Rebar Calculator for Concrete Slabs (Bars, Spacing and Weight)

Quick answer: a 20 ft x 20 ft slab with #4 rebar at 16 inches each way, 3 inches in from the edges, needs 16 bars each way, about 624 ft of rebar in total. That is 32 bars of 20 ft, weighing about 417 lb.

Rebar calculator for slabs

Slab length (ft)

Slab width (ft)

Bar spacing, each way (in) 681012161824

Bar size #3 (3/8")#4 (1/2")#5 (5/8")#6 (3/4")

Edge distance (in)

Stock bar length 20 ft40 ft10 ft

Laps are taken as 40 bar diameters (#3 = 15", #4 = 20", #5 = 25", #6 = 30"), a common rule of thumb; your plans or local code may require more. Weights use standard ASTM A615 values. Cutting waste is not included, so round up when ordering.

Rebar sizes and weights

Bar	Diameter	Weight per ft	20 ft bar weighs	Lap (40 x diameter)
#3	3/8"	0.376 lb	7.5 lb	15"
#4	1/2"	0.668 lb	13.4 lb	20"
#5	5/8"	1.043 lb	20.9 lb	25"
#6	3/4"	1.502 lb	30.0 lb	30"

How the calculation works

1. Bars running the long way = (width − 2 × edge distance) ÷ spacing, rounded up, plus 1.
2. Each bar's length = slab length − 2 × edge distance. Bars longer than the stock length are spliced with a lap.
3. Repeat for the other direction, add everything up, and divide by the stock length to get the number of bars to buy.

Common questions

Where should rebar sit in a slab on the ground?

Raise it on chairs or concrete blocks so it ends up near the middle of a 4 inch slab, not lying on the dirt. Rebar on the ground does almost nothing.

What spacing is common for a patio or driveway?

Many residential slabs use #3 or #4 bars at 12 to 18 inches each way. Follow your plans or local practice, especially for driveways that carry trucks.

How many chairs do I need?

Plan on a chair every 3 to 4 ft in each direction, so roughly one per 12 sq ft of slab.

Checked by: Engr. Abdul Latif, civil and structural engineer. Bar weights are ASTM A615 values; lap length is a common rule of thumb. Estimates for planning only.