Size matching is an important consideration in heart transplantation. It is important to avoid implanting an under-sized heart. Size matching has historically been performed using donor and recipient weight. Equations derived from cardiac MRI studies allow prediction of an individual’s heart mass using gender, age, height and weight. Potential donor-recipient pairs may be accurately size-matched using these equations. This allows identification of donor-recipient pairs who may be at higher risk of primary graft dysfunction or post-operative mortality. This app allows heart transplant surgeons and physicians to quickly and easily calculate the mis-match percentage in a potential donor-recipient pair and inform decisions about whether a particular donor organ is adequately sized for a particular recipient.