Rebar Calculator

How to use the rebar calculator

1. Enter the slab length and width in feet, measured to the inside of the forms.
2. Enter the grid spacing from your plans or permit. Residential slabs are often reinforced at 12, 16 or 18 inches on center each way; driveways and footings follow their own details.
3. Set the edge clearance, the distance from the form to the first bar. Concrete cast against soil needs about 3 inches of cover over the steel (ACI 318), so 3 inches is the default.
4. Pick the bar size and stick length. Home centers sell 10 and 20 ft sticks; supply yards also stock 40 ft.
5. Enter the lap splice length used where two sticks overlap. Your plans govern; a rule of thumb often quoted for small residential work is 40 bar diameters (20 inches for #4).

The rebar formula

Bars one way = floor((width − 2 × edge) × 12 ÷ spacing) + 1

Length of each bar = (length − 2 × edge) + laps

Weight = total feet × unit weight (lb/ft)

The same count is repeated in the other direction, and the two totals are added. When a bar run is longer than one stick, the calculator adds a lap for every joint, then divides the total footage (plus waste) by the stick length. Short offcuts from one run can usually be reused for the next, which is why the stick count works from total feet.

Rebar sizes and weights

U.S. bar numbers are eighths of an inch of nominal diameter. Nominal weights from the ASTM A615 bar table, as published by the Concrete Reinforcing Steel Institute (CRSI):

• #3: 3/8 in, 0.376 lb/ft (7.5 lb per 20 ft stick)
• #4: 1/2 in, 0.668 lb/ft (13.4 lb per stick)
• #5: 5/8 in, 1.043 lb/ft (20.9 lb per stick)
• #6: 3/4 in, 1.502 lb/ft (30.0 lb per stick)

CRSI notes that ASTM allows delivered bars to weigh up to 6% less than nominal, so the weight shown is a safe upper estimate for hauling.

Worked example: 24 × 24 ft garage slab

#4 bar at 18 inches on center, 3 inch edge clearance, 20 ft sticks, 20 inch laps, 5% waste:

• Bar run: 24 − 0.5 = 23.5 ft in each direction
• Bars each way: floor(23.5 × 12 ÷ 18) + 1 = 15 + 1 = 16, so 32 bars total
• Each 23.5 ft run needs two sticks and one 20 inch lap: 23.5 + 1.67 = 25.17 ft
• Total: 32 × 25.17 = 805 ft; with 5% waste 846 ft = 43 sticks of 20 ft
• Weight: 805 × 0.668 = about 538 lb
• Intersections to tie: 16 × 16 = 256

Rebar vs. wire mesh in a slab

Welded wire reinforcement is faster to roll out, but it is hard to keep at mid-depth, and many contractors prefer a rebar grid on chairs for driveways and garage floors because it stays where it is placed. Neither replaces proper base preparation and control joints; reinforcement mainly holds cracks tight once they form.

Placement tips

• Support the grid on chairs or dobies so the steel sits in the middle to upper third of the slab, not on the ground.
• Tie intersections with wire; tying every second or third crossing is common for slabs, but tie all of them where workers will walk.
• Stagger lap splices so joints in neighboring bars do not line up.
• Order concrete separately: the cubic yard calculator gives slab volume, and a 4 inch slab needs about 1.23 cubic yards per 100 sq ft.
• Structural footings, retaining walls and anything on engineered plans should follow those plans; this tool estimates quantities, not design.

Frequently asked questions