How it is calculated
For a grid, the calculator subtracts the concrete cover at the edges, divides by the spacing and gets the bars in each direction. Bars longer than the stock length get a lap of 40 bar diameters. Weight uses standard bar weights. Bars to buy are chosen to keep offcuts small.
Formula
Bars along the length = (width − 2 × cover) ÷ spacing + 1Weight = linear feet × lb per foot (#4 = 0.668)Lap splice = 40 × bar diameterExample
A 20 × 12 ft slab with one grid of #4 at 18 in on center needs 22 bars, 321 linear ft and 214 lb. With 20 ft stock that is 22 bars and about a pound of tie wire.
Tips from practice
- Support the grid on chairs so it sits in the middle of the slab, not on the ground.
- Stagger the laps of neighboring bars.
- For thin slabs on grade, welded wire mesh or fibers may be enough; check your plans.
Questions and answers
How much does #4 rebar weigh?
0.668 lb per foot, so a 20 ft bar weighs 13.4 lb. #3 is 0.376 lb/ft and #5 is 1.043 lb/ft.
What spacing for a slab?
Driveways and garage slabs often use #3 or #4 at 12–18 in on center each way.
How much tie wire?
About 1.5 ft of wire per tie. The calculator counts the ties and converts them to pounds.
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