During starts and turns, between and within laps, the swimmer’s velocity is not constant;​ thus, besides the drag force, the swimmer experiences an additional (inertial) force. Some of the water around the swimmer is set in motion and this can be thought of as an added mass (M_A,p) the swimmer has to accelerate (in addition to body mass, M₀):​ the higher M_A,p, the higher the resistive and inertial forces that oppose the swimmer’s motion during acceleration phases. This study introduces a novel method to determine M_A,p, consisting of a standing start maximal test. Sixteen male swimmers (526.1 ± 65.8 FINA Points) performed maximal sprints during which their instantaneous speed was assessed using an IMU positioned on their sacrum. The estimation of M_A,p was based on the swimmer’s maximum velocity (v_max) and acceleration time (τ), as determined using a standing start test, and the active drag coefficient (k_a) and mean propulsive force (F_P), as determined using the residual thrust method. On average vmax = 1.73 ± 0.11 m.s⁻¹, τ = 1.14 ± 0.11 s, FP = 146.8 ± 20 N and ka = 47.9 ± 5.7 kg.m−1. M_A,p in surface swimming (28.7 ± 15.2 % M₀) is similar to the added mass that can be determined in passive conditions underwater (M_a,p = 25 ± 3 % M₀) but presents a larger variability. This variability could not be attributed to the swimmer’s technical level, e.g the active to passive drag ratio:​ k_a/k_p, where k_p = 26.6 ± 3.3 kg.m−1 (determined using passive towing experiments).