Rebar Calculator
With slab length 20 ft, slab width 12 ft, grid spacing 12 inch, clear cover at the edges 3 inch, rebar comes to 464 ft — total bar length. It is reached in 6 steps, the last of which is 12 * 19.5 + 20 * 11.5, and each one is printed on the page with its numbers filled in. The formula is the one published by Concrete Reinforcing Steel Institute, not an approximation fitted to it.
Bars, total length and weight of reinforcement for a slab at a given grid spacing, with the lap length the code requires.
Formula and sources checked · How we check
Slab length 20 ft, Slab width 12 ft, Grid spacing 12 inch, Bar size #4 — 1/2 in, 0.668 lb/ft
464 ft
Total bar length for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.
It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.
- Bars running lengthwise
floor(11.5 / (12 / 12)) + 112 bars- Bars running crosswise
floor(19.5 / (12 / 12)) + 120 bars- Total bar length
12 * 19.5 + 20 * 11.5464 ft- Lap length, 40 diameters
(40 * 0.5) / 121.667 ft- Weight of steel
464 * 0.668309.952 lb- Intersections to tie
12 * 20240 ties
Several at once
A job is rarely one of anything. Work out the first, add it, change the figures and add the next — the list keeps what each line was, and totals them.
Ask about this in the chatRebar Grid Spacing Chart
Worked example
A 20 × 12 ft slab on a 12 in grid with 3 in cover takes 12 bars one way and 20 the other: 464 linear feet of #4, weighing 310 lb, with 240 intersections to tie. Laps add to that — 40 bar diameters is 20 in for a #4, so every splice costs another foot and a half.
How to work it out yourself
- 1.Subtract the cover from each slab dimension — steel does not run to the edge.
- 2.Divide each reduced dimension by the spacing in feet, take the whole number and add one. That is the bar count in each direction.
- 3.Multiply each count by the length its bars run, and add the two for total linear feet.
- 4.Multiply by the weight per foot for the tonnage, and add 40 bar diameters for every splice.
Bars and weight for a 20 × 12 slab by spacing
| Grid spacing (inch) | Total bar length | Bars running lengthwise | Weight of steel |
|---|---|---|---|
| 6 | 928 ft | 24 bars | 619.9 lb |
| 8 | 696 ft | 18 bars | 464.93 lb |
| 12 | 464 ft | 12 bars | 309.95 lb |
| 16 | 348 ft | 9 bars | 232.46 lb |
| 18 | 317 ft | 8 bars | 211.76 lb |
| 24 | 232 ft | 6 bars | 154.98 lb |
Spacing in inches, #4 bar, 3 in cover. Halving the spacing roughly doubles the steel.
What does it cost?
Typical price is $0.6–$1.6 per ft. Straight stock in 20 ft lengths. Cut and bent to a schedule costs more, and small quantities from a builders merchant cost far more per foot than a yard.
| Item | Typical price |
|---|---|
| #3 rebar, 20 ft stick | $8–$16 per stick |
| #4 rebar, 20 ft stick | $12–$24 per stick |
| #5 rebar, 20 ft stick | $18–$36 per stick |
| Tie wire and chairs | $20–$60 per slab |
Price ranges as of 2026. Materials pricing moves — treat these as a sanity check on a quote, not as a quote.
The formula
- Bars running lengthwise
floor(11.5 / (12 / 12)) + 1 - Bars running crosswise
floor(19.5 / (12 / 12)) + 1 - Total bar length
12 * 19.5 + 20 * 11.5 - Lap length, 40 diameters
(40 * 0.5) / 12 - Weight of steel
464 * 0.668 - Intersections to tie
12 * 20
Source: Concrete Reinforcing Steel Institute, American Concrete Institute — ACI 318 building code
Questions people actually ask
- How far apart should rebar be in a slab?
- Eighteen inches for a patio, twelve for a driveway, six to eight for a footing or anything carrying a load. Spacing is a design decision, not a rule of thumb — an engineered slab has a schedule, and this calculator counts what the schedule says rather than deciding it.
- How much does rebar weigh?
- By bar size: #3 is 0.376 lb per foot, #4 is 0.668, #5 is 1.043, #6 is 1.502. The number is the diameter in eighths of an inch, and the weight goes with the square of it — which is why stepping from #4 to #5 adds 56% to the tonnage.
- How much should bars overlap?
- Forty bar diameters is the ordinary lap for slab-on-grade — 20 in for a #4, 25 for a #5. ACI 318 sets it properly from the concrete strength, the bar spacing and the cover, so a designed structure takes its splice length from the drawings, not from this rule.
- Where in the slab should the steel sit?
- In the middle third for a slab in bending, held there on chairs. Steel lying on the ground does nothing at all: the whole point is that it takes tension on the side of the slab that stretches, and the bottom of a slab on grade is not always that side.
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