Estimation is the most commonly used method to determine blood loss. According to the literature, estimating blood loss is not exact; there are wide margins of error and blood loss tends to be underestimated. Forty-two registered nurses were asked to estimate varied amounts of blood on 20 peripads. Both over- and underestimation occurred; 71% overestimated and 25% underestimated blood loss. There was also disagreement as to what constituted heavy, moderate or slight blood loss. There was no difference in accuracy among variables of education, years of experience, specialty area, or when and how nurses learned to estimate. Contrary to current literature, these nurses: 1 overestimated blood loss; 2 had significant difficulty in estimating very small and very large amounts of blood; 3 although in error, were consistent in estimating repeated samples with the same amounts of blood; 4 were not consistent in estimating heavy, moderate, and slight blood loss.