Every heartbeat moves blood through the cardiovascular system. But how much blood does the heart pump in a minute?

That measurement is called cardiac output. It depends mainly on two variables: heart rate and stroke volume. Understanding the relationship between them makes it easier to see why cardiac output changes when you rest, exercise, or experience changes in cardiovascular function.

What Is Cardiac Output?

Cardiac output (CO) is the volume of blood pumped by the heart in one minute. It is usually expressed in liters per minute (L/min).

The basic relationship is:

Cardiac output = heart rate × stroke volume

Heart rate is the number of heartbeats per minute, while stroke volume is the amount of blood ejected by a ventricle with each beat.

For example, if the heart beats 70 times per minute and ejects 70 mL with each beat:

70 × 70 mL = 4,900 mL/min

That is approximately:

4.9 L/min

Cardiac Output Formula

The standard cardiac output formula is:

CO = HR × SV

Where:

• CO = cardiac output
• HR = heart rate in beats per minute
• SV = stroke volume in mL per beat

If you want the answer in liters per minute, divide the result in milliliters by 1,000.

For example:

HR = 75 bpm

SV = 70 mL/beat

75 × 70 = 5,250 mL/min

5,250 ÷ 1,000 = 5.25 L/min

This is the basic calculation behind a cardiac output calculator.

Heart Rate and Cardiac Output

Heart rate is one of the two direct components of the cardiac output equation.

If stroke volume stays the same, increasing heart rate increases cardiac output mathematically.

For example:

However, the body is more complicated than the formula. A very rapid heart rate does not necessarily produce proportionally greater effective cardiac output because filling time can change as heart rate rises.

Stroke Volume and Cardiac Output

Stroke volume is the amount of blood ejected by a ventricle with each contraction.

It can be represented as:

Stroke volume = end-diastolic volume − end-systolic volume

In simple terms, it is the amount of blood in the ventricle before contraction minus the amount remaining afterward.

Stroke volume is influenced by preload, contractility, and afterload, which can change the amount ejected with each heartbeat.

Because cardiac output depends on stroke volume, changes in stroke volume can directly change cardiac output.

How to Calculate Cardiac Output

To calculate cardiac output, you need two values:

1. Heart rate in beats per minute
2. Stroke volume in milliliters per beat

Then:

CO = HR × SV

Suppose a person's heart rate is 80 bpm and stroke volume is 65 mL:

80 × 65 = 5,200 mL/min

Convert milliliters to liters:

5,200 ÷ 1,000 = 5.2 L/min

So the estimated cardiac output is 5.2 L/min.

This simple calculation is useful for learning cardiovascular physiology. It should not be confused with a clinically measured cardiac output.

Try the Heart Rate Calculator

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The Heart Rate Calculator can help with heart-rate calculations that are relevant to cardiovascular and exercise tracking.

What Happens When Heart Rate Increases?

When heart rate rises, cardiac output can increase if stroke volume is maintained.

This commonly occurs during exercise. Working muscles need more oxygen, so the cardiovascular system increases blood flow to meet the body's demands.

However, increasing heart rate indefinitely is not a way to continuously increase effective cardiac output. At very high rates, the heart has less time to fill between contractions, and cardiac output can become impaired in some circumstances.

The relationship between heart rate and cardiac output therefore depends on the rest of the cardiovascular system.

What Happens When Stroke Volume Changes?

If heart rate remains constant, increasing stroke volume increases cardiac output.

For example:

70 bpm × 60 mL = 4.2 L/min

Compared with:

70 bpm × 80 mL = 5.6 L/min

Stroke volume can change with preload, contractility, and afterload. Exercise, hydration, conditioning, medications, and disease can influence these variables.

Cardiac Output at Rest vs. During Exercise

The body does not need the same amount of blood flow at rest as it does during strenuous activity.

During exercise, muscles require more oxygen and nutrients, so cardiac output rises. Both heart rate and stroke volume can contribute to this increase.

