Helical piles with small sizes are widely used in traditional civil engineering infrastructure application, especially in onshore electrical power industry. The floating offshore wind power industry has taken an interest in helical piles because of their rapid installation and ability to support immediate loading. Compared with onshore structures, wind power structures suffer larger external load and it becomes necessary to increase the overall size of helical piles, leading to considerable load capacity provided by the shaft, which is typically neglected for a light structure. In this paper, the undrained uplift capacity of helical piles was studied using finite-element limit analysis. The analysis explored various diameter ratios of the shaft to the plate and embedment depths, as well as different interface conditions. The local bearing capacity on the plate was also explored for attached and vented interface conditions. The failure mechanisms of a helical pile were presented to explain the mutual influence between plate anchor and shaft on the uplift capacity. Algebraic expressions for the uplift capacity of a helical pile were proposed. Engineers may use these expressions to improve the accuracy of their estimates for the uplift capacity of helical piles.