Rebar Calculator
Enter your slab dimensions, rebar spacing, and bar size to get an instant bar count, linear feet, and weight estimate for your concrete project.
1Slab Dimensions
2Rebar Settings
ACI 318 recommends 12-18" for residential slabs
#4 rebar is standard for most residential concrete slabs
Two-way grid is standard for slabs exposed to freezing or heavy loads
Overlap note: Where bars need to be spliced, overlap them by at least 40 bar diameters. For #4 rebar that is 20 inches. This calculator adds 10% to account for overlaps and edge setback.
Your Rebar Estimate
Grid Layout
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Buying Summary
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Tip: Set rebar at least 1.5 inches from the bottom of the slab — use rebar chairs or dobies to hold it off the ground. Rebar resting on dirt provides almost no reinforcement benefit.
How to Use This Calculator
Using this rebar calculator ensures you purchase the exact amount of steel reinforcement needed to strengthen your concrete slab. Follow these five steps:
- 1Enter slab dimensions. Use a tape measure to determine the length and width of your concrete pour in feet and inches.
- 2Select rebar spacing. Also known as "on center" spacing, this determines how tight the grid is. ACI 318 (American Concrete Institute) guidelines recommend 12 to 18 inches on center for standard residential slabs. Shrink that to 8 inches for driveways or areas supporting heavy loads.
- 3Select rebar size. Number 4 (#4) rebar, which is a half-inch thick, is the absolute most common and universally recommended size for residential concrete slabs.
- 4Select bar length. To correctly calculate how many physical bars you need to purchase, select the length available at your supplier. A 20-foot bar is standard from lumber yards and large big-box stores.
- 5Select grid pattern. Choosing a two-way grid (running bars in both directions) is the professional standard and provides significantly better multi-directional crack control than a one-way layout.
The Formula
This rebar calculator maps out your grid mathematically to determine your material needs. Here is the math in plain English:
Bars Running Lengthwise = (Slab Width / Spacing) + 1
Bars Running Widthwise = (Slab Length / Spacing) + 1
Total Linear Feet = (Lengthwise Bars × Slab Length) + (Widthwise Bars × Slab Width)
Bars to Buy = (Total Linear Feet × 1.10) / Bar Length (rounded up)
First, the calculator divides the slab's width by your chosen spacing to find out how many bars need to run lengthwise down the slab. It repeats this process for the widthwise bars. To find the linear feet required in each direction, the bar count is multiplied by the slab dimension in that specific direction. If you chose a two-way grid, both numbers are added together.
Because a 40-foot slab requires splicing two 20-foot bars together, the calculator automatically adds a 10% material buffer. This ensures you have plenty of extra steel to accommodate lap splices (where two bars overlap each other and are tied together) and edge setbacks. Finally, the total linear feet is multiplied by the specific weight per foot—drawn directly from CRSI (Concrete Reinforcing Steel Institute) standard bar weights—to provide the total weight of your steel order.
Frequently Asked Questions
Does a concrete patio always need rebar, or is wire mesh an alternative?
For light foot-traffic patios, wire mesh (welded wire reinforcement) is often an acceptable alternative to rebar. Wire mesh sits closer to the surface and is excellent at preventing shrinkage cracks from widening. However, if the patio will hold a heavy hot tub, or if it is a driveway supporting vehicles, thick steel rebar is mandatory for structural integrity.
What is the difference between rebar and wire mesh, and when should I use each?
Wire mesh controls temperature and shrinkage cracking near the top of the slab. It prevents the concrete from spiderwebbing but provides minimal heavy load support. Rebar (reinforcing bar) provides immense tensile strength near the middle-bottom of the slab. It allows the concrete to bend slightly under heavy loads and bridge over settling soil without snapping cleanly in half.
How do I support rebar off the ground during the pour?
Rebar sitting directly on the dirt is useless. You must suspend it roughly in the middle of the slab thickness (or about 1.5 inches to 2 inches off the ground for a standard 4-inch slab). The best way to do this is by tying the rebar grid together with wire and resting the grid on cheap plastic "rebar chairs" or small concrete blocks called "dobies."
What does "on center" spacing mean for rebar placement?
"On center" (OC) spacing refers to the distance measured from the absolute center of one rebar pole to the absolute center of the parallel rebar pole next to it. It does not measure the empty gap between the edges of the steel bars.
Do I need different rebar for a 4-inch slab versus a 6-inch slab?
Generally, no. A #4 (half-inch) rebar is the universally accepted standard for both 4-inch patios and 6-inch residential driveways. What changes is the spacing. For a 4-inch patio, you might space the #4 rebar 18 inches apart. For a 6-inch driveway carrying heavy trucks, you tighten that spacing grid to 12 inches or even 8 inches apart.
How do I cut rebar on a job site?
Do not use a hacksaw; it is far too slow and physically demanding. For standard #4 rebar, the best tool is a handheld angle grinder fitted with a metal-cutting cutoff wheel. If you have to cut dozens of bars, renting a manual heavy-duty bolt cutter or an electric rebar cutter is the most efficient option.