Rebar Calculator
Calculate the rebar needed for concrete slabs and footings. Get linear feet, number of bars, total weight, and cost estimates based on slab dimensions, bar size, and spacing.
- Total Rebar
- 826 linear ft
42 standard 20-ft bars · 552 lbs
- Slab Area
- 400 sq ft
- Lengthwise Bars
- 21
- Widthwise Bars
- 21
- Total Weight
- 552 lbs
| 20-ft Bars to Buy | 42 |
| Total Linear Feet | 826 |
| Total Weight | 552 lbs |
| Estimated Cost | $620 |
How to Calculate Rebar for a Concrete Slab
Rebar (reinforcing bar) is critical for preventing concrete cracks and providing tensile strength. Concrete is strong in compression but weak in tension — rebar handles the tension forces that would otherwise cause the slab to crack and fail.
To calculate rebar for a rectangular slab, you need to determine the number of bars in each direction based on spacing, then calculate total linear feet. The formula accounts for both directions of a rebar grid:
The Rebar Formula
Bars lengthwise = (Slab width ÷ Spacing) + 1
Bars widthwise = (Slab length ÷ Spacing) + 1
Total linear ft = (Bars lengthwise × Slab length) + (Bars widthwise × Slab width)
Weight = Total linear ft × Weight per ft (varies by bar size)
20-ft bars needed= Total linear ft ÷ 20 (rounded up)
Rebar Size Guide
Rebar is designated by size number, which represents the bar diameter in eighths of an inch:
- #3 (3/8"): 0.376 lbs/ft. Used for temperature/shrinkage reinforcement, light slabs, and tie wire.
- #4 (1/2"): 0.668 lbs/ft. Most common residential rebar. Standard for 4–6 inch slabs, driveways, and patios.
- #5 (5/8"): 1.043 lbs/ft. Used for thicker slabs, footings, and grade beams.
- #6 (3/4"): 1.502 lbs/ft. For foundation walls, columns, and structural footings.
- #7 (7/8"): 2.044 lbs/ft. Commercial and structural applications.
- #8 (1"): 2.670 lbs/ft. Heavy structural use — beams, columns, foundations.
Spacing Requirements per ACI 318
The American Concrete Institute (ACI) 318 Building Code sets rebar spacing requirements:
- Maximum spacing: 18 inches or 3× slab thickness, whichever is less
- Minimum spacing: 1 inch, or 1.33× maximum aggregate size, or the bar diameter — whichever is greatest
- Residential slabs: 12" OC is standard practice for #4 rebar in both directions
- Structural footings: Spacing per engineer's design, typically 6–12" OC
Rebar Placement and Cover
Proper rebar placement is as important as using the right amount. Key ACI 318 requirements:
- Concrete cover: Minimum 3" for earth-contact surfaces, 2" for weather-exposed formed surfaces, ¾" for interior slabs
- Position in slab: Place rebar in the lower third of the slab for slabs on grade. Use rebar chairs (bolsters) to maintain position during the pour
- Lap splices: Minimum 40 bar diameters for Class B tension splices. For #4 rebar: 40 × 0.5" = 20" minimum. Most contractors use 24" as a standard
- Chairs: Space rebar support chairs every 3–4 feet to keep rebar at the correct height during the concrete pour
Rebar vs Wire Mesh
For light-duty slabs (sidewalks, patios), welded wire mesh (WWF) is an alternative to rebar. Wire mesh (typically 6×6 W1.4×W1.4) comes in rolls or sheets and is easier to install. However, rebar provides significantly more tensile strength and is required for:
- Driveways and garage floors (vehicle loads)
- Slabs thicker than 5 inches
- Structural slabs that span between supports
- Any application where cracking would be a structural concern
Frequently Asked Questions
What size rebar do I need for a concrete slab?
#4 rebar (1/2 inch) at 12-inch on-center spacing is the standard for residential concrete slabs per ACI 318. For driveways and garage floors, #4 at 12" OC both ways provides adequate reinforcement. Thicker structural slabs may require #5 or #6 per engineer design.
How far apart should rebar be spaced?
Standard residential spacing is 12 inches on center (OC) in both directions. ACI 318 limits maximum spacing to 18 inches or 3 times the slab thickness. Structural applications per engineer design may call for 6–8 inch spacing.
How much does rebar cost per foot?
Typical retail prices: #3 at $0.40–$0.60/ft, #4 at $0.60–$0.90/ft, #5 at $0.80–$1.20/ft, #6 at $1.00–$1.50/ft. Standard lengths are 20 feet. Prices fluctuate with the steel market. Buying in bulk from a rebar supplier is cheaper than retail.
How much overlap at rebar splices?
ACI 318 requires minimum 40 bar diameters for Class B tension splices. For #4 rebar that's 20 inches, for #5 it's 25 inches. A common rule of thumb for residential work is 24 inches minimum overlap at all splices.
Do I need rebar in a 4-inch slab?
For basic patios and sidewalks, wire mesh is often sufficient. For driveways, garage floors, or any slab with vehicle loads, #4 rebar at 12" OC is recommended per ACI 360. Local building codes may have specific requirements.
How much cover should be over rebar?
ACI 318 minimums: 3 inches for concrete in contact with earth, 2 inches for weather-exposed formed surfaces, 3/4 inch for interior slabs not exposed to weather. Insufficient cover leads to corrosion and spalling.
Related Calculators
- Concrete Slab Calculator — volume and cost for concrete slabs
- Concrete Calculator — cubic yards and bag counts
- Stud Calculator — framing lumber for walls
Estimates are for planning purposes only. Rebar requirements for structural applications must be designed by a licensed structural engineer per ACI 318 and local building codes. Always verify requirements with your local building department.