Rebar Calculator
Reinforcing a concrete slab? Enter your slab dimensions and desired rebar spacing to find out how many pieces and how many linear feet of rebar you need.
Why Rebar Reinforces Concrete
Concrete has excellent compressive strength but cracks easily under tension. When a slab settles or experiences loads that pull rather than push, unreinforced concrete fails. Rebar solves this by adding tensile strength. The steel rods resist stretching, bridging cracks and holding the slab together even if the concrete splits.
A rebar grid spreads loads evenly across the entire slab. Instead of one concentrated weak point, stress distributes through the network of steel. This prevents small cracks from widening into structural failures.
Placement matters as much as quantity. Rebar should sit in the lower third of the slab, typically 2 to 3 inches above the subgrade. Use rebar chairs or dobies to lift the grid off the ground before pouring. Steel sitting on the dirt provides no reinforcement because it is not embedded in the concrete under tension.
Rebar Grid Layout and Spacing
A standard grid has rebar running in both directions, forming a checkerboard of squares. The spacing between bars determines the square size. Closer spacing means more steel and more strength, but also more labor and cost.
Residential garage slabs and shed floors typically use 18-inch spacing. Driveways and workshops that support heavier equipment often call for 12-inch spacing. Commercial and industrial slabs may require even tighter grids, sometimes 6 inches on center.
Overlap each piece where they meet. If your slab is longer than a single rebar length (usually 20 feet), plan overlaps so the grid remains continuous. A 24-inch overlap ensures the bars act as one continuous reinforcement, not isolated segments.
Choosing the Right Rebar Size
Rebar comes in numbered sizes based on diameter. #3 bar is 3/8 inch thick, #4 is 1/2 inch, #5 is 5/8 inch, and so on. Thicker bars carry more load but are harder to cut, bend, and handle. For typical slabs, #4 rebar offers the best balance of strength and workability.
Thinner #3 rebar works for light-duty slabs like sidewalks or thin patio pours. It is easier to manage and costs less, but it does not provide the same crack resistance as #4. If you are pouring a slab that will support vehicles or heavy equipment, step up to #4 or even #5.
This calculator focuses on linear feet and piece count, not specific bar sizes. Once you know how much total length you need, you can order the appropriate size based on your load requirements and local code. Most suppliers sell rebar in 20-foot lengths, so divide your total linear feet by 20 to estimate how many full bars to buy.
Frequently Asked Questions
What spacing should I use for rebar in a slab?
Standard residential slabs use 18-inch spacing in both directions. Heavier loads may require 12-inch spacing. Check local building codes for your specific project.
How much should rebar overlap at joints?
Overlaps should be at least 20 times the rebar diameter. For #4 rebar (1/2 inch diameter), that means a minimum 10-inch overlap. Most builders use 24 inches for simplicity.
What size rebar is best for a concrete slab?
#3 or #4 rebar works for most residential slabs. #4 (1/2 inch diameter) is the most common choice because it provides good strength without being difficult to handle.
Do I need rebar in a small slab?
Slabs thinner than 4 inches or smaller than 10×10 feet may not require rebar, depending on the soil and load. Wire mesh is a lighter alternative for small, low-load slabs.
How do I tie rebar together?
Use rebar ties (wire twists) at every intersection. Pull tight with pliers or a rebar tying tool. Ties hold the grid in place when concrete flows over it.