Concrete
Rebar Grid Calculator
Enter the slab size, bar spacing, edge cover and bar weight to estimate a two-way rebar grid. The result gives total linear feet, weight and the number of 20 ft bars to buy.
Formula
Bars in one direction = floor((perpendicular dimension in inches - 2 × cover) ÷ spacing) + 1. Each bar is the slab dimension minus cover at both ends. Total length = sum of both directions × (1 + lap and waste%). Weight = length × unit weight from ASTM A615: #3 = 0.376, #4 = 0.668, #5 = 1.043, #6 = 1.502 lb/ft.
Worked example
A 20 ft by 12 ft slab with #4 bars at 12 in each way and 3 in cover: lengthwise bars = floor((144 - 6) ÷ 12) + 1 = 12 bars at 19.5 ft; widthwise bars = floor((240 - 6) ÷ 12) + 1 = 20 bars at 11.5 ft. 234 + 230 = 464 ft, plus 10% = 510.4 ft. Weight = 510.4 × 0.668 = 340.9 lb, or 26 bars of 20 ft stock.
Assumptions and limits
- Single mat, two-way grid of one bar size.
- Lap splices are approximated by the percentage allowance; ACI 318 sets actual lap lengths (often 30 to 40 bar diameters or more).
- For estimation only. A licensed Professional Engineer (PE) must design and stamp any structural, foundation or life-safety element.
Frequently asked questions
What do rebar numbers mean?
US bar sizes are in eighths of an inch of nominal diameter: #4 is 4/8 = 0.5 in, #5 is 5/8 in. Unit weights from ASTM A615 are 0.668 lb/ft for #4 and 1.043 lb/ft for #5.
What spacing is common for slab rebar?
Residential slabs often use #3 or #4 bars at 12 to 18 in on center each way, but structural slabs are designed to specific spacing by an engineer.
How much concrete cover does rebar need?
ACI 318 requires 3 in of cover for concrete cast against and permanently exposed to earth, and 1.5 to 2 in for many formed surfaces exposed to weather, depending on bar size.