Rebar Calculator
Estimate the rebar grid for a slab: bars each way, total length, 20 ft sticks to buy, weight and tie wires.
How to use the Rebar Calculator
- Enter the slab length and pick its unit.
- Enter the slab width and pick its unit.
- Enter the grid spacing (on center) and pick its unit.
- Enter the edge clearance and pick its unit.
- Choose the bar size.
- Choose the stock bar length.
- Enter the lap splice.
- Press Calculate. The result updates automatically as you type, and you can switch the result unit at any time.
About the Rebar Calculator
Residential slabs, patios and driveways are commonly reinforced with #3 or #4 bar on a 12–24 in grid, held up on chairs or dobies so the steel sits in the middle or upper third of the slab rather than on the ground. Keep bars about 3 in from the edges so they are fully covered by concrete and protected from rust.
The bar count assumes bars can be cut from stock lengths with little waste; if the offcuts are too short to reuse, round up the stock bars. Footings, beams and structural slabs must follow the engineer's drawings and local code for size, spacing and lap length.
Formula
Bars running lengthwise = ⌊(Width − 2 × clearance) ÷ Spacing⌋ + 1
Bars running widthwise = ⌊(Length − 2 × clearance) ÷ Spacing⌋ + 1
Total length = Σ bar runs + lap splices
Weight = Total length × unit weight
Bar diameter in inches is the bar number ÷ 8 (#4 = ½ in). Runs longer than one stock bar are spliced with an overlap of 40 bar diameters by default (20 in for #4). Unit weights are the ASTM A615 standard values.
Example calculation
Using the default values:
- Slab length = 20 ft
- Slab width = 20 ft
- Grid spacing (on center) = 12 in
- Edge clearance = 3 in
- Bar size = #4 (½ in, 0.668 lb/ft)
- Stock bar length = 20 ft
- Lap splice = 40 bar diameters
the calculator returns:
- Total rebar length: 780 ft
- Stock bars to buy: 40 bars
- Grid: 20 bars × 19′ 6″ and 20 bars × 19′ 6″
- Rebar weight: 521 lb
- Tie points: 400 ties
Frequently asked questions
How much rebar do I need for a 20 × 20 slab?
A 12 in grid with 3 in edge clearance needs 20 bars each way, each 19.5 ft long: about 780 linear feet, or 40 bars of 20 ft #4 rebar weighing about 520 lb.
How far apart should rebar be in a slab?
Residential slabs typically use 12–18 in spacing; 12 in on center in both directions is a common, conservative choice for driveways.
How much does #4 rebar weigh?
#4 rebar weighs 0.668 lb per foot, so a 20 ft bar weighs about 13.4 lb.
Last updated: · Category: Construction calculators