Given the increasingly important role of breast density as an independent risk factor for breast cancer, and the variable breast imaging tests that potentially provide measures for density. We compared breast tissue density on digital mammography (FFDM), digital breast tomosynthesis (DBT), and magnetic resonance imaging (MRI) using semi-automated automated software. These three imaging modalities have not been previously directly compared for estimating breast tissue density. Following informed consent from all participating women, FFDM, DBT, and MRI were performed. Breast percentage density was calculated with semi-automated software, and compared, for all three imaging modalities. 48 patients (mean age, 41 years; range, 35-67 years) underwent FFDM, DBT, and MRI. Percent FFDM, DBT, and MRI breast density measures showed a positive linear correlation, (r = 0.95 for MRI and DBT, P < 0.0001; r = 0.97, P < 0.0001 for FFDM and DBT; r = 0.87 for FFDM and MRI). Linear regression analysis related to MRI and DBT had a high r (2) = 0.89 (95 % CI = 0.88-0.99, P < 0.001). FFDM overestimated breast density in 15.1 % in comparison to DBT and in 16.2 % in comparison to MRI, or conversely each of DBT and MRI underestimated density (relative to FFDM) by 15.1 or 16.2 %, respectively. Differences in percentage breast density between FFDM and DBT, and between FFDM and MRI, were highly significant (P < 0.0001). Differences in percentage breast density between DBT and MRI were not significant (P > 0.05). Breast density measures using FFDM, DBT, or MRI were generally well-correlated, although differences were noted between estimates when comparing FFDM and DBT, and for estimates comparing FFDM and MRI. No signficant differences in percentage density were observed when comparing DBT and MRI. Our work highlight that differences between FFDM, DBT, and MRI should be considered when measuring percentage breast density.