Cardiac Index Calculator
Normal CO at rest: 4.0–8.0 L/min
Body Measurements (for BSA)
Enter measurements to calculate cardiac index
Results update in real time as you type
This Cardiac Index Calculator estimates CI from cardiac output (or stroke volume and heart rate) plus height and weight. ICU nurses and med students stare at this number daily, but the math trips people up when BSA enters the picture. Read on and you'll also find out what a 2.5 CI actually means, why body size matters, and how cardiac index ties into heart failure monitoring.
From this page, you'll learn:
- How to enter cardiac output or SV/HR with height and weight to get CI instantly
- What cardiac index measures and how it differs from raw cardiac output
- How to read low, normal, and high CI values on a clinical reference table
- Why the same cardiac output can mean different things for a small vs large patient
How to Use the Cardiac Index Calculator
Pick your input method, enter body measurements, and read CI on the right. Results update as you type.
- 1Choose By Cardiac Output if you already have CO in L/min from a monitor or cath lab report.
- 2Or choose By SV & HR. Enter stroke volume (mL) and heart rate (bpm). We calculate CO = SV × HR / 1000.
- 3Enter height and weight with your preferred units (cm/in, kg/lbs). These feed the Haycock BSA formula.
- 4Read cardiac index in L/min/m² on the results panel, plus BSA, CO, and a clinical category badge.
- 5Check the reference gauge. Green zone is roughly 2.5–4.0. Below 2.2 warrants closer clinical review.
Worked Example: Dr. Patel reviews Mr. Chen's hemodynamics
Mr. Chen is 68 years old. His cardiac output reads 4.8 L/min. His body surface area is 1.87 m². Dr. Patel enters CO = 4.8, then height and weight so the calculator confirms BSA. Cardiac index = 4.8 ÷ 1.87 = 2.57 L/min/m².
Cardiac Output
4.8 L/min
BSA
1.87 m²
Cardiac Index
2.57
Category
Normal
That lands in the normal range. Mr. Chen's heart is pumping enough blood relative to his body size. And just like that, the number makes sense in context. Totally worth running before you write the note.
What Is Cardiac Index?
Imagine two patients with the same cardiac output of 5.0 L/min. One is a 55 kg woman. The other is a 110 kg man. Same pump volume. Very different bodies. Cardiac index fixes that by dividing output by body surface area so you can compare heart performance fairly across sizes.
Mr. Chen's 2.57 tells Dr. Patel more than "4.8 L/min" alone. It says his heart delivers about 2.57 liters of blood per minute for every square meter of body surface. That's a normalized snapshot of pump function.
Cardiac index vs cardiac output
Cardiac output (CO) is total flow per minute. Cardiac index (CI) is CO adjusted for body size. A "normal" CO of 5 L/min might be fine for a large patient but inadequate for a small one. CI removes that guesswork. Note that our calculator uses the Haycock BSA formula, which is standard in ICU settings but may differ slightly from Du Bois in some older charts.
What affects cardiac index?
Stroke volume and heart rate drive CO, which drives CI. Heart failure drops CI because the ventricle pumps less per beat. Sepsis, fever, anemia, and pregnancy can push CI high. Age matters too. In most cases, elderly patients run a slightly lower CI at rest, though context always wins over a single cutoff.
What does a 2.3 cardiac index mean? It sits in the low-to-borderline zone. Not automatically shock, but your clinical team would weigh symptoms, blood pressure, and lactate before calling it fine. A 2.5 CI like Mr. Chen's? Right at the lower edge of normal. Comfortable for many patients at rest.
ICU monitors often flash CO first. Smart clinicians glance at CI next. That's the number that actually travels well between patients. Now you know why both appear on the same screen.
The Cardiac Index Formula
Let's walk through Mr. Chen's numbers by hand so the calculator output clicks when you see it.
CI = CO / BSA
CO = SV × HR / 1000
BSA = 0.024265 × H^0.3964 × W^0.5378
- CICardiac index in L/min/m². Your size-adjusted pump performance.
- COCardiac output in L/min. Blood volume the left ventricle ejects per minute.
- BSABody surface area in m² from height (cm) and weight (kg).
- SVStroke volume in mL per heartbeat. Normal roughly 60–100 mL.
- HRHeart rate in beats per minute.
Mr. Chen's calculation step by step
Start with cardiac output
CO = 4.8 L/min (from monitor)
CO = 4.8 L/min
Confirm body surface area
BSA = 1.87 m² (from height + weight)
BSA = 1.87 m²
Divide CO by BSA
CI = 4.8 ÷ 1.87
CI = 2.57 L/min/m²
Pretty easy, isn't it? Of course, you can skip all this counting and let the Cardiac Index Calculator do it instantly.
Cardiac Index Reference Ranges
Use this table to interpret CI results at rest. Ranges vary slightly by source and clinical context.
| Cardiac Index (L/min/m²) | Category | Typical Clinical Meaning |
|---|---|---|
| Below 2.2 | Low | May suggest reduced cardiac function |
| 2.2 – 2.5 | Low-normal | Borderline, context matters |
| 2.5 – 4.0 | Normal | Typical healthy range |
| Above 4.0 | High | May suggest hyperdynamic state |
Mr. Chen's cardiac index of 2.57 L/min/m² falls in the Normal row, meaning his heart is delivering an adequate volume of blood relative to his body size at rest.
