How thick of a rebar do I need?

#3 (⅜") for thin slabs and patios. #4 (½") for driveways and standard slabs. #5 (⅝") for foundations and structural slabs. Always check engineering specs for structural work.

Should I tie or weld the bars?

Tie. Welding rebar can change its tensile properties and is rarely allowed without an engineer signing off.

How much overlap is required at splices?

A common rule of thumb is 40 × bar diameter — about 20 inches for #4 bar, 25 inches for #5. Local codes can vary.

Do I need rebar for a 4-inch patio?

For a non-structural pad, welded wire mesh is often used instead and is faster. Rebar is needed for driveways under heavy load and structural slabs.

How thick of a rebar do I need?

#3 (⅜") for thin slabs and patios. #4 (½") for driveways and standard slabs. #5 (⅝") for foundations and structural slabs. Always check engineering specs for structural work.

Should I tie or weld the bars?

Tie. Welding rebar can change its tensile properties and is rarely allowed without an engineer signing off.

How much overlap is required at splices?

A common rule of thumb is 40 × bar diameter — about 20 inches for #4 bar, 25 inches for #5. Local codes can vary.

Do I need rebar for a 4-inch patio?

For a non-structural pad, welded wire mesh is often used instead and is faster. Rebar is needed for driveways under heavy load and structural slabs.

Rebar Spacing & Quantity Calculator

Linear feet of rebar needed for a slab grid, given dimensions and on-center spacing.

Result

Rebar needed

546 linear ft

34 bars total at 12" O.C.

Long bars (run along length)13 × 21.0 ft = 273 LF
Short bars (run across width)21 × 13.0 ft = 273 LF
Slab area240 ft²

Step-by-step

Long bars: ⌈12 ft ÷ 12" spacing⌉ + 1 = 13 bars, each 21.0 ft (incl. overlap).
Short bars: ⌈20 ft ÷ 12" spacing⌉ + 1 = 21 bars, each 13.0 ft.
Total = 273 + 273 = 546 linear feet.

How to use this calculator

Enter slab length and width.
Set the on-center spacing (typically 12-18 in).
Adjust overlap if your slab spans more than one rebar length (rebar comes in 20-ft sticks typically).

About this calculator

A rebar grid for a slab needs one bar at each spacing interval in both directions, plus one at the edges. The overlap accounts for splice length where two pieces meet (typically 40 × bar diameter, or 1-2 ft for #4/#5 bar). Add waste yourself when you order — 5-10% covers cut-offs.