Breast density (BD) is a risk factor for breast cancer, which makes the accurate measurement of BD a priority. Mammography is most widely used for BD determination (MG-BD) but ionizing radiation prohibits its use. BD derived from fat-water decomposition MRI (FWMRI-BD) has been proposed. Here we developed an optimized FWMRI-BD measurement (FraG+W) and compared it to MG-BD and a previous FWMRI-BD measurement (Fra80/90). Both FWMRI-BD measures were strongly correlated with MG-BD and exhibited superior test-retest reliability. The proposed automated FraG+W, which quantifies the entire fibroglandular and water content of the breast, is more accurate and reliable than the previous Fra80/90.