Volunteering is a helping behavior with more benefits for those who receive it than for those who offer it. After more than two decades of intense research focused on isolated variables, this paper focuses on the subjective balance between costs and benefits that make volunteers remain in non-profit organizations. A short instrument of 22 items is validated using a sample of 205 volunteers engaged in 10 non-profit organizations working in the social sector. Confirmatory Factor Analysis provided a 3-factor model of benefits (Benefits from the activities, Benefits of giving and Benefits of sharing) and a 3-factor model of costs (Costs of impotence, Costs from the organizational context and Costs from lack of competence) with a good fit to the data. The General Index of Benefits presented a positive pattern of significant relationships with psychological and subjective well-being, satisfaction with volunteering, organizational commitment, volunteer engagement, role identity as a volunteer and intention to stay as a volunteer in the same organization. The General Index of Costs presented negative relationships with the same mentioned variables. The numerical difference between both General Indexes was called the Subjective Index of Benefits in Volunteering (SIBiV) and can be interpreted as the positive balance of benefits at any time for any volunteer when it is greater than 0. This instrument can help to manage this positive social phenomenon that benefits those who receive the help, volunteers themselves and society as a whole in many different ways.