Concrete Rebar Calculator 2026 – Rebar Length & Quantity Tool

Rebar Selection and Spacing Guide

Rebar (reinforcing bar) is steel reinforcement placed in concrete to increase tensile strength and prevent cracking. Proper rebar size, spacing, and placement are critical for structural integrity. This guide covers rebar selection, spacing requirements, and calculation methods for residential and commercial concrete projects in the USA.

Rebar Size Selection by Project Type

Rebar is designated by numbers indicating diameter in eighths of an inch. #4 rebar is 4/8 inch (1/2 inch) diameter. For residential concrete slabs 4-6 inches thick, use #3 or #4 rebar. Driveways and garage floors typically require #4 rebar at 18 inches on center. For heavy-duty applications like commercial slabs or areas with heavy vehicle traffic, upgrade to #5 or #6 rebar at closer spacing.

Footings require 2-4 horizontal bars running the length of the footing. Use #4 or #5 rebar for standard residential footings 12-16 inches wide. Walls need vertical and horizontal reinforcement, typically #4 or #5 bars at 12-18 inch spacing. Columns require vertical bars (#5-#8) with horizontal ties every 12-16 inches. For detailed slab calculations, see our concrete slab calculator.

Standard Rebar Spacing Requirements

Spacing closer than 12 inches makes concrete placement difficult. Spacing wider than 24 inches may not provide adequate crack control. Standard residential slabs use 18 inch spacing, which balances material cost with structural performance. For cost estimation of complete projects, visit our concrete driveway calculator.

Rebar Placement and Cover Requirements

Rebar must be positioned at mid-depth of slabs, typically 2 inches from the bottom of a 4-inch slab. Maintain minimum 3 inches of concrete cover from rebar to any exposed surface to prevent corrosion. Use rebar chairs or supports every 3-4 feet to hold rebar at proper height during concrete pouring. For slabs on ground, place rebar on 2-3 inch chairs. For suspended slabs, maintain 3/4 inch bottom cover and 3/4 inch top cover.

Lap Splice Length and Connections

When rebar bars must be joined, they overlap in a lap splice. Standard lap splice length is 40 times the bar diameter for Grade 60 steel. For #4 rebar (0.5 inch diameter), lap 20 inches. For #5 (0.625 inch), lap 25 inches. In high-stress areas or seismic zones, increase lap length to 48-60 times diameter. Always tie lap splices with wire at multiple points to prevent separation during concrete pour.

Place lap splices away from high-stress areas and stagger them so adjacent bars don't splice at the same location. For walls and columns, offset splices by at least 24 inches vertically. Horizontal lap splices in slabs should be offset by at least 2 feet to avoid creating weak planes. For foundation work, check our concrete footing calculator.

Rebar Weight and Cost in 2026

Rebar is sold by the piece (20 ft standard length) or by weight. A 20 ft piece of #4 rebar weighs 13.4 pounds and costs $17-$22 in 2026. Larger orders (1000+ lbs) receive bulk discounts of 10-20%. Prices vary by region, with urban areas typically 15-30% higher than rural markets. For complete material estimates including concrete volume, use our concrete calculator.

Tie Wire and Rebar Chairs

Secure all rebar intersections with 16-gauge tie wire to prevent movement during concrete placement. Each grid intersection requires one tie. A 20×20 ft slab with 18 inch spacing has approximately 169 intersections requiring 169 ties. Tie wire comes in 3.5 lb rolls containing approximately 350 ties, costing $10-$20 per roll.

Rebar chairs maintain proper rebar height within the concrete. For slabs, use chairs every 3-4 feet along each bar. Individual plastic chairs cost $0.25-$0.50 each. Metal continuous chair bar costs $15-$25 per 10 ft section. A 400 sq ft slab needs 80-100 individual chairs or 50-60 ft of continuous chair bar. For wire mesh alternatives, see our wire mesh calculator.