What does rebar size mean (e.g., #3, #4)?

Rebar sizes are designated by number, which corresponds to the bar diameter in eighths of an inch. A #4 bar is 4/8 = 0.5 inches in diameter. Common sizes for slabs are #3 (3/8") and #4 (1/2").

What spacing should I use for a concrete slab?

Typical residential slab spacing is 12 to 18 inches on center. Driveways often use 12" spacing with #3 or #4 rebar. Structural slabs may require tighter spacing — always consult local building codes and your engineer.

What is the waste percentage for?

Rebar must be cut to fit and overlapped at splices (typically 12–18 bar diameters). A 10–15% waste allowance covers these cuts and lap splices in typical projects.

How is rebar weight calculated?

Each rebar size has a standard weight per linear foot (e.g., #4 = 0.668 lb/ft). Total weight equals the total linear feet of all bars multiplied by the weight per foot for the selected size.

Does this calculator include both directions?

Yes. The calculator places bars in both the longitudinal (along the length) and transverse (along the width) directions, as is standard for two-way reinforced slabs.

Can I use this for a wall instead of a slab?

Yes. Treat the wall height as the "width" and wall length as the "length." The spacing and bar count logic is identical for vertical rebar in walls.

Rebar Calculator

Name: Rebar Calculator
Availability: InStock
Author: Nham Vu

Calculate the number of rebar bars, total weight, and spacing for a concrete slab or wall.

Slab / Wall Dimensions

Length (ft)

Width (ft)

Rebar Size

Spacing — Length dir. (in)

Spacing — Width dir. (in)

Waste / Overlap Allowance (%)

Results

Enter dimensions and press Calculate

Summary

Calculate the number of rebar bars, total weight, and spacing for a concrete slab or wall.

How it works

Enter the length and width of your concrete slab or wall in feet.
Select the rebar size (e.g., #3, #4, #5) from the dropdown.
Set the desired spacing between bars in inches for both directions.
Click Calculate to get the number of bars needed in each direction.
Review the total linear feet, estimated weight, and bar count summary.
Add a waste percentage to account for cuts and overlaps.

Use cases

Estimating rebar quantities for a concrete driveway or patio slab.
Planning reinforcement for foundation walls and footings.
Ordering steel for retaining walls and basement floors.
Budgeting material costs for residential or commercial concrete pours.
Calculating rebar weight for structural load calculations.
Comparing bar spacing options to optimize material use.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu