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Rebar Calculator

Enter the slab size, bar spacing and bar size. We lay out the grid both ways, allow for lap splices where runs exceed a stock length, and tell you how many 20 ft sticks to buy.

Diagram of a rebar grid over a slab, bars running both ways with the bar spacing marked
Bars run both ways across the slab; spacing sets how many sticks the grid needs.

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Enter your measurements

How the rebar count is calculated

The grid is counted the way it's laid: bars across a dimension = that dimension ÷ spacing, plus one for the starting bar. At 18 in spacing, a 10 ft width needs ⌊120 ÷ 18⌋ + 1 = 7 bars running one way. The same is done in the other direction, and the two directions are added.

Rebar comes in standard 20 ft lengths. Where a run is longer than one stick, the bars must lap. Following the ACI-based estimating rule (a tension lap of at least 24 in or 40 × the bar diameter, whichever is greater), each extra stick in a run adds 20 ft minus the lap (24 in for #4 bar, 25 in for #5). Sticks = grid bars × sticks per run, rounded up.

Bar numbers are eighths of an inch: #3 = 3/8 in, #4 = 1/2 in, #5 = 5/8 in diameter. The results also show the approximate steel weight, using the standard nominal unit weights (ASTM A615): #3 = 0.376 lb per ft, #4 = 0.668, #5 = 1.043. Keep the grid 2–3 in in from the slab edges (concrete cover). The round-up in the stick count absorbs that on typical slabs. Pouring the slab itself? The Concrete Slab Calculator works out its cubic yards and bag count.

Worked examples

12 × 10 ft slab, #4 at 18 in

7 bars one way + 9 the other, all under 20 ft, so no splices.

Buy 16 × 20 ft sticks (174 ft total)

30 × 20 ft slab, #5 at 12 in

21 bars run the 30 ft way at 2 sticks each (one 25 in lap per run); 31 bars run the 20 ft way as single sticks.

Buy 73 × 20 ft sticks (1,250 ft of bar)

8 × 8 ft pad, #3 at 24 in

5 bars each way, single sticks.

Buy 10 × 20 ft sticks (80 ft total)

Frequently asked questions

How far apart should rebar be in a slab?
18 in each way is the common residential grid for patios and shed slabs; 12 in for driveways and heavier loads; 24 in only for light-duty pads. Your slab design or local code governs. This calculator counts material for the spacing you choose.
How much do you overlap rebar?
The standard estimating rule (from ACI 318 practice) is a lap of at least 24 in or 40 bar diameters, whichever is greater: 24 in for #3 and #4 bar, 25 in for #5. Tie the laps with wire so they stay put during the pour.
What size rebar do I need for a 4 in slab?
#3 or #4 is typical for a 4 in residential slab. #4 (1/2 in) is the most common choice for driveways and garage floors. Structural slabs follow an engineer's design, not a rule of thumb.
How long is a stick of rebar?
20 ft is the standard retail and jobsite length in the US, and what this calculator assumes. Longer lengths (30-60 ft) exist through steel suppliers but rarely suit DIY delivery.
Does this include rebar chairs and tie wire?
No. It counts bar only. Budget a bag of tie wire and a chair or dobie per few feet of grid to hold the steel at mid-slab height while you pour.

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Estimate disclaimer: results are planning estimates based on standard manufacturer yields and coverage figures. Site conditions, product choice and installation method change actual quantities. Verify before you buy, and follow local building code for structural work. This tool does not replace professional engineering advice.

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