A group of 158 pre-registration second year nursing students were given a written test in drug calculation. The students' errors were collectively analysed to ascertain the mathematical basis for errors. The results indicated that a large number of students had limited understanding of basic arithmetic. Many students made errors large in magnitude, which implies potentially dangerous drug doses. The most frequent errors were in the categories of division, formula use, multiplying fractions and approximation. The large number of errors in formula use suggests students' drug calculation difficulties cannot be solved solely by enabling the use of calculators.