Ejection fraction (EF) is a measurement of the percentage of blood leaving the heart each time it contracts. It is the amount of blood pumped out of the ventricles with each contraction. EF is widely used as a measure of the pumping efficiency of the heart and is used to classify heart failure types. The EF of the left heart, known as the left ventricular ejection fraction (LVEF), is calculated by dividing the volume of blood pumped from the left ventricle per beat (stroke volume) by the volume of blood collected in the left ventricle at the end of diastolic filling (end-diastolic volume) . LVEF is an indicator of the effectiveness of pumping into the systemic circulation. The EF of the right heart, or right ventricular ejection fraction (RVEF), is a measure of the efficiency of pumping into the pulmonary circulation.

It is important to note that a normal ejection fraction measurement does not necessarily mean that a person does not have heart failure. Heart failure with preserved ejection fraction (HFpEF) can occur when the hearts muscle has become so thick and stiff that the ventricle holds a smaller than usual volume of blood, even though the ejection fraction falls in the normal range. In severe cases, an ejection fraction measurement under 40 percent might be evidence of heart failure or cardiomyopathy.