Rebar Calculator: Sticks, Linear Feet and Lap Splices
Slab size, spacing, bar size and stock length, and it returns the bars each way, the linear feet, the lap allowance, the sticks to buy and the weight. The stick count is nested rather than divided, which is the difference between an order that arrives complete and one that is 20 percent short.
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Worked example
These are the figures the calculator opens with and the answer it gives. Change anything above and every number below moves with it.
- 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 |
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.
Spacing is the whole answer
A 20 by 20 slab with #4 bar at 18 inches each way is 14 bars each direction, 546 linear feet, 28 sticks and 365 pounds of steel. The same slab at 12 inches is 20 bars each way, 780 feet, 40 sticks and 521 pounds.
Open it to 24 inches and it halves again to 390 feet and 20 sticks. Spacing is close to the only variable that matters, because the bar length is fixed by the slab and the count scales straight off the spacing.
Which spacing is right is not a material question. A residential slab or driveway commonly runs #4 at 18 inches, but the engineer, the drawing or the local amendment decides it, and this calculator prices whatever you tell it rather than recommending a number it has no business recommending.
Sticks are nested, not divided
Divide linear feet by stock length and you will be short on almost every small slab. A 10 by 10 slab at 18 inch spacing is 133 linear feet, and 133 divided by 20 is 7 sticks. The honest answer is 8, because each 9.5 foot run is cut from a 20 foot bar and only two runs come out of one stick.
The calculator nests instead. When a run fits inside a stock bar it says how many runs come out of one stick and never splices a bar that did not need splicing. When a run is longer than a stock bar, it chains sticks with a lap at each joint and carries the offcut into the next run.
That is why the stick line and the linear feet line never agree, and the gap between them is the offcut. On the 20 by 20 at 18 inches it is 546 feet needed against 560 feet bought, which is honest waste rather than a rounding error.
Laps are where the length quietly disappears
Any run longer than your stock bar gets spliced, and a splice is not free. At 40 bar diameters, a #4 lap is 1.67 feet and a #5 lap is 2.08. Those add up: a 24 by 24 slab at 16 inch spacing is 846 feet of bar with no laps and 859 feet once the splices are counted.
Forty diameters is the common residential rule of thumb and nothing more than that. ACI 318 sets the real development length from the concrete strength, the bar position, the cover and the spacing, and it can land shorter or considerably longer. An engineered drawing overrides anything on this page.
The other way to make laps disappear is to buy longer stock. Sixty foot bar exists, it delivers on a flatbed rather than in a pickup, and on a big pour it removes both the splice length and the labor of tying it.
Chairs and ties are the cheap part that gets skipped
That 20 by 20 slab has 196 intersections to tie and wants about 100 chairs at one per four square feet. Together they are a rounding error on the concrete bill and they are the only thing keeping the mat where it belongs.
Rebar lying on the ground does nothing. It has to sit in the lower third of the slab to work in tension, and it gets there on chairs, not by being pulled up with a hook while the concrete goes in. Wet setting is a different method and this list does not price it.
Cover is the other half of it. Three inches from the edge is the default here because that is what concrete cast against ground wants, and steel that ends up too close to a face rusts, expands and spalls the slab off over it.
Questions people ask
How much rebar do I need for a 20 by 20 slab?
At 18 inch spacing both ways with #4 bar, 14 bars each direction, 546 linear feet, 28 sticks of 20 foot bar and 365 pounds of steel. Tighten to 12 inch spacing and it becomes 780 feet, 40 sticks and 521 pounds.
What spacing should rebar be in a driveway?
#4 at 18 inches each way is the common residential practice for a four inch driveway, but it is practice rather than a rule. The drawing, the engineer or your local amendment decides, and soil, thickness and load all move it.
How much does rebar weigh per foot?
#3 is 0.376 pounds a foot, #4 is 0.668, #5 is 1.043, #6 is 1.502, #7 is 2.044 and #8 is 2.670. A 20 foot stick of #4 is 13.4 pounds, and the 546 feet in a 20 by 20 slab is 365 pounds, a little under a fifth of a ton.
How long should a rebar lap splice be?
Forty bar diameters is the common residential rule of thumb, which is 1.67 feet for #4 and 2.08 for #5. ACI 318 sets the actual development length from the concrete strength, the bar position, the cover and the spacing, so an engineered drawing always wins.
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Common sizes, worked out
The same calculator with the numbers already filled in, one page per size, each ending in its own material list.