Cardiac Output: Normal Values, Formula & Measurement

Cardiac output is the headline number of the circulation — how much blood the heart moves each minute. Here's the normal range, the formula, what raises or lowers it, and how it's measured.

What is cardiac output?

Cardiac output is the volume of blood the heart pumps per minute, equal to heart rate multiplied by stroke volume. It links the heart to the tissues: whatever the body's oxygen demand, cardiac output has to rise to meet it, from about 5 L/min at rest to 20–25 L/min in a trained athlete at peak exercise.

Cardiac output formula

Cardiac output = heart rate × stroke volume

If the heart beats 70 times a minute and ejects 70 mL each beat, cardiac output is about 4.9 L/min. Indexed to body size it becomes cardiac index (output ÷ body surface area). Estimate stroke volume with our stroke volume calculator.

Normal cardiac output values

Measure	Normal range
Cardiac output	4–8 L/min
Cardiac index	2.5–4.0 L/min/m²
Stroke volume	60–100 mL
Heart rate	60–100 bpm

What determines cardiac output?

Because output is heart rate times stroke volume, anything that changes either changes output. Stroke volume itself has three drivers:

• Preload — the ventricular filling volume (Frank-Starling: more stretch, stronger contraction, up to a point).
• Afterload — the resistance the ventricle ejects against, largely systemic vascular resistance. Higher afterload lowers stroke volume.
• Contractility — the intrinsic force of contraction, boosted by sympathetic tone and inotropes.

How is cardiac output measured?

Method	How it works
Fick principle	Oxygen uptake ÷ arteriovenous O₂ difference; best in low output
Thermodilution	Cold-saline temperature curve via a pulmonary-artery catheter
Echocardiography	LVOT diameter and velocity-time integral
Pulse-contour / bioreactance	Newer, minimally- or non-invasive continuous monitors

Thermodilution is unreliable in low output, tricuspid regurgitation, and shunts — the settings where Fick is preferred. See the detail in our hemodynamics study guide.

High and low cardiac output

• Low output — heart failure, cardiogenic shock, hypovolemia, severe bradycardia or tachyarrhythmia. Tissues are under-perfused.
• High output — sepsis, severe anaemia, hyperthyroidism, pregnancy, and large arteriovenous shunts. The pump is fine; demand or shunting is the issue.

Key takeaways

• Cardiac output = heart rate × stroke volume; normal is 4–8 L/min.
• Stroke volume depends on preload, afterload, and contractility.
• It is measured by Fick, thermodilution, echo, or newer continuous monitors.
• Indexing to body size gives cardiac index, a better severity marker.

Frequently asked questions

What is a normal cardiac output?

About 4–8 L/min at rest. Indexed to body size, the cardiac index is 2.5–4.0 L/min/m².

What is the cardiac output formula?

Cardiac output = heart rate × stroke volume.

What determines cardiac output?

Heart rate and stroke volume, and stroke volume is set by preload, afterload, and contractility.

How is cardiac output measured?

By the Fick principle, thermodilution through a pulmonary-artery catheter, echocardiography, or newer pulse-contour and bioreactance monitors.

What causes low cardiac output?

Heart failure, cardiogenic shock, hypovolemia, and significant brady- or tachyarrhythmias.

What causes high cardiac output?

High-demand or low-resistance states such as sepsis, anaemia, hyperthyroidism, pregnancy, and arteriovenous shunts.

What is the difference between cardiac output and cardiac index?

Cardiac index is cardiac output divided by body surface area, so it accounts for patient size.