Rebar Calculator

You can size a rebar grid in under a minute. Enter the slab length and slab width in feet, then type your grid spacing in inches — 12 inches is the everyday choice for driveways and patios. Press calculate and you'll see how many rebar pieces to cut, the total linear footage, and how many full sticks to buy. Need to fine-tune it? Open More options to change the edge clearance, pick a rebar size, set your own stock length, switch to metric, or add a price per stick for a quick cost estimate. That's the fastest way to answer "how much rebar for a 20x10 slab" without laying out the grid on paper first.

Rebar in a slab is laid as a two-way grid: bars running the length of the slab and bars running across it, crossing at even spacing. The calculator first trims your slab by the edge clearance on all sides — concrete needs a few inches of cover so steel isn't exposed — which gives the actual grid coverage. It then divides each side of that grid by your spacing and adds one bar (a 10 ft run at 12" spacing needs 11 bars, not 10, because you place one at each end). Multiplying each bar count by its run length gives the total linear length, and dividing that by your stock stick length tells you how many full bars to buy. Because you can't buy a partial stick, that figure is always rounded up.

Grid length = slab length − 2 × edge clearance (same for width).

Bars in a direction = floor(grid side ÷ spacing) + 1, counted for both directions.

Total length = (lengthwise bars × grid length) + (crosswise bars × grid width).

Sticks to buy = round up(total length ÷ stock stick length).

Steel weight uses the nominal weight of the chosen bar size (for example #4 rebar weighs 0.668 lb per foot).

Example 1 – a 20 × 10 ft patio slab. With 12" spacing and the default 3" edge clearance, the grid is 19.5 × 9.5 ft. That's 10 bars running lengthwise and 20 running crosswise — 30 pieces, about 385 ft of rebar, or 20 full 20 ft sticks.

Example 2 – a 24 × 24 ft garage slab. At 16" spacing the grid needs roughly 19 bars each way (38 pieces) and around 900 ft of #4 rebar. Tightening the spacing to 12" pushes it past 1,100 ft, which is why spacing has such a big effect on cost.

Example 3 – a 4 × 8 ft equipment pad. At 12" spacing that's a small 9-piece grid — easy to see how the "+1" end bar rule matters on short runs.

- 12" spacing is the safe default for driveways, patios and most home slabs; drop to 18" only for light-duty pads.
- #4 (1/2") rebar covers the vast majority of residential slabs — use it unless a plan specifies otherwise.
- Add 3" edge clearance so steel keeps its concrete cover and won't rust at the slab edge.
- Buy a few extra sticks. Overlaps at splices should be about 40 bar diameters (roughly 20" for #4), and this calculator counts clear runs, not laps.
- Tie the grid at intersections and prop it on chairs so it sits in the middle third of the slab thickness.