Rebar Calculator

The most important technical rule for rebar is Placement and Support. Rebar only provides strength if it is suspended in the middle of the concrete. If the steel is sitting on the ground when the concrete is poured, it will eventually rust and provide zero structural benefit. Professionals use "Rebar Chairs" or "Dobbies" (small concrete blocks) to hold the steel at the correct height during the pour. Our calculator helps you determine the amount of steel, but your installation must ensure that there is at least 2-3 inches of "Concrete Cover" on all sides of every bar. This cover prevents moisture from reaching the steel and causing "spalling," where rusting rebar expands and shatters the concrete from the inside out.

Understanding Lap Splices and Tie Wire is your next challenge. Rebar is typically sold in 20-foot lengths. If your project is 30 feet long, you must overlap two bars. The "Lap" length is usually 40 times the diameter of the bar (e.g., a 20-inch lap for #4 rebar). Our calculator includes a 10-15% "lap and waste" factor to ensure you have enough steel for these overlaps. You will also need "Tie Wire" (usually 16-gauge) to secure the bars together at every intersection so they don't shift when the heavy concrete is being poured and vibrated. Finally, remember that rebar comes in different "Grades"—Grade 60 is the most common and offers high tensile strength for most residential and commercial applications. Our tool provides the data, but your Local Building Code will dictate the exact size and spacing required for your specific soil type and structure.