Rebar Calculator
A suggested rebar size and spacing for your slab-on-grade — plus the linear feet, sticks, and weight to buy.
Your project
Slab-on-grade flatwork only (patios, walkways, driveways, shed/garage floors resting on prepared soil). The suggested rebar is for crack control, NOT structural load. This does not design footings, foundation or stem walls, monolithic thickened-edge slabs, suspended/elevated slabs, or slabs over problem soils or heavy loads — those must be designed by a licensed engineer per ACI 332/318 and your local code (IRC R506). Always confirm with your building official.
What you need
What to buy
—
Rebar size
—
Spacing (o.c.)
— in
Sticks to buy
—
Total rebar
— ft
Weight
— lb
Shopping list
- —
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How we calculate it
bars each way = floor( (side − 2 × cover) / spacing ) + 1 • total rebar = bars × slab sides • sticks = round up( total / stick length )
Assumptions & sources
- Suggested size and spacing are common CRACK-CONTROL practice for non-structural slabs — not a code-mandated structural design (slab-on-grade steel controls shrinkage cracks; it is "not intended to serve as flexural reinforcement") — source: ACI 360R Guide to Design of Slabs-on-Ground
- Spacing never exceeds the code cap: the lesser of 5 × slab thickness and 18 inches — source: ACI 318-19 §24.4.3.3
- Bar sizes and weights: #3 = 3/8 in (0.376 lb/ft), #4 = 1/2 in (0.668), #5 = 5/8 in (1.043) — source: ASTM A615 / CRSI
- Edge cover 3 in (concrete cast against earth); rebar is sold in 20 ft (or 10 ft) sticks — source: ACI 318-19 Table 20.5.1.3.1
Formulas reviewed against manufacturer spec sheets and industry standards. Last verified 2026.
How to estimate rebar for a slab
For a ground-supported slab, rebar is laid in a grid to control shrinkage cracks. Pick your slab size, thickness, and use, and the calculator suggests a common bar size and spacing (kept within the code’s maximum spacing), then counts the bars in each direction, the total linear feet, and how many sticks to buy.
What to know
- It’s crack control, not structure: Reinforcement in a slab-on-grade holds shrinkage cracks tight — it isn’t carrying structural load. Anything structural (footings, walls, raised slabs) needs an engineer.
- Spacing has a code ceiling: Reinforcement spacing can’t exceed the lesser of 5× the slab thickness and 18 inches. The suggestions here stay within that cap; if you override the spacing, keep it under the ceiling shown.
- Mesh is a common alternative: For light foot-traffic flatwork, welded wire mesh (e.g. 6×6 W1.4) is often used instead of a rebar grid. This tool sizes the rebar option.
Common questions
What size rebar do I need for a 4-inch slab?
For non-structural flatwork, #3 (3/8 in) or #4 (1/2 in) bar in a grid is common — #3 at ~18 in on-center for a patio, #4 at ~16 in for a driveway. This is crack-control practice, not a structural spec; a structural slab must be engineered.
How far apart should rebar be in a slab?
Commonly 12–18 inches on-center for flatwork. The code maximum is the lesser of 5× the slab thickness and 18 inches, so for a 4-inch slab the ceiling is 18 inches.
How much rebar for a 10×10 slab?
At #4 bar, 16 in on-center with 3 in edge cover: 8 bars each way, about 160 linear feet — roughly 8 × 20 ft sticks. Use the calculator to match your exact size and spacing.
Do I need an engineer?
For a simple ground-supported patio, walkway, or driveway on good soil, prescriptive practice is typically fine — but footings, foundation walls, raised slabs, heavy loads, or problem soils must be designed by a licensed engineer, and you should follow local code and your building official.