The 'Uptimer' is a custom-made lightweight battery-operated remote activity monitor that records the amount of time an individual spends in the upright position, which is also known as 'uptime'. The aims of this study were to determine levels of uptime over 24 hours and the relation between uptime and a child's age, sex, height, weight, and body mass index (BMI). Uptime was recorded in 529 normally developing children (318 females, 211 males), aged between 8 and 15 years. All children wore an Uptimer continuously for a 24-hour period that included a typical school day. Mean uptime for children in this study was 5.4 hours (SD 1.3; range 1.5 to 10.3 hours), over a 24-hour period. Uptime followed approximately a normal distribution in this population and did not have a linear relation to age, height, weight, nor BMI. Results of this study may be used as a normative database for the evaluation of uptime in children with physical disabilities.