How much rebar do you need?

How do I calculate how much rebar I need for a slab?

Keep the bars about 3 inches in from the edges, then count the bars each way: the width left over divided by the spacing, plus one. A 10 by 10 ft slab with #4 bars 12 inches apart takes 10 bars each way, each 9.5 feet long — 190 feet of bar, or 10 bars of 20 ft with two pieces cut from each.

How far apart should rebar be in a slab?

For driveways, patios and garage floors, #3 or #4 bars at 12 to 18 inches each way are common. Structural slabs and footings follow the engineer's drawings. Keep the grid off the ground on chairs or bricks so it ends up in the middle of the slab.

How much should rebar overlap?

A common rule of thumb is 40 bar diameters — 20 inches for #4 bar, 15 inches for #3. The exact lap length depends on the concrete, the bar and the load, and is set by ACI 318 and the project drawings. The calculator adds a lap wherever a bar is longer than the stock length.

How much does rebar weigh?

By the ASTM A615 standard, #3 bar weighs 0.376 lb per foot, #4 0.668, #5 1.043 and #6 1.502. A 20 ft #4 bar weighs about 13.4 pounds. In metric, a 12 mm bar weighs 0.888 kg per meter.

How many rebar ties do I need?

One at every crossing if you tie them all: a grid of 10 bars by 10 bars has 100 crossings. Many crews tie every other crossing on slabs on grade — check what your inspector expects.

What is the difference between spacing of 12 and 16 inches for the same slab?

For a 12 × 16 ft slab with #4 bars, 12-inch spacing needs 370 ft of bar (247 lb), while 16-inch spacing needs 278 ft (185 lb). Closer spacing adds about 33% more steel.

How the grid is counted

The bars sit inside a rectangle that is smaller than the slab by the edge cover on each side. In each direction the number of bars is the rectangle width divided by the spacing, rounded down, plus one, because there is a bar at both edges of the grid. The total length is the sum of the lengths of all bars. When the run is longer than the stock bar, the bars are spliced with a lap, which is 40 bar diameters by default: that is 20 inches for #4 bar (0.5 inch diameter). The weight comes from the weight per foot of each bar size, and the number of crossings is the number of bars in one direction times the number in the other.

A worked example

A 12 ft by 16 ft slab with #4 bars at 16 inches and 3 inches of cover has a grid of 11.5 ft by 15.5 ft inside the cover. That takes 12 bars in one direction and 9 in the other, 278 ft of bar in all including laps, 14 stock bars of 20 ft, 185 lb of steel and 108 crossings to tie. The same slab in #3 bar weighs 104 lb, because #3 weighs 0.376 lb per foot against 0.668 for #4.

Common mistakes

Running the bars out to the edge of the slab leaves no concrete cover, and the steel can rust and spall; keep the cover. Forgetting the splices is another: every time a run is longer than the stock bar, the lap adds length. The stock-bar count is total length divided by the bar length, so add a few extra bars for offcuts that cannot be reused. And remember that the grid only works if it sits in the middle of the slab thickness, held up on chairs or bricks, rather than lying on the ground.

Sources: bar weights and diameters from ASTM A615 (the standard for deformed bars); the 40-bar-diameter lap is a rule of thumb, while required lap lengths are set by ACI 318 and the project drawings. Related: concrete calculator, gravel calculator, metal weight calculator and deck footing calculator.