This does not mean that a higher resting cardiac output is automatically better. Cardiac output must be interpreted in relation to the body's needs and the person's cardiovascular condition.

Why Cardiac Output Matters

Adequate cardiac output is necessary to deliver blood to organs and tissues.

When the body's metabolic demands increase, the cardiovascular system must adjust blood flow accordingly. Problems affecting heart rate, contractility, filling, vascular resistance, or blood volume can alter cardiac output.

Clinicians may assess cardiac output when evaluating cardiovascular problems or determining response to treatment. It is only one part of cardiovascular assessment; blood pressure, oxygenation, rhythm, ventricular function, and symptoms may also matter.

Cardiac Index vs. Cardiac Output

Cardiac index adjusts cardiac output for body surface area.

The formula is:

Cardiac index = cardiac output ÷ body surface area

It is usually expressed in L/min/m².

Why make this adjustment? A larger person's normal cardiac output may naturally be higher than that of a smaller person. Cardiac index provides a body-size-adjusted value that can be useful when comparing cardiovascular performance between individuals.

For example, two people could have different cardiac outputs but similar cardiac indexes after accounting for body size.

Limitations of Simple Cardiac-Output Calculations

The formula CO = HR × SV is straightforward, but obtaining an accurate stroke-volume value is not always simple.

A number entered into a calculator is only as reliable as the measurements used to produce it.

For example, an estimated stroke volume may come from an echocardiographic assessment or another measurement method. Clinical cardiac output can be estimated or measured using techniques such as thermodilution, the Fick method, echocardiography, or other specialized approaches.

Therefore, a simple cardiac output calculator should be viewed as an educational or estimation tool rather than a diagnostic device.

How Cardiac Output Is Measured Clinically

Healthcare professionals can use several methods to estimate or measure cardiac output.

Depending on the clinical situation, methods may include:

• Echocardiography
• Thermodilution
• The Fick principle
• Cardiac MRI
• Other hemodynamic monitoring techniques

Some methods are noninvasive, while others require invasive monitoring. The appropriate method depends on the patient's condition and the clinical question.

These methods are considerably different from simply multiplying an estimated heart rate by an estimated stroke volume.

Summary

Cardiac output is the amount of blood pumped by the heart each minute.

The basic formula is:

Cardiac output = heart rate × stroke volume

For example, a heart rate of 70 bpm and stroke volume of 70 mL produces an estimated cardiac output of 4.9 L/min.

Heart rate and stroke volume work together. During exercise, both can contribute to the increase in cardiac output needed to supply active tissues. However, a faster heart rate does not automatically mean better cardiac output, particularly at very high rates.

Cardiac output is different from cardiac index, which adjusts the measurement for body surface area.

A simple calculator can demonstrate the relationship between heart rate and stroke volume, but clinical cardiac output assessment requires appropriate measurements and professional interpretation.

For related health calculations, explore the Heart Rate Calculator, BMR Calculator, and Calorie Calculator.

References

Cleveland Clinic. (2022). Cardiac output. https://my.clevelandclinic.org/health/diagnostics/23344-cardiac-output

King, J., & Lowery, D. R. (2023). Physiology, cardiac output. StatPearls Publishing. National Center for Biotechnology Information. https://www.ncbi.nlm.nih.gov/books/NBK470455/

Patel, N., Durland, J., Awosika, A. O., & Makaryus, A. N. (2024). Physiology, cardiac index. StatPearls Publishing. National Center for Biotechnology Information. https://www.ncbi.nlm.nih.gov/books/NBK539905/

Vincent, J.-L. (2008). Understanding cardiac output. Critical Care, 12(4), 174. https://pmc.ncbi.nlm.nih.gov/articles/PMC2575587/

Khalil, B., Foster, C., & Launico, M. (2026). Physiology, stroke volume. StatPearls Publishing. National Center for Biotechnology Information. https://www.ncbi.nlm.nih.gov/books/NBK547686/