How to use the anchor bolt pullout calculator
When an anchor bolt is pulled in tension, a cone of concrete can break out around it. Enter the effective embedment and the specified concrete strength and the basic breakout strength appears at once; add the projected area ratio and the modification factors and the nominal and design strengths follow.
The page applies the concrete breakout equations of the American Concrete Institute standard (ACI 318) in US units: Nb = 24 x lambda a x the square root of f'c x hef raised to 1.5, reference projected area ANco = 9 x hef squared, and an edge factor of 1.0 when the edge distance is at least 1.5 x hef, otherwise 0.7 + 0.3 x ca,min divided by 1.5 x hef. The coefficient 24 is for cast-in anchors with f'c in psi and hef in inches, giving Nb in pounds; the SI form uses 10 with megapascals and millimeters and agrees within about 0.5%. Forces are shown in pounds with kilonewtons in parentheses. The sources are secondary ones such as software verification examples and university course notes, as of October 2026, so check the original standard as well.
The formula for the projected area ratio, the lightweight concrete factor and the strength reduction factor were not confirmed in an authoritative source, so no values are suggested and you enter your own. Cases where two edges are both close, group spacing effects, and steel failure, pullout and side-face blowout are not covered. Embedment is calculated only up to the 11 in limit of the formula. This calculator is for reference and does not replace design. Review by a qualified engineer is required.
Frequently asked questions
The concrete breakout formula used here is reported as valid up to 11 in of effective embedment. Deeper anchors fall outside that range, so the page shows a notice instead of a result.
It means no area is lost to an edge or a neighboring anchor. The formula for a reduced projected area is not covered here, so enter the value worked out from the standard you follow.
No. This page calculates concrete breakout only, so steel tensile failure, pullout and side-face blowout have to be checked separately.