Rebar Calculator - Slab Rebar Grid Estimator
Calculate the number of rebar pieces and total length needed for a slab, from slab dimensions and rebar spacing.
Total Rebar Pieces
42
Running Each Direction
21 / 21
Total Length
840 ft
How to Calculate Rebar for a Slab
Enter your slab's length and width along with the on-center spacing you want between rebar pieces, and this calculator finds how many bars run in each direction to form a grid, plus the total number of pieces and combined linear length needed.
Example
A 20 ft × 20 ft slab with 12-inch on-center spacing needs ⌊(20×12÷12)⌋+1 = 21 bars running each direction, for 42 bars total. Since each bar in one direction spans 20 ft, the total combined length is 21×20 + 21×20 = 840 linear feet.
Common Use Cases
- Planning a rebar grid for a concrete driveway, patio, or slab foundation.
- Estimating rebar quantities to order for a DIY concrete pour.
- Checking spacing assumptions against a project's structural drawings.
FAQs
Does this include overlap for splicing bars?
No — this calculates the straight total length before adding any lap splice overlap. When two rebar pieces need to be joined end-to-end, they're typically overlapped by roughly 12 to 24 times the bar diameter (check your local code or engineer's spec), so add extra length to your order to cover those splices.
What spacing should I use?
Residential slabs commonly use 12 to 18 inches on-center spacing, but the right spacing depends on the slab's thickness, expected load, and local building code — check project-specific structural drawings or consult an engineer for anything load-bearing.
What if I need different spacing in each direction?
This calculator assumes the same spacing in both directions to form a square grid. If your project calls for different spacing along the length versus the width, run the calculation twice — once for each direction's spacing — and combine the results.