Cardiac Index Calculator
Enter your cardiac output (or stroke volume and heart rate) along with height and weight to calculate the cardiac index. The calculator uses the Haycock formula for body surface area, shows the full working, and flags where the result sits on the clinical range scale. Switch between metric and imperial units; results update as you type.
Formula
Worked example
A patient weighing 70 kg and 175 cm tall: BSA = 0.024265 x 175^0.3964 x 70^0.5378 = 1.848 m². With a cardiac output of 5.5 L/min, CI = 5.5 / 1.848 = 2.98 L/min/m², which falls within the normal resting range.
What is the cardiac index?
Cardiac index (CI) is cardiac output normalized to body surface area. Cardiac output (CO) is the total volume of blood the heart pumps every minute, roughly 4 to 8 litres in a resting adult. Because larger people simply have more tissue to perfuse, a raw CO of 6 L/min means something different in a 45 kg person than in a 120 kg person. Dividing by body surface area gives a size-adjusted figure that allows meaningful comparisons between patients and against population reference ranges. The normal resting CI in adults is 2.5 to 4.0 L/min/m².
How the calculation works
The formula is CI = CO / BSA. Cardiac output can be measured directly (thermodilution, Fick method, echocardiography) or estimated here as stroke volume multiplied by heart rate, then divided by 1000 to convert mL/beat x beats/min to L/min. Body surface area is calculated with the Haycock formula: BSA (m²) = 0.024265 x height (cm)^0.3964 x weight (kg)^0.5378. The Haycock equation is widely used in clinical pharmacology and paediatric dosing because it performs well across a wide range of body sizes.
Clinical significance of abnormal values
A CI below 2.5 L/min/m² at rest suggests reduced cardiac output relative to body demands. Values below 2.0 L/min/m² are associated with cardiogenic shock, a life-threatening state where the heart cannot maintain adequate perfusion pressure. Causes include acute myocardial infarction, severe heart failure, cardiomyopathy, and major valve dysfunction. Elevated CI (above 4.0-5.0) can indicate high-output states such as sepsis, severe anaemia, hyperthyroidism, pregnancy, or arteriovenous fistulae. Isolated CI values must always be interpreted alongside mean arterial pressure, pulmonary wedge pressure, and mixed venous oxygen saturation for a complete haemodynamic picture.
BSA formulas compared
Several equations exist for estimating body surface area. The DuBois and DuBois formula (1916) was the original standard and is still widely quoted: BSA = 0.007184 x height (cm)^0.725 x weight (kg)^0.425. The Mosteller formula (1987) is simpler: BSA = sqrt(height (cm) x weight (kg) / 3600). The Haycock formula (1978) is considered more accurate in children and at extremes of body size, which is why it is used here. In typical-sized adults the three formulas agree closely, usually within 2-3 percent.
Cardiac Index clinical reference ranges
| CI (L/min/m²) | Classification | Clinical significance |
|---|---|---|
| Below 2.0 | Cardiogenic shock range | Critical - immediate assessment needed |
| 2.0 - 2.4 | Low | Possible low cardiac reserve |
| 2.5 - 4.0 | Normal (resting) | Typical healthy adult at rest |
| 4.1 - 5.0 | Elevated | May indicate high-output state (sepsis, anemia, pregnancy) |
| Above 5.0 | High | High-output conditions or measurement artifact |
Typical resting values for adults. Ranges vary by source; always interpret alongside the full clinical picture.
Frequently asked questions
What is a normal cardiac index?
The widely accepted normal range for a resting adult is 2.5 to 4.0 L/min/m². Some sources cite a slightly wider range of 2.6 to 4.2 L/min/m². Values vary with age, fitness level, body position, and the method used to measure cardiac output.
What is the difference between cardiac output and cardiac index?
Cardiac output (CO) is the total litres of blood the heart pumps per minute, regardless of body size. Cardiac index (CI) divides that figure by body surface area (in m²) so that a result from a small person is directly comparable to one from a large person. CI is preferred in research and critical care because it removes the confounding effect of body size.
What does a low cardiac index mean?
A CI below 2.5 L/min/m² suggests the heart is not delivering enough blood relative to body size. Causes include heart failure, acute myocardial infarction, severe valvular disease, cardiomyopathy, or tamponade. Values below 2.0 are associated with cardiogenic shock and require urgent clinical evaluation.
What does a high cardiac index mean?
A CI above 4.0-5.0 L/min/m² usually indicates a high-output state: the heart is pumping more than the resting baseline. Common causes are sepsis, severe anaemia, hyperthyroidism, pregnancy, liver failure, and arteriovenous fistulae. A high CI is not always benign; in septic shock the high output may still be insufficient to meet tissue oxygen demand.
Which BSA formula does this calculator use?
This calculator uses the Haycock formula (1978): BSA = 0.024265 x height (cm)^0.3964 x weight (kg)^0.5378. It is considered more accurate than the older DuBois formula across the full range of body sizes, particularly in children and very small or very large adults.
Can I use stroke volume and heart rate instead of cardiac output?
Yes. Switch the input mode to "Stroke volume + heart rate" and enter stroke volume in mL/beat and heart rate in beats per minute. The calculator multiplies them and divides by 1000 to get cardiac output in L/min, then proceeds with the normal CI calculation.