Ejection fraction is calculated by dividing the amount of blood your heart pumps out per beat (stroke volume) by the total amount of blood in the chamber just before it contracts (end-diastolic volume), then multiplying by 100 to get a percentage. A normal ejection fraction falls between 55% and 70%.
The Formula
The equation is straightforward: EF = (SV ÷ EDV) × 100. SV stands for stroke volume, the amount of blood your left ventricle pushes out with each beat. EDV stands for end-diastolic volume, the total blood sitting in the ventricle after it has fully relaxed and filled.
To get stroke volume, you subtract the blood left behind after the heart squeezes from the blood that was there before it squeezed. In formula terms: SV = EDV − ESV, where ESV (end-systolic volume) is the residual blood remaining after ejection. In a typical adult heart, the ventricle fills to about 120 mL, pumps out roughly 70 mL, and retains about 50 mL. That gives an ejection fraction of about 58%: (70 ÷ 120) × 100.
A Worked Example
Say an echocardiogram shows your left ventricle holds 130 mL of blood when fully relaxed and 45 mL after contracting. First, find stroke volume: 130 − 45 = 85 mL. Then divide by end-diastolic volume: 85 ÷ 130 = 0.654. Multiply by 100 to get 65.4%. That falls squarely in the normal range.
Now imagine someone with a weakened heart muscle. Their ventricle fills to 180 mL (often larger due to stretching) but only ejects 55 mL, leaving 125 mL behind. Stroke volume is 55 mL, so ejection fraction is (55 ÷ 180) × 100 = 30.6%. That number points to significant heart failure.
How Doctors Measure the Volumes
You don’t calculate ejection fraction yourself from raw numbers. The imaging equipment does the math. But the volumes it plugs into the formula come from one of several techniques, each with its own strengths.
Echocardiography (ultrasound) is the most common method. The technician captures images of your heart at different angles, and software uses a technique called Simpson’s method of disks: it slices the ventricle into a stack of thin cross-sections, estimates the area of each disk, and adds them up to approximate the total volume. This is done once when the ventricle is fully expanded and once when it’s fully contracted, giving EDV and ESV. From there, the machine applies the formula automatically.
Cardiac MRI provides the most precise volume measurements because it generates high-resolution 3D images of the heart. It’s considered the gold standard when accuracy matters most, such as before major treatment decisions, but it’s more expensive and time-consuming than ultrasound.
Nuclear imaging (MUGA scan) uses a small amount of radioactive tracer injected into your blood. A camera tracks how the tracer moves through your heart chambers to estimate how much blood is ejected per beat. International cardio-oncology guidelines recommend it as a third-line option when echocardiography or MRI isn’t available, though many cancer treatment centers rely on it routinely because results are highly reproducible from one scan to the next.
What the Numbers Mean
The American Heart Association defines a normal ejection fraction as 55% to 70%. That doesn’t mean your heart pumps out all its blood. Keeping some blood behind is normal and necessary for the ventricle to function properly.
An EF of 41% to 49% is considered mildly reduced. Below 40% typically indicates heart failure or cardiomyopathy. An EF above 75% can also be abnormal, sometimes pointing to hypertrophic cardiomyopathy, a condition where the heart muscle is abnormally thick.
Clinical guidelines break heart failure into categories based on these thresholds:
- Preserved EF (HFpEF): 50% or higher. The heart pumps a normal percentage of blood, but the muscle is stiff and doesn’t fill properly.
- Mildly reduced EF (HFmrEF): 41% to 49%.
- Reduced EF (HFrEF): 40% or below.
These categories matter because treatments differ. At least half of all heart failure patients actually have a preserved ejection fraction, meaning their number looks normal even though their heart isn’t working well. The distinction also affects device eligibility. An implantable defibrillator, for instance, is generally considered for patients with coronary artery disease and a prior heart attack whose EF is at or below 35%.
Why EF Doesn’t Tell the Whole Story
Ejection fraction measures how much blood leaves the chamber, but it doesn’t account for how much heart muscle is actually contributing to that squeeze. Two hearts can produce the same EF through very different mechanics. One might have healthy muscle fibers contracting uniformly, while the other compensates for damaged tissue by over-relying on certain wall segments.
This is because ejection fraction mainly reflects shortening in one direction, the circular squeeze of the ventricle wall. It misses changes in the lengthwise contraction handled by muscle fibers closer to the inner lining of the heart. Those inner fibers are the most vulnerable to early damage from conditions like high blood pressure, diabetes, or chemotherapy.
A newer measurement called global longitudinal strain picks up these subtler problems. It tracks how much the heart muscle deforms during each beat and can detect dysfunction even when ejection fraction still reads as normal. In patients with dilated cardiomyopathy who are receiving optimal treatment, strain measurements have proven to be a stronger predictor of outcomes than ejection fraction alone. Some cardiologists now use both measurements together to get a fuller picture of heart function, particularly when monitoring patients on cancer treatments or evaluating whether heart failure therapy is working.
What Changes Your Ejection Fraction
EF isn’t a fixed number. It shifts with hydration, heart rate, blood pressure, medications, and the progression or treatment of disease. A single low reading doesn’t necessarily lock you into a diagnosis. The 2022 heart failure guidelines include a specific category, “improved EF,” for patients who previously had an EF at or below 40% and later showed improvement above that threshold.
Exercise, medication adjustments, and procedures to restore blood flow to damaged heart muscle can all raise ejection fraction over time. Conversely, a heart attack, uncontrolled high blood pressure, or toxic effects from certain drugs can lower it. Because it fluctuates, doctors typically track EF over multiple measurements rather than making decisions based on a single snapshot.