Rebar Calculator
Calculate rebar for a concrete slab: enter your slab size and grid spacing to get the number of bars each way, total linear feet with lap splices, and how many 20-foot sticks to buy.
Keep bars ~3″ from slab edges
#4 × 20 ft typically $10–$16
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You might also need:Concrete Calculator·Concrete Block Calculator
How rebar quantities are calculated
linear ft = Σ(bar count × run length) × 1.05 lap allowance
20 ft sticks = linear ft ÷ 20, rounded up
A 20 × 12 ft driveway slab on a 16-inch grid needs 10 bars running lengthwise and 15 bars across — about 390 linear feet with lap allowance, or 20 twenty-foot sticks. Runs longer than 20 feet are made from lapped bars; the 5% allowance covers those splices.
Rebar sizes at a glance
| Size | Diameter | Weight per ft | Typical use |
|---|---|---|---|
| #3 | ⅜″ | 0.376 lb | Patios, sidewalks, curbs |
| #4 | ½″ | 0.668 lb | Driveways, slabs, footings |
| #5 | ⅝″ | 1.043 lb | Structural walls, columns |
| #6 | ¾″ | 1.502 lb | Engineered/commercial work |
Installation checklist
- Chair it up: the grid must sit in the slab, not under it — one chair per ~16 sq ft.
- Tie the crossings: every other intersection is enough for slabs; a tie-wire twister tool saves your wrists.
- Mind the clearance: 3″ from edges and at least 1.5″ of concrete cover everywhere, or moisture reaches the steel and rust cracks the slab.
- Pour quantities come from the concrete calculator, and round footings from the Sonotube calculator.
Frequently asked questions
What size rebar do I need for a slab?+
#3 (⅜ inch) rebar suits patios and sidewalks; #4 (½ inch) is the standard for driveways, garage slabs and footings; #5 (⅝ inch) and up appear in engineered structural work. When in doubt for a driveway, #4 on an 16-inch grid is the common residential spec.
What spacing should rebar be in a concrete slab?+
Residential slabs typically use a 12 to 18 inch grid. Tighter spacing (12 inches) adds strength for driveways and slabs on questionable soil; 16–18 inches is common for patios. Your local code or engineer always overrides rules of thumb.
How much should rebar overlap?+
Where two bars continue a run, lap them at least 40 bar diameters — 20 inches for #4 bar, 25 inches for #5 — and tie the lap in two places. This calculator adds a lap allowance automatically when runs exceed stock length.
Does rebar need to be in the middle of the slab?+
Rebar belongs in the middle-to-lower third of the slab thickness, never resting on the ground. Use rebar chairs or dobies every 3–4 feet to hold the grid at height while you pour — about one chair per 16 sq ft.
Is wire mesh or rebar better for a slab?+
Welded wire mesh controls surface cracking in light-duty slabs (sheds, walkways); rebar adds real structural strength for driveways and anything load-bearing. Mesh is cheaper and faster; rebar is stronger. Many pros use rebar in the perimeter thickened edge plus mesh in the field.