Rebar Radius Calculator: Minimum Bend Diameter by Bar Size
Calculate the minimum inside bend diameter and radius for reinforcing bars used as stirrups, ties, main bars, or standard hooks. Values follow ACI 318-19 Tables 25.3.1 and 25.3.2 for bar sizes No. 3 through No. 18. Pair this with the rebar development length calculator when detailing hooked bars.
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Rebar Weight Chart
US Standard Rebar Sizes (#2–#18) with weight per foot, diameter, and cross-sectional area.
View Chart →How the Bend Diameter Rule Works
Pick the Application
Stirrups and ties follow a tighter table than main longitudinal bars because they carry less strain concentration.
Match the Bar Size
The multiplier jumps at No. 6, No. 9, and No. 14 as the code accounts for higher strain in larger-diameter bars.
Get Diameter and Radius
The calculator multiplies bar diameter by the code factor, then halves it for the radius your fabricator needs.
Verify Against Shop Mandrel
Compare your actual mandrel size in the advanced panel to confirm your fabrication setup meets the code minimum.
Bend Diameter Multipliers by Bar Size
ACI 318-19 sets the minimum inside bend diameter as a multiple of the nominal bar diameter (db). The multiplier depends on whether the bar is a stirrup/tie or a main bar, and increases for larger bar sizes.
| Bar Size | db (in) | Stirrup/Tie Min. Diameter | Main Bar Min. Diameter | Main Bar Multiplier |
|---|---|---|---|---|
| #3 | 0.375 | 1.50 in (4db) | 2.25 in (6db) | 6db |
| #4 | 0.500 | 2.00 in (4db) | 3.00 in (6db) | 6db |
| #5 | 0.625 | 2.50 in (4db) | 3.75 in (6db) | 6db |
| #6 | 0.750 | 4.50 in (6db) | 4.50 in (6db) | 6db |
| #7 | 0.875 | 5.25 in (6db) | 5.25 in (6db) | 6db |
| #8 | 1.000 | 6.00 in (6db) | 6.00 in (6db) | 6db |
| #9 | 1.128 | N/A | 9.02 in (8db) | 8db |
| #10 | 1.270 | N/A | 10.16 in (8db) | 8db |
| #11 | 1.410 | N/A | 11.28 in (8db) | 8db |
| #14 | 1.693 | N/A | 16.93 in (10db) | 10db |
| #18 | 2.257 | N/A | 22.57 in (10db) | 10db |
Source: ACI 318-19 Table 25.3.1 (main bars, standard hooks) and Table 25.3.2 (stirrups and ties). Stirrup/tie provisions do not extend to No. 9 through No. 18 bars, which are rarely used as transverse reinforcement.
Bend Diameter vs Bend Radius, and Why It Matters
Rebar fabricators bend bars around a steel mandrel or pin. The inside dimension of that bend is the bend diameter. ACI 318-19 lists minimums as diameters, not radii, so shop drawings sometimes need a quick conversion.
Bending a bar tighter than code minimum stretches the outer fiber of the steel past its safe strain limit. That can crack epoxy coating, reduce ductility at the bend, or fracture the bar outright during fabrication.
Bar size drives the multiplier because strain at the outer fiber scales with diameter for a fixed bend radius. A No. 3 bar bent to a 4db diameter strains less than a No. 11 bar bent to the same multiple, so the code raises the multiplier for bigger bars.
Fabrication Tip
Most rebar fabrication shops set mandrel sizes to the code minimum exactly, rounding up slightly for tolerance. If your shop drawing calls out a smaller pin diameter than the ACI minimum, flag it before bending starts.
Example Scenarios
Scenario 1: No. 4 Stirrup in a Beam
Bar size: No. 4 (db = 0.500 in)
Application: Stirrup, Table 25.3.2
Multiplier: 4db
Minimum bend radius = 2.00 ÷ 2 = 1.00 in.
A 2-inch mandrel or larger satisfies this stirrup bend. Most rebar benders stock a standard 2-inch pin for No. 3 and No. 4 stirrup work.
Scenario 2: No. 10 Column Dowel with 90° Hook
Bar size: No. 10 (db = 1.270 in)
Application: Main bar, 90° standard hook, Table 25.3.1
Multiplier: 8db
Minimum bend radius = 10.16 ÷ 2 = 5.08 in.
Minimum extension = 12db = 12 × 1.270 in = 15.24 in.
A 10.16-inch bend diameter is not a stock mandrel size. Fabricators typically round up to the nearest available pin, commonly 10.5 in or 11 in, which still satisfies the code minimum.
