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Description: What is kindness, and what makes an act kind? Previous research has suggested that the kindness of an act depends on the benefit provided to the recipient, the cost to the actor, and the ratio of cost to benefit. Here we conduct two studies to investigate the relationship between the benefit, cost, cost-benefit ratio and the kindness of a large sample of ‘everyday’ acts. In Study 1 participants from the US and UK (n=15,997) rated 1,692 acts to family, friends, colleagues and strangers. Item-level analysis using mixed models showed that benefit predicts the kindness of acts to family, friends, colleagues and strangers, whereas cost predicts the kindness of acts to family, friends, and colleagues, but not strangers. Cost-benefit ratio was not a positive predictor of the kindness of acts. In Study 2 participants from the US (n=4,801) rated 385 acts to a generic ‘someone’. Item-level analysis showed that benefit, but not cost, was a predictor of the kindness of acts. Cost-benefit ratio was not a positive predictor of the kindness of acts. Participant-level analysis showed that benefit and, to a much lesser degree cost, was a predictor of the kindness of acts. Cost-benefit ratio was not a substantial positive predictor of the kindness of acts. These results suggest that kindness is primarily a matter of providing benefits to others, and secondarily a matter of paying a cost to provide those benefits to familiar others. We discuss the implications of these results for existing theories of kindness, and how this method might be used to further investigate the nature of kindness and other related concepts.