Rebar Material Calculator

Calculate how much rebar you need to reinforce a concrete slab by entering the dimensions below, along with the desired spacing and the spacing between the edge and the rebar grid.


Material Estimate
The number of bars needed and the length of each.
Equal Spacing
The spacing between each bar along the grid length or width.
Grid Size
The size of the rebar grid, accounting for the edge clearance.
Learn how we calculated this below

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How to Estimate How Much Rebar You Need

When installing a concrete slab or structure it is a good idea to add rebar, or reinforcing bar, to reinforce it and prevent it from cracking down the road. Using rebar as reinforcement adds considerable strength to concrete and reduces the overall slab thickness needed.[1]

Calculating the amount of bar needed can be done in a few simple steps.

Step One: Figure Out the Grid Spacing

Rebar is often installed in a grid pattern, and to estimate the amount needed it will be important to first identify the size and spacing is appropriate for your project. For a patio it is common to use a #3 bar spaced every 18″-24″ while for a driveway #4 bar is often used with a 12″-18″ spacing.

Different projects require a different size rebar and will require the rebar to be spaced differently. Most projects require rebar to be laid in a grid pattern with bars running in opposite directions and overlapping.

The use of steel or fiber mesh can be used as an alternate form of reinforcement in some circumstances, and is often used for thinner slab pours. Try our reinforcing mesh calculator to estimate.

Rebar installed in a grid pattern to reinforce concrete slabs and prevent cracking

Step Two: Estimate the Rebar Grid Clearance

Once you have identified the specs for your rebar, you’ll need to identify how much clearance or space is needed between the rebar grid and the edge of the slab. This is needed to determine both the lengths of the rebar and how many rows and columns are needed.

Subtract the clearance from the length and width to find the dimensions of the grid. For example, if the slab is 10′ x 10′ and the clearance is 3″, the size of the rebar grid will be 9′ 6″ x 9′ 6″.

Step Three: Estimate the Rows and Columns

Find the number of bars needed for each direction by dividing the width of the grid by the desired spacing. For example, if the width is 9′ 6″ and the spacing is 18″, the number of columns would be 8.

9′ 6″ = 114″
114″ / 18″ = 6.3
round 6.3 up to 7 to get complete coverage
add 1 additional bar for the last edge

The length of the bars will be the opposite dimension of the grid. Repeat this process to find the number of rows for the opposite direction.

Ordering the Right Amount of Rebar Material

Stacks of rebar used to reinforce a thick concrete slab

Rebar is often available in 20′, 30′, 40′, and 60′ lengths. If your grid dimensions are larger than the lengths available you will need to overlap the bars to get a long enough length.

If this is the case you will need to order additional material to account for the overlap. Use our rebar weight and size calculator to find the total length of rebar needed for your project and estimate the total weight for purchasing. We recommend ordering 10% additional material to account for overlaps, cutoffs, and waste to prevent a shortage when installing.

Additional Concrete Resources

If you’re installing a concrete slab, patio, or driveway, we have additional resources available. Use our concrete calculator to estimate the amount of concrete needed for the slab. You can also find concrete pricing in your area from our network of concrete installation contractors.

See our full suite of masonry calculators and estimation tools.


  1. Beverley Burgess Bell, Purpose of Rebar in Concrete, HomeSteady,