Reduced milk yield due to mastitis has been estimated throughout the literature to be the largest source of loss due to an occurrence of mastitis in dairy cattle, though there is a high amount of variability amongst these estimates. The objective of this paper is to estimate yield loss due to mastitis using a large sample size of cows throughout the United States to reduce the variability of the loss estimates. 38,150 test day observations from December 2013 were used from herds in Wisconsin, New York, Illinois, and Michigan to estimate milk yield loss due to mastitis. A base model and a model with an interaction term between SCC and lactation number based on Bartlett et al. (1990) was used to calculate milk yield loss. Milk yield loss was estimated to be 0.0428 kg/day in the base model and 0.0388 kg/day in the interaction model per 1% increase in the natural log of the SCC. This paper is part of ongoing research to create a mastitis cost calculation tool for United States dairy farmers.