Health calculators

Fick Cardiac Output Calculator

Updated Sep 3, 2026 By Jehan Wadia
Rate Formulas Sources Medical disclaimer
Patient Demographics
Sex
Age Group
Entered in kilograms (typical 1 – 150 kg).
Height
Entered in centimeters (typical 100 – 250 cm).
Clinical Values
Entered in g/dL (typical 7 – 17 g/dL).
Beats per minute at time of sampling
beats/min
As measured on ABG
%
As measured on mixed venous gas from PA catheter
%

Results

Cardiac Output (CO)
—
Normal
Normal range: 4.00 – 8.00 L/min
Cardiac Index (CI)
—
Normal
Normal range: 2.50 – 4.00 L/min/m²
Stroke Volume (SV)
—
Normal
Normal range: 60.00 – 100.00 mL/beat
Intermediate Values
BSA (Haycock)—
Selected VO₂ Index—
Estimated VO₂—
Normalized inputs used—
Step-by-Step Solution
Your Values vs. Normal Ranges
Cardiac Output Sensitivity to SvO₂

Introduction

This Fick Cardiac Output Calculator tells you how much blood the heart pumps each minute. It uses the Fick principle, which links blood flow to how much oxygen the body uses.1

Type in a few values: sex, age group, weight, height, hemoglobin, heart rate, and two oxygen numbers (SaO2 from an arterial blood gas and SvO2 from a PA catheter). The tool then gives you three key results:

  • Cardiac output (CO): liters of blood pumped per minute
  • Cardiac index (CI): cardiac output adjusted for body size
  • Stroke volume (SV): blood pumped with each beat

Each result shows a Low, Normal, or High label next to the normal range. You also get the body surface area, the estimated oxygen use (VO2), a step-by-step math breakdown, and charts that compare your numbers to normal values.

Doctors, nurses, and students use this in cath labs and intensive care units to check heart function when a thermodilution reading is not available or looks wrong. This tool uses an estimated VO2, so treat the answer as a guide, not a final diagnosis. Always use it with the full clinical picture.

How to use our Fick Cardiac Output Calculator

Enter your patient's sex, age group, weight, height, hemoglobin, heart rate, and oxygen saturations. The calculator uses the Fick principle to give you cardiac output (CO), cardiac index (CI), and stroke volume (SV), plus body surface area, estimated VO₂, and a step-by-step solution.

Sex: Tap Male or Female. This sets the assumed oxygen consumption (VO₂) index.

Age Group: Pick "< 70 years" or "≥ 70 years". Older patients use a lower VO₂ index.

Weight: Type the patient's weight. Tap the unit button to switch between kg and lbs.

Height: Type the height in centimeters, or tap the unit button to enter feet and inches. Height and weight are used to find body surface area.

Hemoglobin (Hb): Enter the latest hemoglobin from lab work. Tap the unit button to switch between g/dL and g/L.

Heart Rate (HR): Enter the heart rate in beats per minute at the time the blood samples were taken. This is used for stroke volume.

SaO₂ (%): Enter the arterial oxygen saturation from the ABG.

SvO₂ (%): Enter the mixed venous oxygen saturation from the PA catheter sample. SaO₂ must be higher than SvO₂, or the calculator will ask you to check the values.

Result units: Use the small dropdown on each result card to show CO in L/min or mL/min, CI in L/min/m² or mL/min/m², and SV in mL/beat or L/beat.

Press Calculate to see your results, normal ranges, and charts. Press Reset to clear all fields and start over.

What Is the Fick Cardiac Output Method?

Cardiac output (CO) is how much blood your heart pumps in one minute. The Fick principle is a way to measure it using oxygen. Your body takes in oxygen with each breath, and your blood carries that oxygen to your tissues. If we know how much oxygen the body uses, and how much oxygen the blood picks up and drops off, we can work out how much blood the heart is moving.

