Rebar Calculator: Linear Feet for a Slab Grid

A slab grid is two layers of bars crossing at right angles. The numbers you actually buy on are total linear feet and how many stock-length bars that works out to. Enter your slab size and spacing — results update live. The default is the 12×12 slab on the common 18 in grid.

216 Total linear feet

18 Bars in the grid

11 20 ft stock bars to buy

9 bars run the 12 ft length (12 ft each) and 9 bars run the 12 ft width (12 ft each). Approx. weight: 144 lb of #4 bar.

Rebar Grid Table: Common Slab Sizes at 12, 18 and 24 in Spacing

Bar counts use the standard estimating rule: one bar per spacing interval across the slab, plus one to close the grid. "Stock bars" is the total linear feet divided by 20 ft stock lengths, rounded up — the count to put on the order.

Slab size	Spacing	Bars each way	Total linear feet	20 ft stock bars
10 × 10 ft	12 in	11 + 11	220 LF	11
10 × 10 ft	18 in	7 + 7	140 LF	7
10 × 10 ft	24 in	6 + 6	120 LF	6
12 × 12 ft	12 in	13 + 13	312 LF	16
12 × 12 ft	18 in	9 + 9	216 LF	11
12 × 12 ft	24 in	7 + 7	168 LF	9
10 × 20 ft	12 in	21 + 11	430 LF	22
10 × 20 ft	18 in	14 + 7	280 LF	14
10 × 20 ft	24 in	11 + 6	230 LF	12
12 × 20 ft	12 in	21 + 13	512 LF	26
12 × 20 ft	18 in	14 + 9	348 LF	18
12 × 20 ft	24 in	11 + 7	272 LF	14
20 × 20 ft	12 in	21 + 21	840 LF	42
20 × 20 ft	18 in	14 + 14	560 LF	28
20 × 20 ft	24 in	11 + 11	440 LF	22

Which spacing? 18 in on center is the workhorse grid for residential slabs on grade — patios, shed pads, garage floors. Go to 12 in for driveways and slabs that carry vehicles or point loads. 24 in is light-duty territory: walkways and thin patios where the grid is mostly crack insurance. When in doubt, tighten the spacing rather than upsizing the bar — for slab-on-grade work, coverage beats bar diameter.

How the Math Works

Each direction of the grid is figured the same way: count the bars that fit across the slab, then multiply by the length each bar runs.

Bars one way: divide the slab's cross dimension (in inches) by the spacing, round down, and add 1. That last bar closes the grid at the far edge.
Linear feet that way: bar count × the length each bar runs.
Repeat for the other direction and add the two together.
Stock bars: divide total linear feet by your stock length (20 ft is standard) and round up.

Bars = ⌊dimension (in) ÷ spacing⌋ + 1 · Total LF = bars × run length, both ways

Worked example, a 12 × 12 ft slab at 18 in: each direction fits ⌊144 ÷ 18⌋ + 1 = 9 bars, each 12 ft long, so 9 × 12 = 108 LF per direction and 216 LF total. Stock bars: 216 ÷ 20 = 10.8, so 11 bars — and the leftover cut-offs cover the odd short piece and any mistakes.

Two field notes the clean math leaves out. First, cover: hold the outer bars about 3 in in from the forms so the steel ends up buried in concrete, not rusting at the surface — at the spacings and sizes in the table that changes where the first bar sits, not how many you buy. Second, laps: any run longer than your stock length has to be spliced, and splices overlap by at least 40 bar diameters (20 in for #4 bar; 24 in is the common field rule). Slabs up to 20 ft on a side need no laps with 20 ft stock, which is one more reason that size is so common. On bigger pours, add the lap length for every splice to your linear feet before dividing into stock bars.

And keep the grid up in the concrete, not on the ground: bars lying on the gravel do almost nothing. Set the grid on chairs or dobies so it lands in the middle third of the slab, and tie the crossings with wire so it stays put during the pour.

Figure the Concrete and the Base Too

The grid is one of three orders for a slab pour. The concrete itself is figured with the main calculator (guides: 10×20, 12×12, 12×20), and the crushed stone under it with the gravel calculator.

Open the Concrete Calculator →

Rebar Calculator FAQ

How much rebar do I need for a 12×12 slab?

On an 18 in grid: 216 linear feet — 9 bars each way, bought as 11 × 20 ft stock bars of #4. On a tighter 12 in grid it is 312 LF (13 bars each way, 16 stock bars); on a light 24 in grid, 168 LF (7 bars each way, 9 stock bars).

What size rebar for a 4 inch slab?

#4 (½ in) is the standard choice for residential slabs on grade, with #3 (⅜ in) used on light-duty patios and walkways and #5 (⅝ in) reserved for heavier structural work. For most home slabs the spacing decision matters more than the bar size: a #4 grid at 18 in beats a #5 grid at 24 in.

How much does rebar weigh?

Per linear foot: #3 = 0.376 lb, #4 = 0.668 lb, #5 = 1.043 lb. The 12 × 12 example above at 216 LF of #4 weighs about 144 lb total — light enough to haul in a pickup bed, which is why 20 ft stock is practical for home pours.

Rebar grid or welded wire mesh for a slab?

Both control cracking; neither makes a thin slab strong by itself. Mesh is cheaper and faster on small, light-duty slabs, but it is hard to keep up in the middle of the pour — it gets stepped down to the gravel and ends up doing nothing. A tied rebar grid on chairs holds its position, which is why it is the safer default for driveways, garage floors, and any slab you cannot afford to redo. Plenty of builders also fiber-reinforce the mix and skip steel on light patios.

Estimates Only

Results are planning estimates. Bar counts use the standard estimating rule (one bar per spacing interval, plus one) and 20 ft stock lengths; actual layouts vary with edge cover, lap splices, and how cuts are nested. Reinforcement requirements for structural slabs are set by engineering and local building code — for load-bearing or elevated work, consult a licensed professional.