How Much Rebar for a 20x20 Slab?
Rebar to buy
28 sticks
400 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 length20 ft
- Slab width20 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 buy28 sticks
| Slab area | 400 sq ft |
|---|---|
| Bars running the length | 1419.5 ft each, spaced across the width |
| Bars running the width | 1419.5 ft each, spaced along the length |
| Linear feet, no laps | 546 ft |
| Lap allowance | 0 ft0 splices at 1.67 ft each |
| Total linear feet | 546 ft |
| Sticks at 20 ft | 28560 ft bought, one run per stick |
| Weight | 365 lb0.18 tons |
| Intersections to tie | 196 |
| Chairs | 100one per 4 sq ft |
The material list this produces
| #4 rebar, 20 ft sticks | 28 |
|---|---|
| Tie wire, lb | 2 |
| Chairs | 100 |
| Poly vapor barrier, sq ft | 440 |
Slab width compared
Length held at 20 ft, width swept. The middle column is the lap allowance and it is the whole point of this table: zero on every row until the slab gets wider than a stick, and from there it never goes away again.
| Slab width | Rebar to buy | Lap allowance | Chairs |
|---|---|---|---|
| 20 by 8 ft | 13 sticks | 0 ft | 40 |
| 20 by 10 ft | 14 sticks | 0 ft | 50 |
| 20 by 12 ft | 22 sticks | 0 ft | 60 |
| 20 by 16 ft | 25 sticks | 0 ft | 80 |
| 20 by 20 ft | 28 sticks | 0 ft | 100 |
| 20 by 24 ft | 33 sticks | 5 ft | 120 |
| 20 by 30 ft | 42 sticks | 13.3 ft | 150 |
What else matters here
Four hundred square feet, 546 ft of steel, 28 sticks and not one splice. This is the largest square slab that gets away with that, and the reason is the cover: 3 in off each end brings the bars out at 19.5 ft, which fits a 20 ft stick with six inches to spare.
One foot wider and it stops working. The table below holds the length at 20 ft and sweeps the width, and the lap column reads zero all the way down until the slab passes 20 ft. At 20 by 24 it is 5 ft of lap and at 20 by 30 it is 13.3, because the cross bars have grown longer than a stick and have to be chained.
That is worth knowing while the dimension is still soft, which on a garage floor or a shop pad it usually is. A splice is not a fault, it is how every large slab gets built, but it is steel you buy and place that does no work except hold two bars together, and it is time at every joint. This slab has 196 intersections to tie and 100 chairs under the mat, and 365 lb of steel is one pickup bed.
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 do I need for a 20x20 slab?
14 bars each way at 18 in spacing with #4 bar, 546 ft of steel, 28 sticks of 20 ft bar and 365 lb. Tighten to 12 in spacing and it becomes 780 ft, 40 sticks and 521 lb.
Does a 20x20 slab need lap splices?
No. The bars are 19.5 ft after cover and a 20 ft stick carries each one whole. Go wider than 20 ft and the cross bars outrun the stick, which is where the laps begin.
How many chairs and ties for a 20x20 slab?
100 chairs at one per four square feet, 196 intersections to tie and about 2 lb of tie wire.
The rebar calculator takes any dimensions you like and hands back the same list.