How Much Rebar for a Slab Grid?
A 20ร20 ft slab with #4 bars at 18 in o.c. each way needs 15 bars per direction โ 30 bars of 20 ft, plus lap splices where 20-footers meet the edges. At 0.668 lb per foot for #4, that is about 400 lb of steel plus laps, chairs, and ties โ roughly 460 lb on the truck. Here is the grid method estimators use.
Step-by-Step Takeoff
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1
Confirm bar size and spacing from the plan or span tables (#4 at 18 in o.c. is typical residential).
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2
Bars each way: 20 ร 12 รท 18 + 1 = 15 bars (30 total).
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3
Length: 30 ร 20 ft = 600 lin ft + lap allowance (~40 bar diameters per splice).
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4
Weight: 600 ร 0.668 lb/ft (#4) โ 400 lb, plus ~15% for laps, chairs, and waste.
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5
Chairs: one per 4 sq ft of slab to hold steel mid-depth; tie every intersection on the perimeter.
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6
Keep 2โ3 in of cover top and bottom so bars never daylight and rust.
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7
Model spacing, bar size, and laps exactly in the Rebar Calculator.
Guide FAQ
What spacing for slab rebar?
18 in o.c. each way is typical for residential slabs; 12 in for heavy loads.
How long should lap splices be?
About 40 bar diameters for Grade 60 in normal concrete โ 20 inches for #4 bar.
How much does #4 rebar weigh per foot?
0.668 lb/ft โ a 20-footer is about 13.4 lb, a one-person carry.
Rebar or wire mesh for a slab?
Rebar for structural slabs and poor soils; 6ร6 mesh suffices for light interior floors on grade.
What are chairs and why do they matter?
Chairs hold steel at mid-slab depth during the pour โ steel lying on the ground does nothing.
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