Adding lanes is a common method to increase the capacity at intersections. The additional turning lanes are often limited by the road configuration. The paper presents a probabilistic model to calculate the total capacity at signalized intersections for a full through lane and an additional left-turn lane. The proposed model has an advantage over current estimation methodologies by considering not only stochastic arrivals of vehicles but also the signal timing plan, especially signal phasing sequence. Two blockage situations under this channelization form are taken into account. The proposed capacity model shows that the capacities of the through and left-turn movements are related to the proportion of the through traffic volume, the length of the additional turning lane, and the effective green time of both through and left-turn signal phases. It is also found that the through capacity increases with the increase of the proportion of the through traffic and the length of the additional lane, whereas the left-turn capacity declines with an increase of the proportion of the through traffic but increases with an increase of the length of the additional lane. Both capacities decrease with the increase of the green time when the g=C ratio is fixed. The proposed model is compared with design recommendations and verified using the microscopic traffic software VISSIM as well as field data. The results show that the capacities obtained by theoretical models are consistent with the simulated results, and have an advantage over the design recommendations. The capacity models can provide theoretical suggestions for the design of additional turning lanes and signal timing plans.