With respect to health risk behaviours, many people are unaware of their own risk behaviour and regard their behaviour as more healthy than it really is. This article studied differences between people with and without misperception of their physical activity with respect to several reference points: the social comparison style of people (upward, equal and downward); the body self-image with regard to their weight [body mass index (BMI) and self-rated weight]; and linkages of physical activity with different outcome expectancies (health, appearance, weight, feeling fit, relaxation and stress relief). Results from 516 respondents (response 52%; 56% women) with a mean age of 53.7 years found that respondents who incorrectly think that their physical activity is adequate (overestimators) tend to rate their physical activity more often in comparison to others. Furthermore, overestimators and people who correctly think they exercise enough more often use downward comparison, while underestimators and people who know they exercise too little mostly use upward comparison. People who, rightly or not, think their weight is adequate or who have a lower BMI, more often assume that their physical activity is sufficient or high. People who, rightly or not, think that their physical activity is adequate more often score higher on other reasons to be physically active besides health. Increasing the accuracy of people's self-perceptions of physical activity may be important to incorporate into strategies to promote physical activity in populations at risk of inactivity.