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Construction

Concrete Slab Rebar Calculator

Pouring a concrete slab without rebar (or with the wrong amount) is how you get cracked driveways, sinking garage floors, and patios that heave with the first hard winter. The wire mesh that comes free at the lumberyard isn't enough for anything heavier than a 4" sidewalk. This calculator tells you how many bars long-way, how many bars short-way, total linear feet of rebar, and the number of 20-ft sticks to buy for any rectangular slab — garage floor, driveway, patio, shed pad, monolithic foundation. Pick #4 bar (1/2") for 4" slabs and #5 (5/8") for thicker slabs or driveways with truck traffic.

Total rebar (ft)

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Bars long way
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Bars short way
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20 ft sticks
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Why slabs need rebar (or fiber)

Concrete is strong in compression (it'll happily carry a parked truck) but weak in tension (it cracks the moment something tries to bend it). Soil shifts, frost heaves, and truck wheel loads all bend a slab. Without rebar, that bending opens cracks; with rebar, the steel holds the crack closed and keeps the slab acting like one piece.

A standard residential slab grid: rebar tied at 18" oc each way, both directions, sitting at the middle depth of the slab (not on the ground). For driveways carrying trucks or any slab over 4" thick: 12" oc grid spacing and #5 (5/8") bar instead of #4.

Worked example: a 20x20 slab at 18" oc grid. Bars long-way: 20 ft ÷ 1.5 ft + 1 = 15 bars at 20 ft each. Bars short-way: same = 15 bars at 20 ft each. Total LF = 600. At 20-ft stick = 30 sticks of rebar. At 2025 pricing for #4 (~$8/stick) that's $240 of steel for the slab.

Rebar vs. wire mesh vs. fiber

  • Wire mesh (6x6 W2.9xW2.9): cheapest. Adequate for 4" residential sidewalks and small patios with light foot traffic. Easy to install but only works if it's chaired up off the dirt — not stepped flat into the wet pour.
  • Rebar grid: the standard for garage floors, driveways, slab-on-grade foundations, and any slab that'll see vehicle loads. 4-5x stronger than wire mesh.
  • Fiber reinforcement: polypropylene or steel fibers mixed into the concrete at the truck. Controls hairline shrinkage cracking; doesn't replace rebar for structural loads. Use as a supplement, not a substitute.

How to use this calculator

  1. Slab length and width in feet. For an L-shaped or irregular slab, break it into rectangles and add the rebar counts together.
  2. Grid spacing: 18" for residential default. 12" for driveways with heavy vehicle traffic, monolithic foundations, or slabs over 6" thick.
  3. Output: bars in each direction, total linear feet, and number of 20-ft sticks to buy.
  4. Add 10% for laps and chairs: rebar pieces must overlap by 40 bar diameters at every splice (20" for #4, 25" for #5). Plus you need rebar chairs every 4 ft to hold the grid at proper depth. The calculator returns net bar count — add 10% for waste.

Common scenarios

24x24 garage floor, 4" thick, #4 rebar at 18" oc. 17 bars each way, 24 ft each = 816 LF = 41 sticks of #4. Add 4 sticks for laps = 45 sticks. ~$360 in steel, plus tie wire and chairs.

12x40 driveway, 5" thick, #5 rebar at 12" oc. 13 bars long-way at 40 ft + 41 bars short-way at 12 ft = 520 + 492 = 1,012 LF. 51 sticks of #5. Plus 5-6 sticks for laps. Material around $500-600.

10x12 shed pad, 4" thick, #4 at 18" oc. 9 bars + 7 bars = 16 bars total, 90 + 84 = 174 LF = 9 sticks. Quick job, ~$72 of rebar.

FAQ

Where in the slab thickness should the rebar sit? +
The middle to upper third — about 2" up from the bottom of a 4" slab, or 2.5" up in a 5" slab. Rebar resting on the dirt does nothing structurally and will rust faster. Use 2"-3" rebar chairs (plastic or metal) every 3-4 feet to hold the grid at proper height while you pour.
What does "#4 rebar" mean? +
The number refers to bar diameter in eighths of an inch. #4 = 4/8 = 1/2" diameter. #5 = 5/8". #6 = 6/8 = 3/4". Residential slabs use #4 by default; #5 for thicker slabs or higher loads. Standard length is 20 ft per stick; some yards stock 10-ft and 40-ft sticks too.
Do I need to overlap rebar at splices? +
Yes. Minimum lap is 40 bar diameters at every splice: #4 = 20" overlap, #5 = 25". Tie the overlapping sections together with rebar tie wire (a hand twist tool helps a lot). Splices in adjacent bars should be staggered — don't put all the splices in one line across the slab.
Can I use wire mesh instead of rebar? +
For 4" slabs with light loads (sidewalks, small patios, walkway slabs): yes, 6x6 W2.9xW2.9 welded wire mesh works. For garage floors, driveways, anything with vehicle loads, slab foundations: use rebar. Mesh is too thin to handle vehicle wheel loads over time.
What's the difference between epoxy-coated and plain rebar? +
Plain (black) rebar is fine for interior slabs and slabs not exposed to road salt or chronic moisture. Epoxy-coated (the green stuff) is required for bridge decks, marine structures, and slabs in coastal/salt environments. For a residential driveway in snow country it's overkill but not wrong.
Do I need control joints if I have rebar? +
Yes — rebar holds cracks closed but doesn't prevent the slab from wanting to crack at thermal stress points. Cut control joints (saw-cut 1/4 of slab depth) every 8-10 ft in each direction within 24 hours of the pour. The slab will crack at the joints instead of randomly across the surface.
How thick should my slab be? +
4" for sidewalks, patios, and light-duty floors. 5" for residential driveways and garage floors. 6" for heavy vehicle traffic (RV pad, commercial driveways). 8"+ for industrial floors. Always pour over a properly-compacted base (4-6" of crushed stone) on top of a vapor barrier.
What concrete strength should I order? +
3,000 psi (sometimes labeled 3000) for typical residential interior slabs. 3,500-4,000 psi for driveways and any exterior slab in freeze-thaw country. 4,500-5,000 psi for industrial floors. Air-entrained concrete (5-7% air) is required for exterior slabs that will see freeze-thaw cycles — not optional in northern climates.