How it’s calculated
Example: a 20 × 20 ft slab, 12 in grid, 3 in clearance: each run is 19.5 ft, so ⌈19.5⌉ + 1 = 21 bars each way (20 bars would leave gaps wider than 12 in). Total = 2 × 21 × 19.5 = 819 ft; with 10% → 900.9 ft = 46 bars of 20 ft. In #4 bar that weighs 900.9 × 0.668 = 602 lb.
Cutting matters: on a 16 × 16 ft slab each 15.5 ft piece uses up a whole 20 ft bar (the 4.5 ft offcut can't make another piece), so the 34 pieces need 34 bars, not the 29 that total length alone would suggest.
Bar weights are the ASTM A615 nominal values (#4 = 0.668 lb/ft). Bar size, spacing, cover and lap length for a structural slab must come from your plans or a licensed engineer.
Frequently asked questions
How much rebar do I need for a slab?
For a 12 in grid, roughly 2 ft of bar per square foot of slab plus laps. A 20 × 20 ft slab needs about 819 ft of bar before laps, or around 46 twenty-foot bars with 10% extra.
How much does #4 rebar weigh?
0.668 lb per foot, so a 20 ft bar weighs about 13.4 lb. #3 is 0.376 lb/ft and #5 is 1.043 lb/ft.
What spacing should rebar be in a slab?
Residential slabs often use #3 or #4 bar at 12–18 in each way, but requirements depend on loads, soil and code. Always follow your drawings.
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Sources
- Rebar Properties (ASTM A615/A706 reinforcing bars) — Concrete Reinforcing Steel Institute
- Bridge Design Manual, Section 300: Reinforcing bar sizes and weights table — Delaware Department of Transportation
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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