Rebar Calculator
Estimate the amount of steel rebar needed for a concrete slab grid.
Fill in the fields and press Calculate.
1. Enter Slab & Rebar Details
Select units and enter your project specifications.
2. Rebar Estimate
Enter either value. If both are entered, "Cost per Piece" will be used.
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.
Decision Guide
Turn the slab grid into a buy list
The useful decision here is not just total rebar length. You need piece count, spacing, and a clear buying plan before this turns into a supplier order.
Buy by full stick count first, then sanity-check linear footage.
If the yard only sells full pieces, price this layout as a buy list, not just a length estimate.
Spacing and lap waste decide whether this is one store run or a real supplier quote.
Before you order
- Total pieces matter as much as total linear footage.
- Spacing is part of the structural decision, not just cost control.
- Full-stick buying rules can change the real price quickly.
Next steps
- Confirm bar size and spacing before buying.
- Get a per-piece quote if the supplier cuts nothing.
- Add lap splice, chairs, and tie wire before checkout.
Common mistakes
- Stopping at total length and forgetting piece count.
- Using random example spacing instead of project requirements.
- Pricing by length when the supplier sells full sticks.
Use this guide for planning only. Structural spacing, bar size, and lap details still need code or engineering review.
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.