How do I calculate how much rebar I need for a slab?

Enter the slab length and width, choose your bar spacing, edge offset, and cover depth. The calculator returns bar count in each direction, total length in feet, and total weight in pounds. For a typical residential slab, #4 rebar at 12-inch spacing in both directions is a common starting point — always verify with your local building code or engineer.

What is the weight of rebar per foot by size?

#3 rebar weighs 0.376 lb/ft. #4 rebar weighs 0.668 lb/ft. #5 rebar weighs 1.043 lb/ft. #6 rebar weighs 1.502 lb/ft. #7 rebar weighs 2.044 lb/ft. #8 rebar weighs 2.670 lb/ft.

How much does rebar cost per foot?

Rebar typically costs $0.40-$0.80 per linear foot for common sizes (#3-#5) from a steel supplier in 20-foot sticks. Retail pricing at home improvement stores runs 30-50% higher.

What rebar spacing is required for a concrete slab?

For residential slabs on grade, 12-inch spacing with #4 rebar is a common standard. Structural slabs or areas with heavy loads may require 8-inch or 6-inch spacing with larger bar sizes. Always check your local building code.

Do I need to include lap splices in my rebar calculation?

This calculator focuses on straight-length takeoffs. If your slab or wall is longer than standard 20-foot bar lengths, you will need lap splices - typically 40-60 times the bar diameter (20-25 inches for #4 rebar). Add a 10-15% waste allowance to cover splices, corner hooks, and cuts.

What size rebar do I need for a concrete driveway or patio?

#3 or #4 rebar is standard for residential driveways and patios. #4 (1/2 inch diameter) is better for driveways that carry vehicle weight. Spacing is typically 12 inches on center in both directions.

Rebar Calculator – Quantity, Spacing & Weight for Slabs, Footings & Walls

Unit system

Project type

Slab inputs

Length (ft)

Width (ft)

Bottom cover (in)

Edge offset (in)

Bar spacing (in)

Bar size

Layers

Waste allowance (%)

Concrete Reinforcement Planning Checklist

Reviewed by Maria Ramirez, PE. Covers rebar placement, cover requirements, tie wire, chair spacing, lap splice allowances, and inspection steps before pouring.

Download Checklist (PDF)

Planning reference only. See Methodology and Data Sources. View all project checklists →

Rebar Calculator — Quantity, Weight & Cost Guide

This calculator estimates rebar bar count, total length, weight in pounds, and material cost for slabs, footings, and walls. Use the breakdown by group to generate a material list for ordering. Always confirm spacing and cover requirements with your local building code or a structural engineer.

Rebar Weight & Cost Reference Table

Bar size	Diameter	Weight (lb/ft)	Weight (kg/m)	Est. cost/ft	Common use
#3	3/8"	0.376	0.560	$0.25–$0.45	Light slabs, small footings
#4	1/2"	0.668	0.994	$0.40–$0.65	Residential slabs, driveways, footings
#5	5/8"	1.043	1.552	$0.60–$0.95	Structural slabs, walls, columns
#6	3/4"	1.502	2.235	$0.85–$1.30	Heavy slabs, retaining walls
#7	7/8"	2.044	3.042	$1.10–$1.70	Structural beams, columns
#8	1"	2.670	3.973	$1.40–$2.10	Heavy structural applications

Costs are estimates for standard mill stock purchased from a steel supplier in 20-ft lengths. Retail pricing at home improvement stores runs 30–50% higher.

Common Rebar Spacing Guide

Application	Typical bar size	Typical spacing	Cover
Residential patio slab	#3 or #4	12" each way	1.5"–2" bottom
Concrete driveway	#4	12" each way	2" bottom
Garage slab	#4	12" each way	2" bottom
Strip footing	#4 or #5	3 bars longitudinal + ties @12"	3" all sides
Foundation wall	#4 or #5	16" vertical, 16" horizontal	2" face
Retaining wall	#5 or #6	12" vertical, 16" horizontal	2"–3" face

Tips for accurate rebar takeoffs

Add 10–15% waste for cuts, laps, corner hooks, and offcuts on longer runs
Rebar is sold in 20-foot sticks, plan your layout to minimise offcuts
Chairs and bar supports are separate from the rebar quantity, allow one chair per 4 sq ft for slabs
For projects over 1,000 sq ft, buying from a steel supplier or rebar fabricator saves significantly over home improvement stores
Tie wire is typically sold by the spool, 3.5 lb spools cover roughly 1,000 ties

Frequently Asked Questions

Enter the slab length and width, choose your bar spacing, edge offset, and cover depth above. The calculator returns bar count in each direction, total length, and total weight. For a typical residential slab, #4 rebar at 12-inch spacing in both directions is a common starting point — always verify with your local building code.

Common sizes: #3 = 0.376 lb/ft, #4 = 0.668 lb/ft, #5 = 1.043 lb/ft, #6 = 1.502 lb/ft. See the reference table above for all sizes. Weight matters for ordering by the ton from steel suppliers and for calculating delivery truck loads.

Rebar typically costs $0.40–$0.80 per foot for #3–#5 from a steel supplier in 20-foot sticks. Home improvement stores run 30–50% higher. For large projects (10+ tons), buying directly from a steel service center or rebar fabricator is significantly cheaper.

12-inch spacing with #4 rebar is common for residential slabs on grade. Structural slabs, driveways, or areas with heavy loads may require 8-inch or 6-inch spacing with larger bar sizes. Always check your local building code.

This calculator covers straight-length takeoffs. If your project is longer than standard 20-foot bar lengths, you'll need lap splices — typically 40–60 times the bar diameter (about 20–25 inches for #4). Add a 10–15% waste allowance to your result to cover splices, corner hooks, and cuts.

#4 rebar is standard for residential driveways that carry vehicle weight. #3 works for light patios and walkways. Space at 12 inches on center in both directions. For a 4-inch slab, position one layer centered at 2 inches from the bottom.

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Accuracy & Review

Reviewed by: Maria Ramirez, PE

Maria is a licensed Professional Engineer with experience in residential and light commercial concrete quantity takeoffs. She reviewed the formulas, unit conversions, bar weight data, and default assumptions used in this calculator.

Last updated: Feb 12, 2026

See: Methodology · Data Sources · Review Board

Disclaimer: Results are for general planning only and do not constitute engineering or structural design advice. Actual quantities vary based on site conditions, design requirements, and local building codes. Structural applications should be reviewed by a qualified professional.

See Methodology and Data Sources for details.