Free Rebar Calculator
Calculate how many pieces of rebar you need for a concrete slab or footing based on spacing and dimensions.
On center spacing, typically 12" to 24"
Standard rebar comes in 20 ft lengths
Total Bars (both directions)
28
Total Bars (both directions) vs Slab Length
How to Calculate Rebar Needed
Formula
Bars Along Length = floor(Width / Spacing) + 1
Bars Along Width = floor(Length / Spacing) + 1
Rebar is laid in a grid pattern (both directions). Standard rebar comes in 20-foot lengths. Overlap joints should be at least 24 bar diameters (about 18 inches for #4 rebar). Common sizes are #3 (3/8"), #4 (1/2"), and #5 (5/8").
Example Calculation
A 20×20 ft slab with rebar on 18-inch centers, using 20 ft rebar.
- 01Spacing in feet = 18 / 12 = 1.5 ft
- 02Bars along length = floor(20 / 1.5) + 1 = 14 bars (each 20 ft long)
- 03Bars along width = floor(20 / 1.5) + 1 = 14 bars (each 20 ft long)
- 04Total bars = 14 + 14 = 28 bars
- 05Total linear feet = 14 × 20 + 14 × 20 = 560 lin ft
- 06Rebar sticks = 560 / 20 = 28 sticks
Frequently Asked Questions
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