Order picking is a labor-intensive process in warehouse operations, where even small efficiency gains can yield substantial productivity benefits. This study investigates the effect of weight labeling on manual picking performance through two controlled experiments. In the first experiment, participant completed the tasks first with weight-labeled boxes and then without labeling. Weight labels used a traffic-light color scheme to indicate light, medium, and heavy categories. The labeled condition resulted in a shorter mean picking time compared to condition without labeling, representing an 8.39% improvement. In the second experiment, the sequence was reversed to include both the benefit of labeling and prior task familiarity. Across four participants and 45 box configurations, labeling reduced mean picking time by 15%. A detailed condition-level analysis revealed the greatest improvements for lightweight items located on high or low shelves, with gains exceeding 30% in some configurations. Minimal or negative effects were observed for certain heavy-item scenarios. These results indicate that weight labeling is a low-cost, easily implementable intervention that can significantly improve picking speed, particularly for items in ergonomically challenging locations, and may complement other warehouse efficiency strategies.