Anchor Bolt Pullout (Concrete Breakout) Calculator
This calculator determines the basic concrete breakout tensile capacity Nb for a single cast-in anchor bolt far from edges and in cracked concrete, per ACI 318-19 Equation 17.6.2.1a. The formula Nb = kc * sqrt(fc') * hef^1.5 uses kc = 24 for cast-in anchors (in US customary units: lb and inches). For isolated anchors far from edges with no group effects, Ncb = Nb and the design strength is phi * Nb = 0.75 * Nb. When anchors are close to edges, multiple anchors interact, or the anchor group is in a high-seismic region, more detailed calculations per ACI 318-19 Chapter 17 are required.
Concrete breakout formula (ACI 318-19)
Nb = kc × sqrt(fc') × hef^1.5 [lb, psi, in]
kc = 24 (cast-in anchors)
phi Nb = 0.75 × Nb (cracked concrete, ductile)
Units: fc' in psi, hef in inches, Nb in pounds (divided by 1,000 for kips). This formula is valid for isolated single anchors far from edges. Divide by 1,000 to convert to kips.
Additional checks required for anchor design
- Steel tensile failure: phi * Nsa = phi * Ase,N * futa, where phi = 0.75 for ductile steel elements and futa is the ultimate tensile strength of the bolt.
- Anchor pullout (bearing on anchor head): phi * Np = phi * 8 * Abrg * fc', where Abrg is the bearing area of the anchor head.
- Side-face blowout: governs for deeply embedded anchors close to an edge, per ACI 318-19 Section 17.6.4.
- For groups of anchors, use the group projected area ANc instead of the single-anchor area ANco.
Frequently asked questions
What is concrete breakout (pullout cone) failure?
Concrete breakout in tension occurs when a tensile load on an anchor forms a cone-shaped fracture surface in the concrete. The breakout strength depends on concrete tensile strength, embedment depth, and the projected area of the failure cone. ACI 318-19 Chapter 17 governs this limit state.
What is the basic concrete breakout formula?
Per ACI 318-19 Section 17.6.2.1, the basic nominal concrete breakout strength for a single anchor in tension is Nb = kc * sqrt(fc') * hef^1.5, where kc = 24 for cast-in anchors (lb and inch units), fc' is concrete compressive strength in psi, and hef is the effective embedment depth in inches.
What modifications apply to Nb?
The full nominal strength Ncb = (ANc/ANco) * Psi_ed,N * Psi_c,N * Psi_cp,N * Nb. ANc/ANco is the projected area ratio (equals 1.0 for an isolated anchor far from edges). Psi_ed,N corrects for edge distance; Psi_c,N for cracking condition; Psi_cp,N for post-installed anchors. For cast-in anchors far from edges in cracked concrete, all modifiers = 1.0.
What is the design (factored) strength?
The ACI 318 LRFD design breakout strength is phi * Ncb, where phi = 0.75 for concrete controlled (ductile) failure, or phi = 0.70 for brittle/non-ductile failure per ACI 318-19 Table 17.5.3. This must equal or exceed the factored tensile load Nu.
What embedment depth is typically required?
Minimum embedment depth is the larger of 6 anchor diameters or 2.5 in for cast-in anchors (ACI 318-19 Table 17.2.3). In practice, embedments of 8-12 bolt diameters are common to achieve adequate breakout and pullout capacity. Always check both breakout and pullout (bearing) separately.
Official sources
- American Concrete Institute: ACI 318-19 Building Code Chapter 17 Anchoring to Concrete.
- American Institute of Steel Construction: AISC Design Guide 1 Column Base Plates.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.