How this instrument works
Cardiac index takes cardiac output — the litres of blood the heart circulates per minute — and divides it by body surface area, the total skin surface of the person whose heart is being measured. A raw output figure on its own is hard to interpret: 4.9 L/min might be entirely normal for someone of average build and genuinely low for someone considerably larger, since a bigger body has more tissue to perfuse. Dividing by body surface area puts every reading on the same scale, in litres per minute per square metre.
Body surface area itself is usually estimated rather than measured directly, most often with the Mosteller formula from height and weight — this site's own BSA calculator computes that figure, which then feeds directly into the index. A commonly cited normal range for the index at rest runs roughly from 2.5 to 4.0 L/min/m², with figures below that threshold flagging output that may be inadequate for the body it's serving, a distinction a raw output number alone cannot make.
As with output alone, this index is one number among many that clinicians weigh together, not a standalone verdict. It says nothing about why a reading is low or high — only whether it's proportionate to body size. StatPearls' physiology reference on the topic covers the underlying reasoning and its role in critical care monitoring in more depth than a calculator page can.
- Enter Cardiac output (L/min) — pull this from the cardiac output calculator, or from a monitored or measured value.
- Enter Body surface area (m²) — from the BSA calculator on this site, or a directly measured figure if available.
- Read Cardiac index (L/min/m²) — the calculator divides cardiac output by body surface area.
- Compare the result against the roughly 2.5-4.0 L/min/m² range typically cited as normal at rest.
- Re-run the figure whenever either input changes, since cardiac index shifts with both the heart's performance and any change in the body surface area estimate.
Worked example — 4.9 L/min over 2.0 m²
A cardiac output of 4.9 L/min over a body surface area of 2.0 m²: 4.9 ÷ 2.0 = 2.45 L/min/m² — inside the roughly 2.5-4.0 range generally considered normal, though sitting right at its lower edge here.
Raise the output to 6.0 L/min while keeping the same 2.0 m² body surface area, and the index becomes 6.0 ÷ 2.0 = 3.0 L/min/m², comfortably mid-range. Now take a smaller person, body surface area 1.6 m², with a cardiac output of 4.0 L/min: 4.0 ÷ 1.6 = 2.5 L/min/m² — again at the low edge of normal, despite a lower raw figure than the first example. The smaller body needs less total flow to reach the same relative adequacy.
Questions
Why not just use cardiac output on its own?
Because the same raw figure can mean different things depending on body size. A cardiac output of 4.9 L/min might be entirely unremarkable for one person and borderline low for someone with a considerably larger body surface area — the index removes that ambiguity by scaling for size.
What is a normal cardiac index?
Roughly 2.5 to 4.0 L/min/m² at rest, per commonly cited clinical reference ranges. Figures meaningfully below that band can indicate inadequate heart performance relative to body size, though the number is always interpreted alongside blood pressure, symptoms, and other measurements rather than in isolation.
Where does the body surface area figure come from?
Most often from a formula estimating surface area from height and weight — the Mosteller formula is the most widely used, and this site's BSA calculator computes it directly. A measured or more precise BSA figure can be substituted here if one is available.
Does cardiac index change with exercise the same way cardiac output does?
Yes — since body surface area stays fixed for a given person, the index rises and falls in step with the underlying output. The 2.5-4.0 L/min/m² range cited here describes a resting state, so exercise readings running higher reflect increased demand rather than an abnormal result.
Can two people have the same cardiac output but different cardiac index?
Yes, and this is precisely the point of the calculation. A smaller person and a larger person can share an identical output while their index values differ meaningfully, because the smaller body surface area divides into the same numerator to produce a higher result.
Is a low cardiac index always a medical concern?
Not automatically — the number sits within a range, and values near the lower edge aren't the same as clearly abnormal ones. Interpretation depends on the clinical picture as a whole, including how the figure compares to a person's baseline and what other findings accompany it, so this calculator supports that judgment rather than replacing it.
References
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.