Where Cardiac Index Gets Used
Heart failure in the ICU
When CI drops below 2.2 L/min/m², teams worry the heart isn't keeping up. Heart failure patients often run low CI because a weakened ventricle pushes less blood per beat. Doctors track CI alongside blood pressure, urine output, and oxygen levels. One number never tells the whole story, but a falling CI is a red flag worth chasing.
Why is my cardiac index high?
CI above 4.0 can show up with sepsis, hyperthyroidism, severe anemia, or post-op stress. The heart compensates by pumping faster and harder. Context is everything.
Age and the elderly
Consider Mrs. Okonkwo, 79, with CI 2.1 at rest. For her age and medication list, that might be acceptable. A 30-year-old with the same number? Different conversation. Elderly patients typically have lower CI, but symptoms like fatigue, swelling, and shortness of breath matter more than the digit alone.
Can you improve cardiac index?
In clinical settings, fluids, medications, and devices (like balloon pumps) can raise CI when it's dangerously low. Lifestyle changes that strengthen the heart over time may help long-term pump function. We're not your doctor, so treat this as background info, not a treatment plan.
Cardiac Index vs Cardiac Output: The Mix-Up
Here's where most people get confused about cardiac index: they read CO and assume the patient is fine. Patient A has CO 5.2 L/min and BSA 2.4 m². CI = 2.17. Patient B has CO 4.1 L/min and BSA 1.5 m². CI = 2.73. Patient A has higher output but worse index. See the difference?
Sound familiar? Textbooks love cardiac output because it's easy to measure. Bedside rounds lean on CI because it levels the playing field. Our calculator shows both so you never mix them up on a worksheet or a monitor strip.
That's why we compute BSA from height and weight automatically instead of asking you to look it up in a nomogram chart from 1987.
FAQs
How do you calculate cardiac index?
Divide cardiac output by body surface area. Here's the step-by-step: (1) Get CO in L/min from a monitor or calculate it as SV × HR / 1000. (2) Calculate BSA from height and weight using the Haycock formula. (3) Divide CO by BSA. Example: 4.8 ÷ 1.87 = 2.57 L/min/m². Our calculator runs all three steps in real time.
What is a good cardiac index?
For most adults at rest, a cardiac index between 2.5 and 4.0 L/min/m² is considered normal. Below 2.2 is low and may suggest reduced pump function. Above 4.0 is high and may point to a hyperdynamic state. Mr. Chen's 2.57 sits comfortably in the good range.
What does a 2.5 cardiac index mean?
A CI of 2.5 L/min/m² is at the lower boundary of the normal range. It means the heart pumps about 2.5 liters of blood per minute for each square meter of body surface. For many patients at rest, that's acceptable. If symptoms or other vitals are off, your care team would investigate further.
What is the difference between low and high cardiac index?
Low CI (below 2.2) means the heart isn't delivering enough blood relative to body size. You might see this in heart failure, cardiogenic shock, or after a large heart attack. High CI (above 4.0) means the heart is working overtime, often due to infection, blood loss, or hypermetabolic states. Normal CI means the pump matches the body's needs at rest.
How does cardiac index relate to heart failure?
Heart failure weakens the ventricle's ability to pump, which typically lowers stroke volume and cardiac output. Because CI normalizes for body size, it's one of the key numbers ICU teams watch in heart failure patients. A CI that keeps dropping despite treatment is a warning sign that the heart is falling behind, even if blood pressure hasn't crashed yet.
Related calculators: Target Heart Rate Calculator · BMI Calculator · BRI Calculator
References
- Patel N, Durland J, Awosika AO, Makaryus AN. Physiology, Cardiac Index. StatPearls. Treasure Island (FL): StatPearls Publishing; 2024.
- King J, Lowery DR. Physiology, Cardiac Output. StatPearls. Treasure Island (FL): StatPearls Publishing; 2023.
- Oberman R, Shumway KR, Bhardwaj A. Physiology, Cardiac. StatPearls. Treasure Island (FL): StatPearls Publishing; 2023. (Heart rate 60–100 bpm.)
- Haycock GB, Schwartz GJ, Wisotsky DH. Geometric method for measuring body surface area: a height-weight formula validated in infants, children, and adults. J Pediatr. 1978;93(1):62-66. PMID: 650346.
- Du Bois D, Du Bois EF. A formula to estimate the approximate surface area if height and weight be known. 1916. Nutrition. 1989;5(5):303-311. (Seminal reprint of the 1916 Archives of Internal Medicine paper.) PMID: 2520314.
- Baran DA, Grines CL, Bailey S, et al. SCAI clinical expert consensus statement on the classification of cardiogenic shock. Catheter Cardiovasc Interv. 2019;94(1):29-37. PMID: 31104355. (Endorsed by ACC, AHA, SCCM, STS.)
- Naidu SS, Baran DA, Jentzer JC, et al. SCAI SHOCK Stage Classification Expert Consensus Update. J Am Coll Cardiol. 2022;79(9):933-946. (Endorsed by ACC, AHA, ESC ACVC, and others.)
- Singh S, Sharma S. High-Output Cardiac Failure. StatPearls. Treasure Island (FL): StatPearls Publishing; 2023.
- Cioccari L, Luethi N, Glassford NJ, Bellomo R. The normal cardiac index in older healthy individuals: a scoping review. Crit Care Resusc. 2019;21(1):9-17. PMID: 30857507.
- Jasińska-Gniadzik K, Szwed P, Gąsecka A, Zawadka M, Grabowski M, Pietrasik A. Haemodynamic monitoring in acute heart failure – what you need to know. Postepy Kardiol Interwencyjnej. 2022;18(2):90-100. PMID: 36051835. PMC: PMC9421519.