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Rebar Calculator

Calculate rebar quantity and spacing for concrete slabs.

Last updated: June 13, 2026

Calculator

Slab Length

feet

Slab Width

feet

Rebar Spacing

inches

Results

Total Rebar Pieces42

Lengthwise Bars21 × 20 ft

Widthwise Bars21 × 20 ft

Total Linear Feet840 ft

20ft Bars to Buy42

How to Use This Calculator

1Enter slab length and width in feet.
2Enter desired rebar spacing in inches.
3View total bars needed and linear footage.

Formula Explanation

Rebar is laid in a grid pattern. The number of bars in each direction depends on the slab dimension divided by spacing.

Bars = (Dimension ÷ Spacing) + 1 (in each direction)

Variables:

DimensionsSlab length and width in feet

SpacingDistance between rebar bars in inches

Worked Examples

20×20 ft slab, 12" spacing

1. Lengthwise: (20/1)+1 = 21 bars × 20ft
2. Widthwise: (20/1)+1 = 21 bars × 20ft
3. Total: 42 bars, 840 linear feet

Result: 42 bars, 42 twenty-foot pieces to buy

Why Use This Calculator

Calculate exact rebar quantities.
Minimize waste and over-ordering.
Ensure proper spacing for code compliance.
Estimate material costs.

336 × 280 Large Rectangle

300 × 250 Medium Rectangle

Tips & Best Practices

Standard rebar spacing for residential slabs is 12-18 inches.
#4 rebar (1/2") is most common for residential work.
Overlap bars by 24-30 diameters at splices.
Rebar should sit at 1/3 from the bottom of the slab.

Common Mistakes to Avoid

❌ Not accounting for overlap at bar splices.

✅ When bars need to be joined, they must overlap by at least 24 bar diameters (12" for #4 rebar). Add 10% extra material.

❌ Placing rebar directly on the ground.

✅ Use rebar chairs or supports to position rebar at the correct height within the slab (usually 1/3 from the bottom).

Frequently Asked Questions

What size rebar for a driveway?

#4 rebar (1/2" diameter) at 12-18" spacing is standard for residential driveways and patios. Heavier loads may require #5 rebar.

Do I need rebar in a concrete slab?

For driveways and structural slabs, yes. For small decorative pads or walkways, wire mesh or fiber reinforcement may suffice. Check local building codes.

336 × 280 Large Rectangle

300 × 250 Medium Rectangle

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Disclaimer: This calculator is provided for informational and educational purposes only. Results are estimates and should not be used as a substitute for professional advice. RediCalc.com makes no guarantees regarding the accuracy, completeness, or suitability of any calculations. Always consult a qualified professional before making financial, health, construction, or other important decisions.

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