The present invention relates to a method of calculating a proton density fat fraction, PDFF, from a water and fat separated magnetic resonance imaging, MRI, based on fat-referenced lipid quantification in a region of interest (ROI) and using determination of a reference tissue. The method comprises the step of determining: F·βf/R, wherein F is the fat signal in the ROI provided from the MRI, βf is a function providing a ratio between T1 saturation values of the fat signals in the reference tissue and in the ROI; and R is a representation of the sum of fat and water signals on an intensity scale where the saturation of each of the fat and water signals equals the saturation of fat in the reference tissue.