For the diagnosis, prevention and therapy of malnutrition, it is important to estimate the energy and fluid requirements of an individual patient. To our knowledge, it is unknown how accurately medical doctors can estimate the energy and fluid requirements of patients in a clinical routine situation. Hence, we conducted the following survey. A written face-to-face survey about the energy and fluid requirements of and tube feeding and fluid recommendations for a typical patient was performed with 179 medical doctors. An estimation error of >15% was defined as relevant. The results revealed substantial variations in estimating the energy and fluid needs of the patient. A total of 25% of the participants underestimated the energy requirements, and 47% of the participants underestimated the fluid requirements. In addition, 68% of the participants recommended a daily dose of tube feeding that was <85% of the reference value. A substantial proportion of medical doctors show a lack of knowledge concerning energy and fluid requirements, which demonstrates a need for better medical education with regard to nutrition.