How effortful an action feels critically shapes everyday decisions. Despite the importance of perceptions of effortfulness for making choices, the behavioral and neural representations of the subjective cost of physical effort are not well understood. We used functional magnetic resonance imaging (fMRI) to monitor brain activity while participants engaged in risky choices for prospective physical effort independent of reward. Behaviorally we found that participants exhibited increasing sensitivity to changes in subjective effort as objective effort levels increased. Moreover, the ventromedial prefrontal cortex (vmPFC) encoded the subjective cost of prospective effort options and integrated these representations to facilitate decisions regarding effort expenditure. These results provide insight into human decision-making by showing how neural representations of feelings of effortfulness engender choices about exertion.