In stroke-induced motor paralysis, impairment of finger individuation often persists over the long term. Traditional finger function assessments in clinical rehabilitation rely on ordinal scales and subjective evaluation, limiting the ability to capture detailed recovery processes. This study proposes a novel quantitative method to evaluate finger individuation by assessing how much a single finger can move individually without involuntary movements of the other fingers. Fifty healthy adults and ten stroke patients with hemiparesis were instructed to flex each finger, starting from all fingers in fully extended position. Data from healthy participants were used to define the ranges of involuntary movement in the four non-instructed fingers during each instructed finger movement. For patient evaluation, “individuation index” was used, which was defined as the maximum flexion angle achievable within the ranges obtained from healthy participants. This index was expressed as a percentage of the maximum range of motion for each finger. In a patient with minimal motor paralysis, all fingers achieved individuation index of 100%. In patients with mild and moderate paralysis, fewer fingers achieved an individuation index of 100%, reflecting the severity of motor impairment. This method accurately detected differences in finger individuation, providing a quantitative and objective measure that could serve as a useful index for assessing motor control in stroke patients.