Rebar Calculator: Plan Your Slab Reinforcement

A slab gets a grid of steel bars running both directions. The bars sit inside the concrete edges by the edge clearance, usually 3 inches, then divide the remaining span by the on-center spacing. Counting floor(span / spacing) + 1 bars each way puts a bar at both inner edges with even gaps between.

Each bar runs the full inner length of the other direction, so total steel length is bars-one-way times length plus bars-the-other-way times width. Multiply by the bar size's weight per foot for total pounds, add waste for cuts and laps, and divide by the stock length to get bars to buy.

US bar sizes number the diameter in eighths of an inch: #4 is 4/8, or 1/2 inch. Weight per foot is fixed by ASTM A615 and grows with the square of the diameter, so #5 weighs about 56 percent more than #4. Most residential slabs use #3 or #4; #5 and up serve driveways, footings, and structural work.

Pick the size your drawings specify, not the size that seems sturdy. Bigger bars at wide spacing are not equivalent to smaller bars at tight spacing for crack control, and inspectors check what is on the plans.

Take a 20 by 20 ft slab, #4 bar, 18-inch spacing, 3-inch edge clearance, 20-ft stock, 10 percent waste. The inner grid is 19.5 by 19.5 ft. Bars each way: floor(19.5 / 1.5) + 1 = 14, so 14 by 14 = 28 bars. Total length: 28 x 19.5 = 546 ft. Weight: 546 x 0.668 = 364.7 lb. With waste: 600.6 ft, divided by 20-ft stock = 31 bars to buy.

Tighten spacing to 12 inches and the count jumps: floor(19.5 / 1) + 1 = 20 bars each way, 40 bars, 780 ft, 521 lb before waste. Spacing is the most powerful input, which is why drawings dimension it explicitly.

Spacing is center-to-center, not the clear gap between bars. Cover is the concrete between the bar and the slab edge or soil, protecting steel from moisture and corrosion; 3 inches against earth is the common minimum. Where two bars meet end to end, the lap splice overlaps them about 40 bar diameters so load transfers across the joint.

Waste covers cut-offs, lap overlaps, and the odd bent bar. Ten percent suits simple rectangles; L-shapes, curves, and heavy lap conditions deserve 12 to 15 percent. Chairs and ties that hold the grid at height are separate items, usually estimated per bar.

The most common mistake is entering the clear gap instead of center-to-center spacing, which understates the bar count. The second is forgetting edge clearance, which overstates it. The third is skipping the waste allowance and coming up short on delivery day.

A fourth mistake is choosing bar size by feel instead of by the drawings. And a fifth is ignoring laps on slabs longer than the stock length: every splice adds overlap length that the straight-bar math does not include, which is part of what the waste allowance covers.

Order by stock bars, rounded up, and confirm the yard stocks your bar size in the length you planned. Twenty-foot bars are standard, but delivery on a small truck may limit you to 10-foot lengths, which doubles the splices. Have ties, chairs, and a cutter or bender on site before the steel arrives.

Once the steel is planned, the concrete calculator estimates the concrete volume for the same slab in cubic yards, and the concrete block calculator covers block walls that often accompany slab projects.