Ultrasound machines are frequently used by physiotherapists. Thus, it is imperative that machines are accurate for safe and effective treatments. This study aimed to examine the accuracy of therapeutic ultrasound machines in terms of power output and timing function, and to determine factors that relate to the degree of machine accuracy. An observational design was used. Sixty-four machines were sampled. Machines were tested at three power settings, two frequencies, and pulsed and continuous modes. Timers were assessed for 5- and 1 0-minute durations. Ultrasound machines were considered inaccurate if they deviated by more than ±20% and timers if they deviated by ± 10%. Fifty-nine percent (291/492) of power tests were inaccurate, with 79% of these producing less power output than depicted on the dial. Thirty-seven percent of timers were inaccurate, with mechanical timers more inaccurate than digital timers (p < 0.05). Older ultrasound machines were more likely to be inaccurate (p<0.05). Length of time since machine calibration was also associated with machine inaccuracy at 3 MHz (p< 0.05). There: is a high level of power and timer inaccuracy in machines that may be related to machine age and frequency of calibration. This study highlights the need for therapists to be aware of the potential for machine inaccuracy and for stricter guidelines on machine calibration to be introduced.