Rebar Calculator

How to Calculate Rebar Quantity

Estimating rebar for a concrete slab involves calculating how many bars run in each direction based on the spacing.

1. Count Bars Per Direction: Divide the perpendicular dimension (in inches) by the spacing, then add 1.
   Bars = (Dimension (in) ÷ Spacing (in)) + 1
2. Total Linear Feet: For each direction, multiply the bar count by the slab dimension in that direction.
   Total Length = (Long Bars × Length) + (Short Bars × Width)
3. Apply Waste & Layers: Multiply by number of layers (1 for single mat, 2 for top + bottom) and add 5–10% for lap splices and cutoff waste.

Example Calculation

For a 20 ft × 12 ft slab with #4 rebar at 18 inches on-center:

• Long bars: (12 ft × 12 ÷ 18) + 1 = 9 bars × 20 ft = 180 linear ft
• Short bars: (20 ft × 12 ÷ 18) + 1 = 14 bars × 12 ft = 168 linear ft
• Total: 180 + 168 = 348 linear ft
• Weight: 348 ft × 0.668 lb/ft (#4) ÷ 2000 = 0.12 tons
• With 5% waste: 365 linear ft, ~0.13 tons

Common Rebar Mistakes

• Rebar too close to the surface: Rebar must be placed at mid-depth of the slab (2 inches from bottom for a 4-inch slab). Use concrete chairs or dobies to support bars off the ground.
• Insufficient overlap at splices: When bars need to be extended, the lap splice must be at least 40 bar diameters. For #4 rebar, that is 20 inches of overlap.
• Forgetting corner reinforcement: Slab corners are the first place cracks appear. Add diagonal bars or corner reinforcing to prevent 45-degree corner cracks.

Frequently Asked Questions