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Rebar Calculator

Rebar grid for a slab: bars each way, linear feet with laps, sticks to buy and weight.

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Rebar needed

Enter your numbers above.

Set the rebar on chairs, not on the dirt

Rebar only helps if it ends up inside the slab, usually near the middle or upper third. Bars laid on the ground and "pulled up" during the pour usually stay at the bottom. Plastic chairs or small concrete dobies are cheap and hold the grid at the right height.

How to figure rebar for a slab

Rebar in a slab is laid as a grid. The bars stop short of the edges by the edge cover, usually about 3 in., so each bar runs the slab dimension minus twice the cover. The number of bars in each direction is (available width in inches ÷ spacing) + 1, rounded down, so there is a bar at each edge.

Where a run is longer than a stock bar, bars overlap in a lap splice. A common rule of thumb is 40 bar diameters, which is 15 in. for #3, 20 in. for #4 and 25 in. for #5; your engineer or local code may specify something different. Total linear feet divided by the stock length gives the number of sticks, before any savings from reusing offcuts.

Weight uses the Concrete Reinforcing Steel Institute's standard unit weights: 0.376 lb/ft for #3, 0.668 lb/ft for #4 and 1.043 lb/ft for #5. Weight matters when rebar is priced by the pound or when you are hauling it in a pickup.

Where these numbers come from

The formulas and default values in this calculator are based on:

  • Concrete Reinforcing Steel Institute (CRSI) standard bar sizes and unit weights
  • ACI 318 concrete cover guidance
  • 40-bar-diameter lap length as a common field rule of thumb

These are planning estimates. Check product labels, your local building code, and a licensed contractor before you buy materials or build.

Frequently asked questions

Do I need rebar in a 4-inch patio slab?

Many residential patios and walks use welded wire mesh or fiber-reinforced concrete instead, and some use nothing but control joints. Rebar helps most on driveways, garage slabs, poor soil, and any slab carrying heavy loads. Local practice and your building department are the best guide.

What spacing should I use?

Twelve to 18 in. on center each way is typical for residential slabs with #3 or #4 bar. Closer spacing adds strength and crack control; wider spacing saves steel. Structural slabs should follow an engineer's drawing.

Why don't the bars go all the way to the edge?

Concrete cover protects the steel from moisture and rust. ACI 318 calls for about 3 in. of cover where concrete is cast against earth, so bars stop short of the forms and sit up off the ground.

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