Rebar Takeoff for a Slab: Grid Spacing, Linear Feet, and Weight
October 21, 2026
Reinforcing bar (rebar) is steel placed in concrete to resist tension and control cracking. Concrete is strong in compression but weak in tension, so slabs, footings, and walls use rebar in a grid or cage. The amount, size, and spacing are specified by the structural drawing or building code; this article shows how to turn that spec into a count and a weight so you can order materials.
As an example, take a 20 by 20 foot slab with #4 bars (1/2 inch diameter) spaced 12 inches on center in both directions. In each direction, the bars are 20 feet long. The number of bars in each direction is 20 ÷ 1 + 1 = 21 (or 20 if you start half a spacing in from each edge), so the total is 42 bars of 20 feet each, 840 linear feet.
Skip the math and get your answer instantly:
Open the Concrete Calculator →Counting Bars
Number of bars in one direction = (slab dimension in inches ÷ spacing) + 1, rounded up. For spacing of 12 inches over 240 inches: 21 bars. The same number goes in the other direction. Subtract cover at the edges: reinforcement is kept 2 to 3 inches inside the edges, so the bars are 19 feet 4 inches long if they stop 3 inches from each edge. Total length = bars per direction × bar length × 2 directions. If the spacing is 16 inches, the count is 16 bars each way (240 ÷ 16 + 1 = 16), total 640 feet. At 18 inches, 14 bars each way, 560 feet. Closer spacing means more steel.
| Spacing | Bars each way (20 ft slab) | Total linear feet | Weight with #4 bar (0.668 lb/ft) |
|---|---|---|---|
| 12 in | 21 | 840 | 561 lb |
| 16 in | 16 | 640 | 428 lb |
| 18 in | 14 | 560 | 374 lb |
| 24 in | 11 | 440 | 294 lb |
Bar Sizes and Weights
Rebar sizes are designated by the number of eighths of an inch in nominal diameter: #3 is 3/8 inch, #4 is 1/2 inch, #5 is 5/8 inch, #6 is 3/4 inch. Weights per foot: #3 0.376 lb, #4 0.668 lb, #5 1.043 lb, #6 1.502 lb, #7 2.044 lb, #8 2.670 lb. Standard stock lengths are 20 feet, sometimes 40 or 60 feet, and shorter cut lengths are available. To order, divide the total linear feet by the stock length and round up: 840 ÷ 20 = 42 bars. Add 5% to 10% for waste and laps. Rebar is also sold by the ton; 561 pounds is about 0.28 tons.
Lap Splices and Corners
When a bar must be extended, bars overlap at lap splices. The required lap length depends on the bar size, concrete strength, and code, commonly 30 to 40 bar diameters; for #4 bars, that is 15 to 20 inches. For a 20-foot slab with 20-foot bars, no lap may be needed within the slab, but larger slabs need additional bars. Corners and edges use bent bars or L-shaped pieces. Tie bars at intersections with wire ties at each crossing or at alternate crossings, and support the grid on chairs or dobies so that it sits at the right height in the slab, usually the middle or upper third, rather than lying on the ground.
Alternatives: Welded Wire Mesh and Fibers
For lightly loaded slabs, welded wire reinforcement (such as 6 x 6 W1.4 x W1.4) comes in sheets or rolls and is faster to install, though it is harder to keep at the right height. Synthetic fibers added to the concrete mix control plastic shrinkage cracking but do not replace structural reinforcement. Residential patios often use #3 bars at 18 to 24 inches or mesh. Garages and driveways frequently use #4 bars at 16 to 18 inches. Always follow the engineer's drawings or local building codes.
Cost and Ordering Tips
Rebar is priced by weight or by the piece, and prices fluctuate with steel markets. A takeoff of 561 pounds at roughly $0.80 to $1.20 per pound is $450 to $670, not counting delivery, chairs, and ties. Ask the supplier about cutting and bending services, and inspect the bar for heavy rust or oil, which can affect bonding. Plan for inspection before the pour, since many jurisdictions require it.
Frequently Asked Questions
How many pounds is a 20-foot piece of #4 rebar?
About 13.4 pounds (0.668 lb per foot).
How do I count rebar for a slab?
Divide the dimension by the spacing, add one, and multiply the count by the bar length for each direction.
What is a lap splice?
An overlap of two bars to transfer load, typically 30 to 40 bar diameters long.
Where should rebar sit in the slab?
At the middle or upper third of the thickness, supported on chairs.
How much extra rebar should I order?
About 5% to 10% for waste and laps.