Rebar calculator
How many bars each way, how many linear feet, and how many sticks to buy for a slab grid.
Calculator
Detailed breakdown
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How to use this calculator
Measure
Measure the slab in feet and pick your grid spacing in inches. Eighteen inches is common for residential slabs, twelve for driveways.
Calculate
Once you hit calculate, results update as you type. Change any number to compare options instantly.
Order
Print the estimate and take it to your supplier. Quantities are rounded up so you don't come up short.
Counting a rebar grid without missing the last bar
A rebar grid is two sets of bars at right angles. The trap is the count: bars running the length are spaced across the width, and you need one more bar than the number of gaps. A 10 foot width at 12 inch spacing has ten gaps and eleven bars — miss that and you'll be one bar short on every run.
Run the numbers on a 20 by 10 foot slab at 12 inches on center: eleven bars running lengthwise at 20 feet each is 220 linear feet, and twenty-one bars running widthwise at 10 feet each is 210 linear feet. That totals 430 linear feet. Note that the stick count is not simply that divided by 20: bar is cut, not poured, so a 12 foot bar uses a whole 20 foot stick and the remaining 8 feet is usually scrap. Here the numbers happen to work out cleanly to 22 sticks, with 231 intersections to tie — but on a 12 by 12 slab the same arithmetic needs 26 sticks, not 16.
Lap splices are the one thing this calculator deliberately leaves out. Wherever two bars are joined end to end, code generally wants them overlapped by about 40 bar diameters — 20 inches for #4 bar, 30 inches for #6. On a slab wider than your stick length those laps add up fast, so count your splices and add the overlap before ordering.
Not every slab needs bar. A 4 inch patio, shed floor or walkway on stable ground is commonly poured with welded wire mesh or fiber reinforcement instead, both of which control shrinkage cracking without the tying. Rebar earns its place where the slab carries real load or spans soft ground: driveways, garage floors, footings and anything a vehicle sits on. If you are pouring a footing rather than a slab, the bar runs the length in two or three continuous lines rather than in a grid, so this calculator is the wrong shape for that job.
Placement matters as much as quantity. Rebar does nothing sitting on the ground: it has to sit in the middle third of the slab thickness, held up on chairs or dobies. Pulling the bar up with a hook as the concrete goes in is the traditional method, and it's unreliable — the bar ends up wherever it happens to land.
Frequently asked questions
How much rebar for a 20 by 10 foot slab?
At 12 inch spacing both ways, about 430 linear feet — eleven 20 foot bars one direction and twenty-one 10 foot bars the other. That's 22 sticks of standard 20 foot rebar.
What spacing should rebar be in a slab?
Twelve to eighteen inches on center is typical for residential slabs. Driveways and anything carrying vehicle loads are usually at 12; patios and sheds are fine at 18.
What size rebar for a concrete slab?
#3 (3/8 inch) or #4 (1/2 inch) covers most residential slabs, with #4 the common choice for driveways. Structural work is sized by an engineer, not by a rule of thumb.
How much overlap do rebar splices need?
Around 40 bar diameters where bars are lapped — roughly 20 inches for #4 and 30 inches for #6. This calculator counts bar only and doesn't include splice overlap.
How high off the ground should rebar sit?
In the middle third of the slab thickness, supported on chairs or dobies before the pour. Rebar lying on the subgrade adds essentially nothing.
Do I need rebar in a 4 inch patio slab?
Usually not. A patio or walkway on stable ground is normally poured with welded wire mesh or fiber reinforcement, which handles shrinkage cracking. Save the bar for driveways, garage floors and footings.
Sources
Formulas and material assumptions on this page follow published industry guidance: