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.
Frequently asked
The distance from the center of one bar to the center of the next. 12" O.C. means a bar every 12 inches.
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