Rebar Calculator
Estimate the amount of steel rebar needed for a concrete slab grid.
1. Enter Slab & Rebar Details
Select units and enter your project specifications.
2. Rebar Estimate
Your results will appear here.
Enter dimensions to calculate.
* Estimates are for a single layer grid and do not include lap splice length or waste. Always consult an engineer for structural projects.
How to Calculate Rebar for a Slab
Rebar (reinforcing bar) is crucial for adding tensile strength to concrete, preventing cracks and structural failure. This calculator helps you estimate the total amount of rebar needed for a standard grid pattern in a concrete slab.
Calculation Steps:
- Determine Grid Area: Enter the total length and width of your concrete slab.
- Set Spacing: Input the on-center spacing for your rebar grid (e.g., 18 inches). This is the distance from the center of one bar to the center of the next.
- Calculate Bar Count: The calculator determines how many rebar pieces are needed for each direction of the grid. For example, for a 20ft slab with 18-inch spacing, you need `(20 ft * 12 in/ft) / 18 in = 13.33`, which rounds up to 14 bars. An extra bar is often added, so we calculate `floor(dimension / spacing) + 1`.
- Total Length & Weight: It multiplies the number of bars by the length of each run to find the total linear footage. Using the selected rebar size's standard weight-per-foot, it calculates the total weight.
Key Considerations:
- Lap Splice: Rebar comes in standard lengths (e.g., 20 or 40 feet). If your slab's dimension exceeds this, you must overlap or "splice" bars. The required lap length (typically 40-60 times the bar diameter) should be added to your total. This calculator currently assumes continuous runs.
- Cover: Ensure the rebar grid is elevated by "chairs" so it's surrounded by at least 1.5-3 inches of concrete on all sides (top, bottom, and edges). Do not lay it directly on the ground.
- Structural Requirements: For load-bearing structures like foundations or garage slabs, always follow the plans specified by a structural engineer.