Definition
Rebar (reinforcing bar) is a steel bar used as a tension device in reinforced concrete and masonry structures. Rebar provides tensile strength that concrete lacks, preventing cracking and structural failure. Rebar is sized by diameter in 1/8 inch increments-#3 bar is 3/8 inch, #4 is 1/2 inch, #5 is 5/8 inch, and so on through #18. Grade 60 (60 ksi yield strength) is standard for most construction, with Grade 40 and Grade 75 also available.
Formula
For a 24x24 foot slab with rebar at 18 inches OC grid: 24 feet = 288 inches, 288 / 18 = 16 spaces per direction, requiring 32 bars. At 20 feet long each, total is 640 linear feet. Add 10% for lap splices and waste. Use #3 bar for slabs 4-6 inches thick, #4 for footings and heavier slabs.
Example
Rebar Calculation Example
A 20x40 foot driveway slab requires #4 rebar at 18 inches OC each way. For the 20-foot direction: 20 / 1.5 = 13.33, round up to 14 spaces = 15 bars x 40 feet = 600 linear feet. For the 40-foot direction: 40 / 1.5 = 26.67, round up to 27 spaces = 28 bars x 20 feet = 560 linear feet. Total = 1,160 linear feet plus 10% for laps = 1,276 feet.