How to use the rebar calculator
- Enter the slab length and width.
- Enter the grid spacing and the edge clearance (the distance the bars stop short of the slab edges).
- Choose the bar size, stock bar length and lap splice length.
- Add a waste allowance and, optionally, a price per bar.
How the calculation works
Rebar running the length of the slab are spaced across its width: their count is the clear width (width − 2 × clearance) divided by the spacing, rounded down, plus one. Bars running across the width are counted the same way along the length. Each bar is as long as the clear dimension it spans.
If a run is longer than a stock bar, bars are spliced and each splice adds one lap length. Short runs are cut several to a bar. Weight uses the nominal weights for ASTM A615 deformed bars (for example 0.668 lb/ft for #4).
Formula
Worked example
20 × 12 ft slab, #4 bars at 12 in, 3 in clearance
Clear dimensions: 19.5 × 11.5 ft (234 × 138 in). Bars along the length: ⌊138 ÷ 12⌋ + 1 = 12 bars of 19.5 ft. Bars across: ⌊234 ÷ 12⌋ + 1 = 20 bars of 11.5 ft.
Total = 12 × 19.5 + 20 × 11.5 = 464 ft; with 10% waste 510.4 ft, about 341 lb. With 20 ft stock: 12 + 20 = 32 bars, 36 with waste.
Assumptions and limitations
- This is a takeoff estimate, not a structural specification. Bar size, spacing, cover, lap length and placement must come from your project drawings, building code or a structural engineer.
- Dowels, hooks, chairs, tie wire and edge or thickened-section reinforcement are not included.
- Offcuts from long runs are not reused in the count, which can slightly overestimate bars.
Frequently asked questions
How many feet of rebar do I need per square foot of slab?
For a grid at 12 in each way, roughly 2 linear feet per square foot; at 18 in, about 1.33 ft per square foot, before laps and waste.
How much does #4 rebar weigh?
Nominally 0.668 lb per foot, so a 20 ft bar weighs about 13.4 lb.
What is a lap splice?
Where two bars must be joined, they overlap by a specified length so load transfers between them. The required lap depends on the bar size, concrete strength and code; the calculator adds the length you enter for each splice.
Sources and references
- ASTM A615/A615M, Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement (nominal bar weights).