Rebar Calculator
Calculate total rebar needed for concrete slabs. Enter slab dimensions and spacing to get linear feet, bar count, weight and estimated cost.
Slab Specifications
Reinforcement Results
Rebar Calculation Formula
Rows in one direction = Slab dimension ÷ Spacing (in feet)
Total linear ft = (Rows × Length) + (Rows in perpendicular × Width)
How to Calculate Rebar for a Concrete Slab
Calculating the grid for reinforcement requires converting your target spacing into feet to determine the number of rows needed. Divide the length of the slab by the spacing, then multiply by the slab's width to get the linear footage required for that direction. Repeat the process for the opposite direction and add the two sums together to reach your total grid size.
Because rebar is most commonly sold in standard 20-foot lengths, our calculator takes your total linear feet, includes a standard 10% waste/overlap margin, and divides the total by 20. This gives you the exact bar count required so you can accurately quote your materials list before ordering.
Rebar Spacing Guide by Application
| Application | Recommended Spacing |
|---|---|
| Residential patio | 18 inches |
| Driveway | 12 inches |
| Garage floor | 12 inches |
| Structural slab | 6–8 inches |
| Sidewalk | 18–24 inches |
Rebar Size and Weight Reference
| Bar # | Diameter | Weight per Linear Foot |
|---|---|---|
| #3 | 3/8 in | 0.376 lb/ft |
| #4 | 1/2 in | 0.668 lb/ft |
| #5 | 5/8 in | 1.043 lb/ft |
| #6 | 3/4 in | 1.502 lb/ft |
Frequently Asked Questions
How do I calculate rebar for a concrete slab?
Divide the slab length by the spacing (converted to feet) to get the number of rows in one direction. Multiply those rows by the slab width for total linear feet in that direction. Repeat for the perpendicular direction and sum both totals.
How much rebar do I need for a 10×10 concrete slab?
A 10×10 ft slab with 12-inch spacing requires approximately 110 linear feet of rebar for a single-layer grid, or about 6 standard 20-foot bars per direction.
What size rebar for a residential concrete slab?
#4 rebar (1/2 inch diameter) is the standard choice for most residential slabs including driveways, patios and garage floors. #3 works for light-duty patios.
How far apart should rebar be in a concrete slab?
12 inches on center is the most common spacing for residential slabs. Patios can use 18 inches. Structural slabs and driveways subject to heavy loads should use 6–12 inches.