How we worked it out
Updated as you change the numbers.
- Bars running lengthwise = (width − 2 × clearance) ÷ spacing + 1 = (20 − 0.5) ft ÷ 1.5 ft, rounded down, + 1 = 14, each 19.5 ft.
- Bars running widthwise = (20 − 0.5) ÷ 1.5, rounded down, + 1 = 14, each 19.5 ft.
- Every run fits in one 20 ft bar; short runs are cut two or more to a bar where they fit.
- Total = 28 stock bars, 546 linear ft × 0.668 lb per ft = 365 lb.
Method and assumptions
The grid has bars both ways at the chosen spacing, stopping short of the edges by the clearance. Each direction needs (span ÷ spacing, rounded down) + 1 bars. Bar weights are the ASTM A615 nominal values: 0.376 lb/ft for #3, 0.668 for #4 and 1.043 for #5.
Where a run is longer than a stock bar, bars are overlapped (lap spliced). The calculator uses 40 bar diameters, a common rule of thumb for slabs on ground. Structural drawings and your local code take precedence over this figure.
Tips
- Stagger lap splices so they don’t all fall in one line across the slab.
- Buy a few extra bars for bending at corners and for chairs.
Questions
How much rebar for a 20 × 20 slab?
With bars every 18 in each way and 3 in clearance, a 20 × 20 ft slab needs 14 bars each way, each 19.5 ft long: 28 bars of 20 ft. In #4 bar that is 546 linear feet, about 365 lb.
Sources
- Concrete Reinforcing Steel Institute (CRSI): reinforcing bar sizes and weights
- ASTM A615: deformed carbon-steel bars for concrete reinforcement
Method and figures checked on . Every worked answer on this page is covered by an automated test. Found a mistake? Tell us.