Rebar Calculator
Size the steel grid for a concrete slab: bar count, total length, weight, and sticks to buy — from your slab dimensions and spacing. Runs 100% in your browser.
Rebar Calculator
About this rebar calculator
Rebar math is a grid problem: bars run both directions across the slab, spaced evenly, and the count comes from dividing each dimension by the spacing. This calculator does that division in both directions, then converts bars to total linear feet, weight (so you know the haul), and 20-ft sticks to buy — with a waste allowance for cuts and lap splices. Our rebar guide walks through bar sizes, spacing rules, and a fully worked slab example.
Bar size is the first choice. #3 (3/8 in) suits light-duty slabs; #4 (1/2 in) is the residential default for patios, driveways, and shed floors; #5 (5/8 in) is for heavier slabs and anything an engineer specs. Weight per foot matters at the store: a 10×10 ft slab at 18-inch spacing needs about 107 lb of #4 bar — plan the trip accordingly.
Spacing is the second choice. 18 inches on center is the standard residential grid for 4-inch slabs; go to 12 inches for driveways and heavy loads, and 24 inches only for light-duty work like garden walkways. And remember the grid must sit in the concrete on chairs — rebar lying on the dirt does nothing.
How to use
- Enter your slab length and width in feet.
- Choose your bar spacing (18 in is standard) and bar size (#4 is the common default).
- Pick a waste allowance for cuts and lap splices, and optionally a price per stick.
- Press Calculate for bar count, length, weight, and sticks to buy.
Good to know
- Covers slabs on grade only — footings, foundation walls, retaining walls, and structural slabs need engineering, not a rule of thumb.
- Assumes a rectangular slab and a uniform grid; irregular shapes should be broken into rectangles.
- Stick length is assumed 20 ft (the common home-center length); confirm with your supplier.
- This is a materials estimate, not structural advice. Keep 3 inches of concrete cover between rebar and soil.
Frequently asked questions
How much rebar do I need for a 10x10 slab?
A 10×10 ft slab with #4 bar at 18-inch spacing needs 16 bars (8 each way), 160 linear feet total, about 107 lb — eight 20-ft sticks cut in half. Add 5% waste and order 9 sticks.
What size rebar do I need for a concrete slab?
#3 (3/8 in) or #4 (1/2 in) covers most residential slabs on grade, with #4 the common default. Anything structural — footings, retaining walls, suspended slabs — should be sized by an engineer.
How far apart should rebar be spaced in a slab?
12 to 24 inches on center is typical for slabs on grade; 18 inches is the common residential default. Use 12-inch spacing for heavier loads and thinner slabs.
How much does rebar weigh per foot?
#3 bar is 0.376 lb/ft, #4 is 0.668 lb/ft, and #5 is 1.043 lb/ft. Multiply by total linear feet for the haul weight.
Can I use wire mesh instead of rebar?
For light-duty slabs, 6×6 welded wire mesh works — but only if chaired up into the middle of the slab. Mesh left on the ground does nothing. Rebar is stronger and more forgiving for driveways and loaded slabs.