Rebar Calculator: How to Calculate Rebar for Concrete Projects
Calculate rebar quantity, spacing, and weight for concrete slabs, walls, and footings. Covers standard rebar sizes, minimum concrete coverage, and IBC/ACI 318 requirements.
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Standard Rebar Sizes and Properties
Rebar (reinforcing bar) is designated by a number that represents the nominal diameter in 1/8-inch increments:
Rebar Spacing and Grid Calculation
For a concrete slab, rebar is typically installed in a grid pattern. The number of bars in each direction:
Number of bars (one direction) = (Length − 2 × Edge Distance) ÷ Spacing + 1
Total bars = Bars in X direction + Bars in Y direction
Total rebar length = Total bars × Length of each bar
Worked Example: 20×30 ft Concrete Slab
- Slab dimensions: 20 ft × 30 ft
- Rebar: #4 at 12-inch on center each way
- Edge setback: 3 inches from each edge
20-ft direction (bars run 30 ft long): - Bars: (20 ft × 12 in/ft − 2 × 3 in) ÷ 12 in + 1 = (240 − 6) ÷ 12 + 1 = 20.5 → 21 bars - Each bar: 30 ft long
30-ft direction (bars run 20 ft long): - Bars: (30 × 12 − 6) ÷ 12 + 1 = (360 − 6) ÷ 12 + 1 = 30.5 → 31 bars - Each bar: 20 ft long
Total rebar: - 21 × 30 ft = 630 linear ft - 31 × 20 ft = 620 linear ft - Total: 1,250 linear feet of #4 rebar
Add lap splices: When bars must be joined, overlap by 1.3 × lap length. For #4: minimum 15 inches (ACI 318, Class A lap). Add 5–10% for laps.
Total weight: 1,250 × 0.668 lb/ft × 1.07 (laps) = 895 lbs ≈ 0.45 tons
Concrete Coverage Requirements (ACI 318)
Concrete must cover rebar to protect against corrosion. ACI 318 minimum cover:
Common Rebar Specifications by Application
4-inch residential slab (foot traffic, light vehicles): - #3 or #4 at 18-inch on center each way - Welded wire mesh (WWM) is an acceptable alternative for light slabs
6-inch residential slab (driveways, heavy vehicles): - #4 at 12-inch on center each way - Double-mat (#4 top and bottom) for slabs over 6 inches
Foundation wall (8-inch CMU or concrete): - #5 vertical at 24–48 inches on center (code-dependent, seismic zone) - Horizontal: #4 at 24 inches or joint reinforcement every 2 courses
Continuous footing: - 2–3 × #4 horizontal continuous bars top and bottom - Stirrups of #3 at 12–18 inch spacing (for deeper footings)
Rebar Cost Estimate
Prices fluctuate with steel market. Rebar is sold by linear foot or by the 20-foot stick. Order 5–10% extra for cuts and overlaps.
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Frequently Asked Questions
- How do I calculate rebar for a concrete slab?
- Determine spacing (typically 12 or 18 inches on center). Calculate bars in each direction: (slab length − edge setbacks) ÷ spacing + 1. Multiply by bar length. Add 5–10% for lap splices. For a 20×30 ft slab with #4 at 12-inch spacing: approximately 1,250 linear feet of rebar.
- What size rebar is used for a concrete slab?
- Residential slabs typically use #3 or #4 rebar. #3 (3/8 inch) at 18-inch spacing for lightly loaded 4-inch slabs. #4 (1/2 inch) at 12-inch spacing for driveways, garage floors, and 5–6 inch slabs. Structural engineers specify larger bars for load-bearing slabs.
- How far apart should rebar be spaced in a slab?
- 12 inches on center for most residential applications (driveways, garage slabs). 18 inches for lightly loaded interior slabs. ACI 318 specifies maximum spacing of 18 inches for primary reinforcement in slabs. Seismic zones may require closer spacing — check local codes.
- How much concrete coverage does rebar need?
- ACI 318 minimums: 3 inches for concrete cast directly against earth (footings), 2 inches for concrete exposed to weather, 3/4–1.5 inches for interior slabs not exposed to weather. Use plastic rebar chairs or wire supports to maintain proper coverage during concrete pour.
- How much does rebar cost per linear foot?
- #3 rebar: $0.17–$0.30 per linear foot. #4: $0.32–$0.60 per foot. #5: $0.50–$0.90 per foot. Prices vary by steel market conditions, region, and quantity. Rebar is typically sold in 20-foot lengths — price per stick ranges from $3.50 (#3) to $18 (#5) depending on current market.
Last updated 7/28/2026