Rebar Calculator
Lay out the reinforcing steel for a rectangular slab. Enter the slab dimensions and your on-center bar spacing to get how many bars run each direction, the total linear feet including a lap-splice allowance, and how many 10 or 20 ft stock bars to put on the order.
Your project
Estimate
370.2lin ft
Rebar needed · Includes 10% for lap splices
- Bars running lengthwise
- 9bars
- Bars running widthwise
- 14bars
- Stock bars to buy
- 19bars
- Estimated rebar cost
- $351.64
Estimates are guidance only and exclude tax, labor, and site conditions unless stated. Confirm with a local pro.
How it's calculated
No black boxes — here is the exact math behind your estimate, so you can check it or adapt it for your own quote.
- 1
Bars lengthwise = ⌊ width × 12 ÷ spacing ⌋ + 1, each (length − 0.5 ft) long - 2
Bars widthwise = ⌊ length × 12 ÷ spacing ⌋ + 1, each (width − 0.5 ft) long - 3
The 0.5 ft is 3 inches of concrete cover held back at each end of every bar - 4
Total linear feet = (both bar runs added) × 1.10 for lap splices - 5
Stock bars = ⌈ total linear feet ÷ stock bar length ⌉ - 6
Cost = total linear feet × price per foot
Worked example
A 20 ft × 12 ft garage slab reinforced with a grid at 18 inches on center, bought in 20 ft bars.
Inputs
- Slab length
- 20 ft
- Slab width
- 12 ft
- Bar spacing (O.C.)
- 18
- Stock bar length
- 20
- Rebar price
- 0.95 $/ft
Result
- Rebar needed
- 370.2 lin ft
- Bars running lengthwise
- 9 bars
- Bars running widthwise
- 14 bars
- Stock bars to buy
- 19 bars
- Estimated rebar cost
- $351.64
Tools & materials for this project
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- Rebar tie wire and twister Fast, consistent ties at every grid intersection and lap.
- Rebar chairs / slab bolsters Keep the grid at mid-slab height instead of on the subgrade.
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Frequently asked questions
What size rebar and spacing does a residential slab need?
#4 bar — ½ inch — at 18 inches on center each way is the workhorse grid for patios, shed pads, and garage floors. Light-duty patios sometimes drop to #3 at 16–18 inches; driveways that see trucks, or slabs on soft soil, move to #5 at 12 inches. In a 4-inch slab the steel belongs in the middle to upper third, which means sitting on 1.5–2 inch chairs, not pulled up by hand after the pour. Spacing and bar size are engineering decisions: your local code, and an engineer where one is required, govern the final call.
How much do I lap rebar where two bars meet?
The common structural rule is about 40 bar diameters for a tension lap, which is roughly 20 inches for #4 bar and 25 inches for #5. Many residential specs simply state a 12-inch minimum lap, but 40 diameters is the safer default and what most inspectors expect on a slab. Tie every lap in at least two places with wire. This calculator adds a flat 10% to total footage to cover laps — on a slab built from 20 ft bars with one 20-inch lap per run, actual lap waste lands near 8%, so 10% leaves a small cushion.
Why do the bars stop 3 inches short of the slab edge?
Steel needs concrete around it or it rusts, and rust expands and cracks the slab from the inside. The code cover requirement is 3 inches where concrete is cast directly against earth, and 1.5–2 inches for formed faces exposed to weather. Holding every bar back 3 inches from each edge — 6 inches off each slab dimension — satisfies the side cover with room for the forms to be slightly out of square. That is why a 20 ft slab uses 19.5 ft bars, and it is the single most common thing DIY layouts get wrong.
Rebar, welded wire mesh, or fiber mesh?
Rebar on chairs is the only one of the three that reliably stays where you put it and carries real load transfer across a crack, which is why driveways and garage floors use it. Welded wire mesh is cheaper per square foot but sheets and especially rolls tend to end up trampled onto the subgrade, where they do nothing. Fiber mesh — polypropylene fibers batched into the mix at roughly 1.5 lb per cubic yard — controls plastic shrinkage cracking in the first hours of curing but is not structural and does not replace rebar.
How many stock bars should I actually buy?
The stock-bar figure divides total footage by your bar length, which assumes you can use every offcut. In practice you cannot: a 12 ft-wide slab cut from 20 ft bars leaves an 8.5 ft drop that may or may not fit anywhere else. Add 5–10% more bars when your slab dimensions do not divide cleanly into the stock length, or buy 10 ft bars for the short direction. Twenty-foot bars mean fewer splices and less tying, but they need a trailer or a roof rack rather than a car.