Rebar Calculator
Calculate rebar for a concrete slab: bars in each direction, total linear feet, and how many stock bars to buy from slab size, grid spacing, and edge clearance.
How this calculator works
Slab reinforcement is a grid of bars running both directions. The calculator fits that grid inside the slab after removing edge clearance, counts the bars each way, sums their length, and divides by your stock bar length to tell you how many to buy.
Formula: Bars across a dimension = ⌊(usable length ÷ spacing)⌋ + 1, where usable length subtracts clearance from both edges. Total linear feet = (lengthwise bars × slab length) + (widthwise bars × slab width).
Worked example
A 20 ft × 12 ft slab, 12-inch grid, 3-inch edge clearance, 20-foot stock bars:
- Bars running lengthwise (spaced across the 12 ft width): 12 bars
- Bars running widthwise (spaced across the 20 ft length): 20 bars
- Total linear feet: (12 × 20) + (20 × 12) = 480 ft
- Stock bars to buy: ⌈480 ÷ 20⌉ = 24 bars
Reference
| Grid spacing | Bars per 10 ft dimension |
|---|---|
| 12 in | 11 bars |
| 16 in | 8 bars |
| 18 in | 7 bars |
How to measure
Measure the slab’s finished length and width, and confirm the required grid spacing and cover from your plan or local code. Add lap length at any point two bars meet end to end, and round up when buying.
Structural reinforcement is code-governed. For load-bearing slabs and footings, confirm bar size, spacing, and cover with a licensed engineer.
Frequently asked questions
What spacing should I use for slab rebar?
Residential slabs commonly use #4 (½-inch) bar on a 12- to 18-inch grid. Tighter spacing means more steel and more crack control; your local code or engineer sets the requirement for structural slabs. This calculator lets you enter whatever spacing your plan calls for.
Why subtract edge clearance?
Rebar must sit inside the slab with concrete cover on all sides — typically 2 to 3 inches from the edge — so the steel does not rust. The calculator removes that clearance from each side before spacing the bars, so the grid fits inside the slab, not flush to the form.
How does the calculator count bars in two directions?
A slab grid runs bars both lengthwise and widthwise. It counts how many bars fit across the width (each spanning the length) and how many fit across the length (each spanning the width), then adds their spans for total linear feet.
Does this include lap splices?
The stock-bar count is based on total linear feet divided by your bar length, which does not add overlap for splices. Where bars must be spliced end to end, add roughly 40 bar diameters of lap at each joint — about 20 inches for #4 bar — and buy a little extra.
What size rebar is standard for a driveway?
Driveways typically use #3 or #4 bar on a 16- to 18-inch grid, or welded wire mesh as a lighter alternative. Heavier loads and thicker slabs move up to #5 bar on tighter spacing.
Estimates only. Verify quantities with your supplier before purchasing.