How to use the anchor concrete breakout calculator
Concrete breakout strength in tension follows ACI 318-19 Chapter 17. The basic single-anchor strength is Nb = kc·λa·√f'c·hef^1.5 in pounds with psi and inches, where kc is 24 for cast-in anchors and 17 for post-installed anchors unless a qualification test in the product evaluation report gives a higher value. The f'c used in the calculation is capped at 10,000 psi for cast-in and 8,000 psi for post-installed anchors, and λa is taken as 1.0 for normalweight concrete. Embedment enters to the 1.5 power, so going deeper buys more capacity than going to a larger diameter.
Three modifiers turn Nb into Ncb. The projected area ratio ANc/ANco compares the actual failure area with the full 9hef² square, clipping it wherever an edge is closer than 1.5hef. The edge factor ψed,N drops to 0.7 + 0.3·ca,min/(1.5hef) once the nearest edge is inside 1.5hef. The cracking factor ψc,N is 1.0 where cracking is expected at service load, and in uncracked concrete 1.25 for cast-in or 1.4 for post-installed anchors. The splitting factor ψcp,N for post-installed anchors in uncracked concrete needs the product's critical edge distance cac, so it is taken as 1.0 here, and when three or more edges lie within 1.5hef a reduced h′ef must replace hef, which this two-edge tool does not handle. For single cast-in headed studs and headed bolts with hef from 11 in to 25 in, ACI 318-19 permits Nb = 16·λa·√f'c·hef^(5/3), which gives up to about 14 percent more than the 1.5-power equation; since both are permitted, the calculator uses the larger of the two in that range and shows the other next to it. Hooked bolts and post-installed anchors stay on the 1.5-power equation.
The Ncb shown is a nominal strength. Design strength requires a strength reduction factor φ that depends on anchor type and on whether supplementary reinforcement is present (Condition A or B): 0.75/0.70 for cast-in, 0.75/0.65 for Category 1, 0.65/0.55 for Category 2 and 0.55/0.45 for Category 3 post-installed anchors, so pick the matching option above. The basis is ACI 318-19 Section 17.6.2 (concrete breakout strength) and Table 17.5.3 (strength reduction factors), as of 2026. This calculator is for reference only; check the evaluation report and your load condition. Anchor groups, combined tension and shear, and anchor reinforcement all need separate treatment, and safety-critical baseplates should be confirmed by a licensed structural engineer.
Frequently Asked Questions
No. ACI 318 Chapter 17 requires steel strength, pullout, side-face blowout, and concrete breakout in tension, plus a separate set of shear limit states and a tension-shear interaction check. This tool covers concrete breakout only, so design to the smallest governing value.
The breakout cone spreads at roughly 35 degrees and projects a square area 3hef on a side. When an edge is closer than 1.5hef that area is clipped, and the ψed factor of 0.7 + 0.3·ca,min/(1.5hef) reduces the strength further.