Concrete Slab Rebar Calculator — Bar Count & Weight
Calculate rebar for a concrete slab grid in both directions. Returns bar count, total lineal footage, and weight per ACI CODE-318-25 spacing rules.
Enter Dimensions
Add 10% for standard cuts and overlaps
Results
Bars Needed
38bars
Total Lineal Feet
608lin ft
Total rebar length including waste
Total Weight
406lb
#4 rebar weight per CRSI
Slab Area
240sq ft
Editorial Standards
Each calculator is reviewed for formula accuracy, unit consistency, and alignment with current U.S. building practices before publication. We verify outputs against published engineering references and real-world project data. Learn more about our methodology.
Reinforcing a concrete slab with rebar prevents random cracking, keeps cracks tight when they do occur, and lets the slab span small soft spots in the subgrade. For most residential slabs — patios, garage floors, driveways, basement floors — a single mat of #3 or #4 rebar at 16 inch on-center grid spacing is the ACI-recommended standard.
This calculator returns bar count in each direction, total lineal footage, and total weight using the published Concrete Reinforcing Steel Institute (CRSI) weight tables. The 10% waste factor covers lap splices, bends, and edge cuts.
How to Estimate Rebar for a Concrete Slab
Bars each direction = ⌈Slab Dimension ÷ Spacing⌉ + 1. Total lineal feet = (long-way bars × short dimension) + (short-way bars × long dimension). Weight = total lineal feet × bar weight per foot (CRSI).
Common Slab Rebar Sizes (CRSI Weights)
- #3 (3/8″): 0.376 lb/ft — light-duty slabs, patios.
- #4 (1/2″): 0.668 lb/ft — residential default for slabs and footings.
- #5 (5/8″): 1.043 lb/ft — driveways, heavier loaded slabs.
- #6 (3/4″): 1.502 lb/ft — structural slabs, foundation walls.
Frequently Asked Questions
What size rebar for a residential concrete slab?
#3 (3/8″) or #4 (1/2″) rebar is standard for 4″ residential slabs — patios, sidewalks, garage floors. #5 (5/8″) is used in driveways and slabs with vehicle traffic. Spacing is typically 16–18″ each way for residential work per ACI 332.
How far apart should rebar be in a slab?
16 inches on-center each way is the most common residential slab spacing. Tighter spacing (12″) for higher loads (workshops, garages). Wider spacing (24″) only for lightly loaded slabs like garden walkways.
How deep should rebar be in a 4 inch slab?
Rebar should sit in the middle third of the slab — for a 4″ slab, that's 1.5–2″ from the bottom. Set on chairs or bricks; never pull-up after pouring. ACI 318 requires ≥ 3″ cover for slabs cast on grade.
How much rebar for a 20×20 ft slab at 16 in OC?
Bars in each direction = (240″ ÷ 16″) + 1 = 16 bars. Total bars = 32. Total lineal feet ≈ 32 × 20 ft = 640 ft. At 0.668 lb/ft for #4 rebar, weight ≈ 428 lb (you'd order ~22 bars of 20 ft length plus laps).
Do I need rebar or just wire mesh for my slab?
For slabs 4″ or thinner with no heavy loads, 6×6 W2.9×W2.9 welded wire mesh is acceptable per ACI 360. For slabs supporting vehicles, workshop equipment, or in expansive soils, use #3 or #4 rebar grid.
Looking for the general calculator?
Calculate rebar quantity, spacing, and total weight for concrete slabs, footings, and walls. Covers #3 through #8 bar sizes using CRSI published weight tables.
Open the Rebar Calculator: Bar Count & Weight →