A convenience sample of 110 registered nurses in four western states completed a demographic questionnaire and a 20-item medication calculation test to investigate errors in medication calculation that contribute to medication error rates. Intravenous questions were most difficult, then oral, then intramuscular/subcutaneous items. Nurses erred more when more than one calculation was required and when milligram to grain conversion was needed. Nurses who rated their skill and comfort with medication calculation above average scored higher. Yet 81 percent of the nurses were unable to correctly calculate medications 90 percent of the time and 43.6 percent of the test scores were below 70 percent accuracy. Strategies are suggested which may be used in staff development in identify registered nurse medication calculation abilities and to enhance these skills in practicing nurses.