Rebar Calculator
Calculate rebar linear footage, grid bar counts, lap splice allowances, and stock 20-ft bars for slabs and continuous footings.
Rebar Calculator Interactive Tool
Enter your project dimensions below. Results update instantly.
Rebar sizing (#3, #4, #5), placement depth, edge clearances, and lap splice overlaps are critical structural engineering decisions governed by local residential building codes and soil conditions. This calculator provides estimates only.
Results
770 ft
Base: 700 ft (+10% lap/waste)39
Full commercial bars to purchase (rounded up)15 bars
Each 24 ft long (360 linear ft)17 bars
Each 20 ft long (340 linear ft)Grid & Footing Reinforcement Formulas
Bars_L = ⌈Width ÷ Spacing⌉ + 1 | Bars_W = ⌈Length ÷ Spacing⌉ + 1 | Total Linear Ft = (Bars_L × L) + (Bars_W × W) | Stock Bars = ⌈Total Linear Ft × (1 + Lap %) ÷ Stock Length⌉Formula Variables:
- Grid Spacing:Center-to-center spacing between parallel reinforcement bars (commonly 12", 18", or 24").
- Lap Splice %:Extra allowance for overlap splices (30× to 40× bar diameter) and cutting scrap (typically 10%–15%).
- Stock Bar Length:Standard commercial delivery lengths of deformed steel reinforcement bar (standard 20 ft).
Rebar requirements are a structural design decision and this is an estimate only. Always confirm bar size (#3, #4, #5) and spacing with local building codes.
Step-by-Step Worked Example
Example: 24 × 20 ft slab reinforced with #4 rebar on an 18-inch grid (20-ft stock bars)
Calculating rebar requirements for a 24 ft by 20 ft garage slab at 18-inch grid spacing with 10% lap splice allowance and 20-foot stock bars.
- Bars parallel to 24-ft length: ⌈20 ft ÷ 1.5 ft⌉ + 1 = ⌈13.33⌉ + 1 = 15 bars, each 24 ft long (15 × 24 = 360 ft).
- Bars parallel to 20-ft width: ⌈24 ft ÷ 1.5 ft⌉ + 1 = ⌈16.00⌉ + 1 = 17 bars, each 20 ft long (17 × 20 = 340 ft).
- Base linear footage: 360 ft + 340 ft = 700 linear feet.
- Total with 10% lap splice & waste: 700 × 1.10 = 770 linear feet.
- Stock 20-ft bars to purchase: 770 ÷ 20 = 38.5 → 39 stock bars (rounded up).
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Related Material & Construction Guides
In-depth guidance on measurement standards, formulas, and material depths:
Input Explanations & Measurement Guidelines
Slab Length & Width
Typical: typical value, change to match your product: 24 × 20 ftInternal dimensions of the concrete formwork.
Grid Spacing
Typical: typical value, change to match your product: 18 inchesCenter-to-center spacing between perpendicular reinforcement bars.
Lap Splice & Waste %
Typical: typical value, change to match your product: 10%Overlap allowance where bar runs meet plus end trimming scrap.
Stock Bar Length
Typical: typical value, change to match your product: 20 feetCommercially available deformed steel bar length.
Assumptions & Limitations
- Assumes single-layer two-way reinforcement grid elevated on plastic chairs or concrete dobies.
- Bars are held 2 to 3 inches in from perimeter forms for concrete cover and corrosion protection.
- Rebar requirements are a structural design decision and this is an estimate only.
Safety & Professional Notes
- Always place plastic OSHA-approved safety caps on all protruding vertical or exposed rebar ends to eliminate impalement hazards.
- Wear heavy split-cowhide leather gloves when tying and cutting steel rebar to prevent cuts and punctures from burred ends.
Frequently Asked Questions
How do you calculate rebar for a 10x10 slab?
For a 10x10 ft slab at 18-inch spacing: ⌈10/1.5⌉ + 1 = 8 bars each way (16 bars total, 160 linear feet). With 10% waste (176 ft), you need 9 stock 20-foot bars.
What size rebar is best for a residential concrete slab?
#4 rebar (1/2-inch diameter) is the industry standard for residential driveways, garage slabs, and heavy patios. #3 rebar (3/8-inch) is occasionally used for lightweight walkways.
What is a rebar lap splice?
Because standard rebar comes in 20-foot lengths, longer runs must overlap. Building codes typically require a lap splice equal to 30 to 40 times the rebar diameter (15 to 20 inches for #4 bar).