Estimate rebar for a slab grid — how many bars, how many linear feet, and what it weighs — from your slab size, bar spacing, and bar size.
Quick answer: A 10 × 10 ft slab with #4 bars at 12 inches on center and 3-inch edge clearance needs 10 bars per direction × 9.5 ft = 95 ft per direction, or 190 ft total ≈ 127 lbs of rebar. Order about 209 ft with a 10 percent allowance for laps and cuts.
Estimate, not engineering. Spacing and bar size here are typical planning values for on-grade slabs — a structural engineer decides reinforcement on load-bearing or structural work.
| Slab use | Bar size | Spacing on center |
|---|---|---|
| Patio / 4-in slab | #4 | 18–24 in |
| Driveway / 4–5-in slab | #4 | 12–18 in |
| Garage / shop / 5–6-in slab | #4 – #5 | 12–18 in |
| Structural slabs / foundations | — | Engineer of record decides |
Spacing is planning guidance for typical on-grade slabs, not a structural prescription. Tighter spacing and larger bars carry more load; structural slabs and foundations follow the engineer of record, not a table.
| Step | Math | Result |
|---|---|---|
| Bars per direction | ⌊(120 − 2 × 3) ÷ 12⌋ + 1 | 10 bars |
| Bar length | 10 ft − 2 × 0.25 ft | 9.5 ft |
| Footage per direction | 10 × 9.5 ft | 95 ft |
| Total footage | 2 × 95 ft | 190 ft |
| Weight (#4 = 0.668 lbs/ft) | 190 × 0.668 | ≈ 127 lbs |
| With 10% waste allowance | 190 × 1.10 | 209 ft |
Straight answers, no fluff. Then run your own numbers above.
The number is the bar diameter in eighths of an inch: #4 is 4/8 inch, or 1/2 inch. It weighs 0.668 lbs per linear foot (ASTM A615) and is the common bar for residential slabs. #5 is 5/8 inch, #3 is 3/8 inch.
Bars per direction = floor((dimension − 2 × edge clearance) ÷ spacing) + 1. A 10 ft slab with #4 bars at 12 inches on center and 3-inch clearance needs floor((120 − 6) / 12) + 1 = 10 bars per direction.
Residential slabs are commonly 12 to 24 inches on center depending on the slab's use — see the spacing table above. A slab on grade with 4,000 psi concrete and light loads can run wider spacing than a slab carrying point loads.
Yes — rebar does nothing sitting on the ground. It belongs in the lower third of the slab (within cover requirements), so support it on chairs, dobies, or continuous bolsters on roughly 4-foot centers, and keep the spacing even while the concrete is placed.
When concrete is exposed to chlorides or corrosive conditions — deicing salts, marine spray, brackish soil. Interior residential slabs on grade typically use plain carbon steel bar; a structural engineer or the local code specifies coating where it is needed.
Anything carrying structural loads: foundations, footings, walls, elevated slabs, and slabs supporting equipment or heavy point loads. This calculator is a material-quantity estimator for typical on-grade slabs — the engineer of record decides bar size and spacing on structural work.
Sizing the pour the rebar goes in? Use the concrete calculator for yards and bags, and the gravel base guide for the compacted stone that goes under the slab.