How much rebar do I need for a slab?
Rebar in a slab is a grid. Take off the edge clearance on each side, divide each direction by the spacing, and add one bar. Multiply the bars by their length, add laps where a run is longer than a stick, and divide by the stick length.
Bars each way = (slab width − 2 × edge clearance) ÷ spacing, rounded down, + 1
A 20 × 20 ft slab with #4 bar at 18 in each way and 3 in edge clearance: the grid is 19.5 ft square. 19.5 × 12 ÷ 18 = 13, plus 1 = 14 bars each way. That's 28 bars at 19.5 ft, or 546 linear feet. Each bar fits on one 20 ft stick, so buy 28 sticks. The grid has 14 × 14 = 196 intersections to tie, and the steel weighs about 365 lb.
Rebar calculator for slab: laps and sticks
When a run is longer than one stick, bars are overlapped and tied. The overlap is set in bar diameters. 40 diameters is a common figure for slabs, which is 20 in for #4 bar. Your plans or engineer may call for a different lap, so change it to match.
A 30 × 20 ft slab at the same spacing needs 14 bars running the long way, each 29.5 ft. Each one takes two sticks with a 20 in lap, so it uses 31.2 ft of bar. Add 20 short bars at 19.5 ft and the total is 826 linear feet, 42 sticks and 14 lap splices.
The calculator counts sticks piece by piece. If two short bars fit on one stick, it uses one stick for both. Add a few percent extra if you expect to cut around openings or bends.
Rebar weight and size
| Bar | Diameter | Weight per ft | Weight per 20 ft stick | 40-diameter lap |
|---|---|---|---|---|
| #3 | 3/8 in | 0.376 lb | 7.5 lb | 15 in |
| #4 | 1/2 in | 0.668 lb | 13.4 lb | 20 in |
| #5 | 5/8 in | 1.043 lb | 20.9 lb | 25 in |
The bar number is its diameter in eighths of an inch. Weights are the standard figures for ASTM A615 bar.
Rebar spacing
Residential slabs often use #3 or #4 bar at 12 to 18 in each way, but there isn't one right answer. Spacing, size and whether you need rebar at all depend on the slab, the soil and the loads. Take them from your plans or ask your building department. The calculator counts material for the layout you enter. It doesn't design the slab.
Ties and chairs
Most crews tie every intersection, or every other one on a large grid with bars that won't be walked on much. Pre-cut ties or a reel of 16 gauge wire both work. A 3.5 lb coil holds about 330 ft of wire, which the calculator counts as 400 ties. Change it if you use longer ties.
Chairs hold the grid up so the steel ends up inside the concrete, not on the ground. The calculator places one about every 3 ft each way. Use taller chairs or dobies for thicker slabs and soft base.
Pricing a rebar job
Concrete contractors usually carry rebar as a line on the slab quote: sticks, tie wire and chairs at cost plus markup, with labor to cut, place and tie. Switch to contractor mode, then add the slab itself from the concrete calculator to the same job and send it as an estimate.
Questions
How much does rebar weigh per foot?
#3 bar weighs 0.376 lb per foot, #4 weighs 0.668 lb and #5 weighs 1.043 lb. A 20 ft stick of #4 is about 13.4 lb.
How much should rebar overlap?
A common figure is 40 bar diameters, which is 20 inches for #4 bar. Follow your plans or engineer if they give a different lap.
How many ties do I need for rebar?
One per intersection if you tie every crossing. A 20 x 20 ft slab with bars 18 inches apart each way has 196 intersections.
How far should rebar be from the edge of a slab?
Edge clearance of about 2 to 3 inches is common so the steel stays covered by concrete. Check your plans for the required cover.