The nature of the relationship between the time people spend on paid work and volunteering remains debated in the social sciences. Time constraint theory suggests a negative relationship because people can allocate only as much time to volunteering as their work responsibilities permit. However, social integration theory suggests a more complex inverse U-shaped relationship because paid work not only limits people’s free time but also plays a key role in their social integration. Departing from these competing theories, this study uses two-wave panel data from Denmark to examine the relationship between hours of paid work and volunteering. In support of time constraint theory, the results suggest that hours of paid work have a significant negative effect on the total number of hours that people spend volunteering, not mainly because paid work hours affect people’s propensity to volunteer but because they affect the number of hours that volunteers contribute.