By Zencalcy Editorial TeamLast reviewed About us
Cardiac index adjusts cardiac output for body size by dividing it by body surface area. The result is expressed in litres per minute per square metre and is more comparable between people of different sizes than cardiac output alone. This calculator performs that division and compares the result with the usual reference range.
Cardiac output scaled to body surface area.
CI = CO ÷ BSA
A cardiac index of roughly 2.5–4.0 L/min/m² is commonly cited as normal at rest. Values below 2.5 are sometimes described as low and values above 4.0 as high. The index depends on the BSA estimate used, so serial measurements should use the same formula and the same height and weight inputs.
| Symbol | Meaning |
|---|---|
| CO | Cardiac output in litres per minute. |
| BSA | Body surface area in square metres. |
| CI | Cardiac index in litres per minute per square metre. |
Cardiac index = 5.0 ÷ 1.8 = 2.78 L/min/m², within the usual range. With CO 4.0 L/min and BSA 1.6 m², the index is 2.50 L/min/m², at the lower edge. The same CO of 4.0 L/min in a BSA of 2.2 m² gives 1.82 L/min/m², which is low.
Cardiac output is a flow, and flow requirements grow with body mass. A large athlete and a small older adult can have very different outputs at rest without either being abnormal. Indexing divides out that size effect and makes a bedside number easier to compare with reference values. It is not a perfect correction: adipose tissue is less metabolically active than muscle, and body surface area formulas are estimates rather than measurements. Extremes of body size, oedema and amputation all reduce the accuracy of the correction. Even so, the index is widely used in critical care and cardiology because it answers a practical question: is this flow adequate for this body? Interpret it with perfusion markers and the clinical trajectory, and use the same BSA method when following a patient over time.
Continue with the Mean Arterial Pressure Calculator, Pulse Pressure Calculator, or the QTc Calculator — or browse more cardiology tools and all health calculators.
Cardiac output scales with body size, so a value that is normal for a large adult may be low for a small child and vice versa. Dividing by body surface area makes comparisons more meaningful across different body sizes.
Blood pressure, ECG and risk scores are screening aids, not diagnoses. A single reading can be misleading; trends and proper technique matter. Scores estimate population risk and do not replace clinical assessment, anticoagulation decisions or emergency care.
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Last reviewed: September 2026. See how we build and check every tool on the about page.
Content updated: September 2026