About the Cardiac Output Calculator
Cardiac output is the volume of blood the heart pumps each minute. This calculator works it out two ways. The direct method multiplies heart rate by stroke volume, for when stroke volume is known from echocardiography or a monitoring device. The Fick method divides oxygen consumption by the difference in oxygen content between arterial and mixed venous blood, as done in the cardiac catheterisation laboratory.
For the Fick method, enter haemoglobin, arterial and mixed venous oxygen saturations (and optionally PaO₂ and PvO₂ for the small dissolved-oxygen fraction). Oxygen consumption can be entered if it was measured; otherwise the calculator uses the common assumed value of 125 mL/min per m² of body surface area — the “assumed Fick”. Real resting VO₂ varies widely (one study of adults measured 70–176 mL/min/m²), so an assumed value can introduce large errors.
Height and weight give body surface area (Mosteller), which converts cardiac output into cardiac index and stroke volume into stroke volume index so patients of different size can be compared. A typical resting adult cardiac output is about 4–8 L/min and cardiac index about 2.5–4.0 L/min/m². The tool is for education and clinical calculation, not for diagnosis.
With the default inputs, the cardiac output is 5.25 L/min. Change any value above to recalculate instantly.
How to use the cardiac output calculator
- 1Choose HR × stroke volume or the Fick method.
- 2Enter heart rate and, for the direct method, stroke volume.
- 3For Fick, enter haemoglobin, SaO₂ and SvO₂ (and VO₂ if measured).
- 4Enter height and weight to get cardiac index and stroke volume index.
- 5Compare the cardiac index with the typical resting range.
Formula and method
The direct method multiplies beats per minute by millilitres per beat and divides by 1,000 to give litres per minute. The Fick principle states that oxygen taken up by the body equals blood flow multiplied by the arteriovenous oxygen content difference. Oxygen content (mL O₂ per dL) is 1.34 mL per gram of saturated haemoglobin plus 0.003 mL per mmHg of dissolved oxygen; multiplying the difference by 10 converts dL to L.
When VO₂ is not measured, 125 mL/min/m² is a widely used assumption for resting adults. Body surface area uses the Mosteller formula, √(height cm × weight kg ÷ 3600). Cardiac index (CO ÷ BSA) and stroke volume index (SV ÷ BSA) normalise for body size.
- CO
- Cardiac output (L/min)
- HR, SV
- Heart rate (bpm) and stroke volume (mL)
- VO₂
- Oxygen consumption (mL/min)
- CaO₂, CvO₂
- Arterial and mixed venous oxygen content (mL O₂/dL)
- BSA
- Body surface area (m²)
Worked examples
Direct method, healthy adult
75 beats × 70 mL = 5,250 mL/min = 5.25 L/min. BSA = √(175 × 75 ÷ 3600) ≈ 1.91 m², so the cardiac index is about 2.75 L/min/m², within the typical resting range.
Assumed Fick in the cath lab
O₂ content difference = 1.34 × 12 × (0.96 − 0.60) ≈ 5.79 mL/dL. BSA ≈ 1.94 m², so assumed VO₂ ≈ 243 mL/min. CO = 243 ÷ 57.9 ≈ 4.20 L/min and CI ≈ 2.16 L/min/m², just below the typical range.
Measured VO₂ with dissolved oxygen
CaO₂ = 1.34 × 10 × 0.99 + 0.003 × 100 = 13.57; CvO₂ = 1.34 × 10 × 0.65 + 0.003 × 35 = 8.815. The difference is 4.75 mL/dL, so CO = 250 ÷ 47.5 ≈ 5.26 L/min, about 58 mL per beat at 90 bpm.
Frequently asked questions
What is a normal cardiac output?+
A healthy resting adult typically has a cardiac output of about 4 to 8 L/min. Because larger people need more flow, the cardiac index (output divided by body surface area), normally about 2.5–4.0 L/min/m², is a better comparison.
What is the difference between cardiac output and cardiac index?+
Cardiac output is total flow in litres per minute. Cardiac index divides that by body surface area, so a small and a large patient with the same index have proportionally similar heart performance.
How accurate is the assumed Fick method?+
Assuming VO₂ of 125 mL/min/m² is convenient but unreliable: in a Stanford study of 57 adults, measured VO₂ ranged from 70 to 176 mL/min/m² and assumed values gave large, unpredictable errors. Measured oxygen consumption or thermodilution gives more reliable values.
How do you calculate stroke volume from cardiac output?+
Stroke volume equals cardiac output divided by heart rate. For example, 5 L/min at 70 bpm is 5,000 ÷ 70 ≈ 71 mL per beat. Normal adult stroke volume is roughly 60–100 mL.
Why does the Fick method need mixed venous blood?+
Mixed venous blood from the pulmonary artery reflects oxygen extraction by the whole body. Blood from a peripheral or central vein represents only part of the circulation and can overestimate or underestimate the true value.
For healthcare professionals and education only. This calculator implements a published formula or score but is not a diagnosis and does not replace clinical judgment. Verify inputs and results against the original source before using them in patient care.