Rebar Calculator

Lay out a rebar grid for a concrete slab: how many bars run each way, the total length and weight, how many stock bars to buy, and the cost.

Center to center, the same both ways.
Distance from the slab edge to the end of each bar.
Bar Size
Stock Length
Filled in at 40 bar diameters. Use your plans if they say otherwise.
$
Fractions work too: try 10 1/2.
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Stock Bars
Bars along length
Bars along width
Total rebar
Weight
Splices
Cost
Add a price to estimate

Slab List

Slabs you add are saved in this browser.

SlabGridLinear Ft.WeightStock BarsCostRemove

How to Calculate Rebar for a Slab

A slab grid has bars running the length of the slab and bars running across it. Take the edge clearance off both ends to get the length of each bar, then work out how many fit across at your spacing. Add one, because a grid needs a bar at each end.

Bars running lengthwise = ⌊(width − 2 × clearance) ÷ spacing⌋ + 1
Length of each = slab length − 2 × clearance

Do the same the other way, then add up the total length. Bars longer than your stock length are spliced by overlapping them (the lap splice). Short bars are cut from stock lengths, with as many pieces as fit from each bar.

Worked Example

Rebar Sizes and Weights

Standard U.S. bar sizes. The number is the diameter in eighths of an inch.

BarDiameterLb. per Foot20′ Bar Weighs40 × Diameter Lap

Tips for Placing Rebar

Rebar only works when it sits inside the concrete, not on the ground under it. Set bars on chairs or concrete blocks so they end up in the middle to upper part of the slab, and tie the crossings with wire. Keep bars back from the edges so they're covered by concrete and don't rust. Spacing, bar size and lap length for anything structural should come from your plans or local code.