Common Error: Confusing Radius with Diameter
Bar size: No. 8 (db = 1.000 in)
Mistake: Reading "6db minimum" and entering 6.00 in as the radius instead of the diameter
Correct minimum radius = 6.00 ÷ 2 = 3.00 in, not 6.00 in.
Doubling the mandrel size by mistake wastes material and can throw off hook geometry on shop drawings. Always confirm whether a spec sheet lists diameter or radius before ordering a mandrel.
Common Calculation Errors
Using the Stirrup Table for Large Main Bars
Table 25.3.2 stirrup and tie provisions only cover No. 3 through No. 8 bars. For No. 9 and larger bars bent as ties or in any main bar application, use Table 25.3.1 main bar multipliers instead.
Forgetting the Extension Length on Hooks
A standard hook is not just a bend diameter. Per ACI 318-19 Table 25.3.1, a 90-degree hook also needs a 12db straight extension, and a 180-degree hook needs a 4db or 2.5-inch extension, whichever is greater, measured from the point of tangency.
Applying One Multiplier Across All Bar Sizes
The main bar multiplier is not constant. It steps up from 6db to 8db at No. 9, and from 8db to 10db at No. 14. Using 6db for a No. 10 or No. 14 bar understates the required bend diameter.
Ignoring Coated Bar Fabrication Limits
Epoxy-coated and galvanized bars can crack their coating at the code minimum bend diameter in some cases. Fabricators sometimes use a slightly larger mandrel for coated bar to protect the coating, beyond the bare ACI 318-19 minimum.
Fabrication and Inspection Context
Rebar bending typically happens in a fabrication shop before delivery, not in the field. Shop drawings specify bend diameters, and fabricators size mandrels to match.
Field inspectors and third-party special inspectors check bend diameters against the approved shop drawings and against ACI 318-19 minimums during placement inspection, especially for hooked bars at slab edges, footings, and beam-column joints.
Per IBC 2024 §1704.2.5 and ACI 318-19 Chapter 26, reinforcement inspection is required before concrete placement on most structural work, including verification that hooks and bends meet the specified geometry.
Frequently Asked Questions
Per ACI 318-19 Table 25.3.1, main bars No. 3 through No. 8 require a minimum inside bend diameter of 6 times the bar diameter. No. 9, No. 10, and No. 11 bars require 8 times the bar diameter, and No. 14 and No. 18 bars require 10 times the bar diameter.
Bend diameter is measured across the inside of the bend at the mandrel or pin. Bend radius equals half the bend diameter. ACI 318-19 tables list minimum values as diameters, not radii, so the radius is calculated by dividing the diameter by two.
Per ACI 318-19 Table 25.3.2, stirrup and tie bends for No. 3 through No. 5 bars require a minimum inside diameter of 4 times the bar diameter. No. 6, No. 7, and No. 8 bars used as stirrups or ties require 6 times the bar diameter.
Per ACI 318-19 Table 25.3.1, a standard 90-degree hook requires a minimum straight extension of 12 bar diameters beyond the point of tangency of the bend.
Per ACI 318-19 Table 25.3.1, a standard 180-degree hook requires a minimum extension of 4 bar diameters or 2.5 inches, whichever is greater, beyond the point of tangency of the bend.
Larger bar diameters concentrate more strain on the outer fiber of the bend. ACI 318-19 Table 25.3.1 increases the multiplier from 6db to 8db for No. 9 through No. 11 bars, and to 10db for No. 14 and No. 18 bars, to limit strain and prevent fracture during fabrication.
No. Bending a bar tighter than the ACI 318-19 minimum can crack the bar coating, reduce ductility, or fracture the bar during fabrication. Rebar fabricators size mandrels to the code minimum and do not go smaller without engineer approval.
Sources and Methodology
- American Concrete Institute, ACI 318-19, "Building Code Requirements for Structural Concrete," Table 25.3.1 (standard hooks and main bar bend diameters) and Table 25.3.2 (stirrup and tie bend diameters).
- ASTM A615/A706, Standard Specification for Deformed and Plain Carbon-Steel Bars and Low-Alloy Steel Deformed Bars for Concrete Reinforcement (nominal bar diameters used in this calculator).
- Concrete Reinforcing Steel Institute (CRSI), Manual of Standard Practice, 30th Edition (2023), fabrication guidance on standard bar bend diameters.
- International Building Code (IBC) 2024, Section 1704.2.5, special inspection requirements for reinforcing steel placement.
Last reviewed: July 2026. Reviewed by site author.
Disclaimer
This calculator provides estimates for planning purposes. For permitted structural work, foundations, multi-story construction, retaining walls over 4 feet, and commercial projects, calculations must be verified by a licensed structural engineer per IBC 2024 §1604. ConcreteCalculate.com is not liable for structural decisions made from these estimates.
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