Preventable medical errors may be due to incorrect drug dosage based on poor weight estimation. This study was to examine the accuracy of patient weight estimations in an emergency medical setting. This prospective study enrolled a convenience sample of medically stable adults. The patient's attending physician, resident physician, nurse, a paramedic, and the patient estimated the patient's weight. Of 394 patients enrolled, patients erred in the estimation of their weight by greater than 20% only 1.5% of the time. The group values were 14.7% for attending physicians, 13.4% for resident physicians, 15.9% for nurses, and 17.4% for paramedics. Our study suggests that emergency department staff estimation of a patient's weight is often inaccurate. When available, the patient's own estimate can be used as their actual weight. When the patient is incapacitated, measurement of the patient's weight is the proven method to avoid this type of dosage error.