The solicitation of charitable donations costs billions of dollars annually. Here, we introduce a virtually costless method for boosting charitable donations to a group of needy persons: merely asking donors to indicate a hypothetical amount for helping one of the needy persons before asking donors to decide how much to donate for all of the needy persons. We demonstrated, in both real fund-raisers and scenario-based research, that this simple unit-asking method greatly increases donations for the group of needy persons. Different from phenomena such as the foot-in-the-door and identifiable-victim effects, the unit-asking effect arises because donors are initially scope insensitive and subsequently scope consistent. The method applies to both traditional paper-based fund-raisers and increasingly popular Web-based fund-raisers and has implications for domains other than fund-raisers, such as auctions and budget proposals. Our research suggests that a subtle manipulation based on psychological science can generate a substantial effect in real life.