Cardiac Output Calculator

Cardiac output is the volume of blood the heart pumps per minute, and it comes down to just two things: how fast the heart is beating, and how much blood it ejects with each beat. Multiplying those together gives the full picture that either number alone can't¹.

What This Calculator Measures

This calculates cardiac output from heart rate and stroke volume, and cardiac index (output adjusted for body size) if height and weight are also provided. It's a physiology teaching tool, not a substitute for actual clinical cardiac assessment.

The Formula

Cardiac Output (L/min) = Heart Rate × Stroke Volume ÷ 1000

Cardiac Index (L/min/m²) = Cardiac Output ÷ Body Surface Area, calculated when height and weight are provided, using the Mosteller formula for BSA, the same method used on the Body Surface Area Calculator².

How This Calculator Computes Your Result

Enter heart rate and stroke volume. If you also enter height and weight, we calculate body surface area using the Mosteller formula and divide cardiac output by it to give cardiac index, which adjusts for body size and is more comparable across people of different sizes than raw cardiac output alone.

Why Cardiac Index Matters More Than Raw Cardiac Output for Comparison

Two people can have identical cardiac output but very different cardiac demands simply because they're different sizes, a larger body has more tissue to perfuse. Cardiac index accounts for this by dividing output by body surface area, which is why clinical reference ranges for adequate perfusion are typically expressed as cardiac index rather than raw cardiac output. It's the reason this calculator surfaces cardiac index as a second, more comparable number rather than stopping at the raw output figure.

Typical Resting Values

The table shows typical values for healthy adults at rest. Stroke volume varies with body size, sex and fitness⁴, and cardiac output varies with body size and activity level¹.

These numbers climb with exercise. During intense activity, elite athletes can reach a cardiac output of about 40 L/min¹.

A Worked Example

A heart rate of 70 bpm and a stroke volume of 70 mL give 70 × 70 = 4,900 mL per minute. Dividing by 1,000 gives a cardiac output of 4.9 L/min.

If the same person is 170 cm tall and weighs 70 kg, body surface area is √(170 × 70 ÷ 3600) = 1.82 m², and cardiac index is 4.9 ÷ 1.82 = 2.70 L/min/m². Both sit inside the typical resting ranges above. The same 4.9 L/min in a larger person with a body surface area of 2.2 m² gives a cardiac index of 2.23 L/min/m², below the typical range. Same output, different meaning.

The calculator shows cardiac output to one decimal place, and cardiac index and body surface area to two. In imperial mode it converts feet, inches and pounds to centimeters and kilograms first. If height or weight is left blank, it shows cardiac output only.

What Drives Stroke Volume and Heart Rate

Stroke volume depends on preload (how much blood fills the ventricle before it contracts), contractility (how forcefully the muscle contracts) and afterload (the resistance the ventricle pushes against). More filling and stronger contraction raise stroke volume, while higher afterload lowers it³. Sympathetic nerve activity and catecholamine hormones such as adrenaline raise heart rate and contractility, and parasympathetic activity does the opposite³.

Cardiac output is also linked to blood pressure: it equals mean arterial pressure divided by total peripheral resistance³.

How Cardiac Output Is Measured in Practice

In clinical care, cardiac output is measured rather than worked out from remembered numbers. The direct Fick method, based on oxygen use and the oxygen difference between arterial and venous blood, is the gold standard but needs a pulmonary artery catheter³. Thermodilution, which tracks a temperature change in blood through a catheter, is another invasive method³. Stroke volume is usually assessed with imaging, and cardiac MRI is considered the gold standard among noninvasive methods, with better reproducibility than echocardiography⁴.

This calculator cannot diagnose a heart condition. A typical-looking result does not rule one out, and an unusual one may only reflect an estimated input.