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MaterialCalc — Rebar Grid & Spacing Calculator

Estimate prepared 09/01/2026. Estimates only; verify against your plans before ordering.

Rebar Grid & Spacing Calculator

On center, both directions. 18 in is typical for a residential slab.

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Covers cut-offs and the odd bent bar.

Result

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A rebar mat is two sets of bars at right angles, and the bar count follows from spacing rather than area — halving the spacing roughly doubles the steel. This gives you the bar count in each direction, total linear feet, how many 20 ft sticks to buy, and what the change from an 18 in to a 12 in grid actually costs.

Assumptions used

These are the numbers behind the result. Where one of them is soft, the cost range widens rather than pretending to a precision the data does not have.

Concrete cover, earth-cast
3 in

ACI 318 cover requirements — Published product data, checked 09/01/2026

Lap splice length
40 × bar diameter

A common rule of thumb. Engineered work uses the value on the drawings, not this one.

ACI 318 development length, typical case — Rough figure — widens the estimate range, checked 09/01/2026

Prices verified 09/01/2026 (US national average, big-box retail shelf price and regional ready-mix quotes). Regional adjustment: Blended from RSMeans city cost index bands and BLS construction wage differentials.

The formula

Bars in one direction = (opposite dimension ÷ spacing) rounded down, plus one for the closing bar. Total footage = bars across × slab length + bars along × slab width. Divide by the 20 ft stick length and round up. Weight uses the ASTM nominal figures: 0.376 lb/ft for #3, 0.668 for #4, 1.043 for #5.

Worked examples

24x24 garage slab, #4 at 18 in

Slab length: 24 Slab width: 24 Grid spacing: 18 Bar size: 4 Waste: 5

Thirty-four bars and roughly 816 linear feet — about 43 sticks. This is the standard detail under a two-car garage.

Same slab at 12 in spacing

Slab length: 24 Slab width: 24 Grid spacing: 12 Bar size: 4 Waste: 5

Fifty bars and about 1,200 feet — roughly 50% more steel and 50% more tying for the same slab. Use the comparison table to see the cost of that decision before you make it.

16x20 patio, #3 at 16 in

Slab length: 20 Slab width: 16 Grid spacing: 16 Bar size: 3 Waste: 5

Lighter bar on a wider grid for a slab carrying no vehicles. Under a patio this is often more steel than strictly needed — mesh would do.

Choosing materials

  • #4 bar on an 18 in grid is the workhorse residential slab detail. #3 at 12 in gives similar steel area for less money per stick but far more tying labor.
  • Rebar must sit in the middle third of the slab, which means chairs or bolsters. Pulling bars up with a hook as the concrete goes in is the traditional method and it is unreliable — half the mat ends up on the ground.
  • Lap splices run about 40 bar diameters: 15 in for #3, 20 in for #4, 25 in for #5. Order for the laps, not just the bar runs, or you finish one stick short at the end of every line.
  • Keep 3 in of cover from earth and 1.5 in from formed faces. Bar closer to the surface than that rusts, and rusting steel expands and spalls the concrete off.
  • Epoxy-coated bar is worth it near salt — coastal work and anywhere with de-icing chemicals. Everywhere else it is a cost with no return.

Common questions

What spacing should rebar be in a slab?

Eighteen inches on center each way with #4 bar is the common residential slab detail. Go to 12 in for heavy vehicle loads, expansive soils, or slabs thicker than 6 in. An engineer sets it properly against your soil and loading.

How much rebar for a 24x24 slab?

At 18 in on center with #4 bar, about 34 bars totaling roughly 816 linear feet — 43 sticks of 20 ft. At 12 in spacing it rises to about 50 bars and 1,200 feet.

Rebar or wire mesh?

Mesh is cheaper and adequate for patios, shed floors and sidewalks on a good base. Rebar is the right call under vehicles, on poor soil, and in slabs over 5 in thick. Rebar also stays where you put it, which mesh notoriously does not.

How far apart should rebar laps be?

Lap length is typically 40 bar diameters — 20 in for #4 bar. Stagger the splices so laps in adjacent bars do not line up, and tie each lap in at least two places.

How much does rebar weigh?

Per linear foot: #3 is 0.376 lb, #4 is 0.668 lb, and #5 is 1.043 lb. A 20 ft stick of #4 weighs about 13.4 lb, which matters when you are working out how much your truck can carry.

Do I need rebar chairs?

Yes. Rebar only reinforces concrete if it sits within the middle third of the slab thickness. Chairs or bolsters at roughly 4 ft intervals hold it there. Steel resting on the subgrade contributes nothing structurally and rusts.

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