 40 diameter lap on #4 bar is 1.67 ft, and it is there so the two bars behave as one piece through the joint. Shorten it and the mat has a hinge in it exactly where you least want one. The calculator carries the offcut from one run into the start of the next, which is why thirty two runs produce 8 splices rather than one each.
Spacing is where the money is on a pad this size. The table below runs 12, 16, 18 and 24 in. At 12 in this slab is 58 sticks and 576 intersections to tie; at 24 in it is 30 sticks and 144. That is close to double the steel and four times the tying labor, and for a floor carrying a car rather than a loaded truck the tighter mat is usually not what is called for. The drawing decides that, not this page.
What this calculator assumes
- Bars are figured as a grid running both ways at the same spacing, which is how a residential slab, a driveway and a footing mat are normally drawn.
- Weights are the published ones: #3 is 0.376 lb per foot, #4 is 0.668, #5 is 1.043, #6 is 1.502, #7 is 2.044 and #8 is 2.670.
- The stick count is nested, not divided. When a run fits inside a stock bar, the count says how many runs are cut from one stick and never splices a bar that did not need splicing. When a run is longer than a stock bar it is built from a chain of sticks with a lap at every joint, and the offcut from one run starts the next. That is why the stick line and the linear feet line never agree, and the gap between them is the offcut you pay for.
- A 40 diameter lap is the common residential rule of thumb. ACI 318 sets the real development length from the concrete strength, the bar position and the cover, and an engineered drawing overrides anything on this page.
- Chairs are one per four square feet, which keeps the mat off the ground on an ordinary pour. Wet setting bar into poured concrete is a different method and this list does not price it.
Questions people ask
How much rebar for a 24x24 garage slab?
16 bars each way at 18 in with #4 bar, 752 ft of bar plus 13.3 ft of lap, 40 sticks of 20 ft steel and 511 lb.
Why does a 24x24 slab need splices when a 20x20 does not?
Because the bars come out at 23.5 ft after 3 in of cover at each end and a stock stick is 20 ft. On a 20 by 20 the bars are 19.5 ft and each one fits a stick whole.
How long should a rebar lap splice be?
Forty bar diameters is the common residential rule of thumb, which is 1.67 ft for #4. ACI 318 sets the real development length from the concrete strength, the bar position and the cover, so an engineered drawing always wins.
The rebar calculator takes any dimensions you like and hands back the same list.