Rebar Size Calculator — Choose Bar Diameter
Determine the right rebar size (#3 through #8) for any slab, footing, or column application based on load requirements and ACI 318 code provisions.
Enter Dimensions
Add 10% for standard cuts and overlaps
Results
Bars Needed
38bars
Total Lineal Feet
608lin ft
Total rebar length including waste
Total Weight
406lb
#4 rebar weight per CRSI
Slab Area
240sq ft
Editorial Standards
Each calculator is reviewed for formula accuracy, unit consistency, and alignment with current U.S. building practices before publication. We verify outputs against published engineering references and real-world project data. Learn more about our methodology.
Picking the right rebar size is the first material decision on any concrete project. Too small: insufficient steel for the load, risk of structural failure. Too large: wasted material, harder to place, and you've over-engineered the project. The right choice balances code minimums, load demands, and labor practicality.
This calculator references the typical residential and light commercial rebar sizes (#3 through #6) and the applications they serve. For structural/engineered concrete, defer to the project structural drawings.
How to Choose Rebar Size
Rebar Size Selection by Application
- #3 (3/8″ dia, 0.376 lb/ft): light slabs, garden walls, decorative concrete, ties and stirrups.
- #4 (1/2″ dia, 0.668 lb/ft): THE residential default — slabs on grade, light footings, free-standing CMU retaining walls.
- #5 (5/8″ dia, 1.043 lb/ft): wider footings, load-bearing CMU walls, reinforced concrete walls under 8 ft.
- #6 (3/4″ dia, 1.502 lb/ft): heavy footings, columns, reinforced concrete walls 8+ ft, commercial slabs.
- #7-#8 (7/8″-1″ dia): structural columns, engineered beams, basement walls with high surcharge — always specified by an engineer.
Frequently Asked Questions
What size rebar for a 4 inch slab?
#4 rebar (1/2 inch diameter) at 12-18 inch on-center grid is standard for residential 4 inch slabs on grade. #3 (3/8 inch) is acceptable for very light loads (sheds, walkways). #5 (5/8 inch) for heavier loads or larger slabs.
What size rebar for footings?
#4 minimum for 12 inch wide residential footings. #5 for 18-24 inch wide footings. #6 for 30 inch+ wide footings or where the engineer specifies higher steel area.
What size rebar for retaining walls?
Vertical bars in CMU retaining walls: #4 or #5 at 16-32 inch on-center spacing per engineered design. Horizontal bars in bond beams: #4 in every other course. Higher walls (8+ ft) and surcharge loads call for #5 to #7.
Which is cheaper, more small bars or fewer large bars?
Tightly-spaced small bars (#4 at 6 inch OC) often beat widely-spaced large bars (#6 at 12 inch OC) on material cost because larger bars cost more per pound. But labor (more bars to place and tie) flips the balance — verify based on local labor rates.
Is rebar size measured in inches or millimeters?
U.S. rebar uses imperial sizing: bar number = 1/8 inch. #4 = 4/8 = 1/2 inch diameter. #5 = 5/8 inch. #6 = 6/8 = 3/4 inch. Metric rebar uses actual mm diameter (M10, M15, etc.) but is rare in U.S. residential work.
Looking for the general calculator?
Calculate rebar quantity, spacing, and total weight for concrete slabs, footings, and walls. Covers #3 through #8 bar sizes using CRSI published weight tables.
Open the Rebar Calculator: Bar Count & Weight →