Cardiac Output vs Cardiac Index

Cardiac output and cardiac index measure the same thing at different scales. Here's the difference in one screen — and why clinicians often prefer the index.

The difference in one line

Cardiac output is the total blood the heart pumps per minute; cardiac index is that output divided by body surface area, so it adjusts for body size. A 6 L/min output means something very different in a small child than in a large adult — indexing makes the number comparable.

Side by side

	Cardiac output	Cardiac index
Definition	Blood pumped per minute	Cardiac output ÷ BSA
Formula	Heart rate × stroke volume	CO ÷ body surface area
Units	L/min	L/min/m²
Normal	4–8	2.5–4.0
Adjusts for size?	No	Yes

Why cardiac index is often preferred

Because it accounts for body size, cardiac index grades the severity of low-output states more fairly than raw output. A cardiac index below 2.2 L/min/m² is the classic threshold for cardiogenic shock, regardless of whether the patient is petite or tall. Read the full detail in our cardiac index and cardiac output guides.

Work it out

Convert between them instantly with the cardiac index calculator, or derive the whole panel at once with the all-in-one hemodynamic calculator.

Key takeaways

• Cardiac output = heart rate × stroke volume (L/min).
• Cardiac index = cardiac output ÷ body surface area (L/min/m²).
• Normal output is 4–8; normal index is 2.5–4.0.
• Cardiac index grades shock severity better because it accounts for size.

Frequently asked questions

What is the difference between cardiac output and cardiac index?

Cardiac output is the total blood pumped per minute (L/min); cardiac index is that output divided by body surface area (L/min/m²), which adjusts for body size.

What are the normal values?

Cardiac output is 4–8 L/min and cardiac index is 2.5–4.0 L/min/m².

Why use cardiac index instead of cardiac output?

Because it accounts for body size, it grades low-output states more fairly — for example, a cardiac index under 2.2 L/min/m² defines cardiogenic shock in any patient.

How do you convert cardiac output to cardiac index?

Divide cardiac output in litres per minute by the body surface area in square metres.