Background The aim of this study was to determine the relationship between prosthesis coverage, and postoperative hidden blood loss (HBL) in primary total knee arthroplasty (TKA). Methods A total of 120 patients who had undergone unilateral TKA from August 2012 to May 2013 were retrospectively studied. The Gross formula was used to calculate the amount of HBL. Routine standard anteroposterior (AP) and lateral X-ray films of the knee joint were taken postoperatively and used to measure the percentages of coronal femoral and of coronal and sagittal tibial prosthetic coverage. Then Pearson's correlation analysis was performed to assess the correlations between the percentages of prosthetic coverage for each AP and lateral position and HBL on the first and third postoperative days. Results The volumes of HBL on the first and third postoperative days after TKA were (786.5 +/- 191.6) ml and (1 256.6 +/- 205.1) ml, respectively, and lateral X-ray film measurements of percentages of coronal femoral, tibial coronal, and sagittal prosthetic coverage were (87.9 +/- 2.5)%, (88.5 +/- 2.2)%, and (89.1 +/- 2.3)%, respectively. Pearson's correlation analysis showed statistically significant correlations between percentages of total knee prosthetic coverage for each AP and lateral position and volumes of HBL on the first and third postoperative days (P <0.05). Conclusions HBL after TKA correlates with degree of prosthetic coverage. To some extent, the size of the surfaces exposed by osteotonny determines the amount of HBL. Choice of the appropriate prosthesis can significantly reduce postoperative HBL. Designing individualized prostheses would be a worthwhile development in joint replacement surgery.