The Fick Equation

The main formula is:

CO = VO₂ ÷ [13.6 × Hb × (SaO₂ − SvO₂)]

  • VO₂: oxygen used by the body each minute (mL/min).
  • Hb: hemoglobin, the protein in red blood cells that carries oxygen (g/dL).
  • SaO₂: oxygen saturation in artery blood, taken from an arterial blood gas.
  • SvO₂: oxygen saturation in mixed venous blood, taken from a pulmonary artery (Swan-Ganz) catheter.
  • 13.6: a constant that comes from how much oxygen one gram of hemoglobin can hold (1.36 mL), times 10 to fix the units. That carrying capacity, the Hüfner constant, varies between 1.34 and 1.39 mL per gram under physiological conditions.1

The gap between SaO₂ and SvO₂ is the key part. If the heart pumps slowly, the tissues pull out more oxygen, so venous blood comes back with less oxygen and the gap gets wide. A wide gap means a low cardiac output.

Direct vs. Indirect Fick

In the direct Fick method, VO₂ is measured with special breathing equipment, the same kind of gas-exchange testing behind the VO2 Max Calculator. That is accurate but slow and hard to do in a busy cath lab. The indirect (or "estimated") Fick method uses a standard VO₂ value based on body size, sex, and age instead. That is what this tool does. VO₂ is estimated as an index in mL/min/m², then multiplied by body surface area (BSA):

  • Male, under 70 years: 125 mL/min/m²
  • Male, 70 years or older: 110 mL/min/m²
  • Female, under 70 years: 110 mL/min/m²
  • Female, 70 years or older: 100 mL/min/m².

The 125 mL/min/m² figure is the assumed oxygen consumption published by Dehmer and colleagues.3 The lower indexes for older and female patients are this calculator's convention rather than a published table, so a measured VO₂ is the better input whenever one is available.

BSA here comes from the Haycock formula: BSA = 0.024265 × height(cm)0.3964 × weight(kg)0.5378.2

Cardiac Index and Stroke Volume

Cardiac index (CI) is cardiac output divided by body surface area. It adjusts for body size, so a small adult and a large adult can be compared fairly. Stroke volume (SV) is how much blood the heart pushes out with each single beat. It equals cardiac output divided by heart rate.

Normal Ranges in Adults

  • Cardiac output: about 4–8 L/min1
  • Cardiac index: about 2.5–4.0 L/min/m²4
  • Stroke volume: about 60–100 mL/beat
  • SvO₂: about 60–80%.

A low cardiac index (under about 2.2 L/min/m²) is a warning sign and can point to cardiogenic shock.4 High output can happen with sepsis, fever, anemia, hyperthyroidism, or a shunt.

Where It Is Used

Doctors use Fick cardiac output during right heart catheterization and in the ICU. It helps them check heart failure, shock, valve disease, and pulmonary hypertension, and it is also used to calculate shunt fractions and vascular resistance. Cardiac output feeds directly into the PVR Calculator for pulmonary vascular resistance.

Things That Can Throw Off the Result

  • The estimated VO₂ may be wrong in patients who are very sick, sedated, obese, or on a ventilator.
  • The venous sample must be true mixed venous blood from the pulmonary artery, not from a central line.
  • Blood samples for SaO₂ and SvO₂ should be drawn at the same time, in a steady state.
  • The method is least accurate when cardiac output is high, because the oxygen gap is small.
  • Old or outdated hemoglobin values will shift the answer.

Note: This tool gives an estimate for education and quick reference. It does not replace measured values or the judgment of a trained clinician.

Formulas used

Body Surface Area (Haycock formula) 2
\text{BSA} = 0.024265 \times H_{\text{cm}}^{0.3964} \times W_{\text{kg}}^{0.5378}
Estimated Oxygen Consumption (VO2) 3
\dot{V}O_2 = \text{VO}_2\ \text{index} \times \text{BSA}
VO2 Index Selection by Sex and Age
\text{VO}_2\ \text{index} = \begin{cases} 125 & \text{Male},\ \text{age} < 70 \\ 110 & \text{Male},\ \text{age} \ge 70 \\ 110 & \text{Female},\ \text{age} < 70 \\ 100 & \text{Female},\ \text{age} \ge 70 \end{cases}
Cardiac Output (Fick principle)
CO = \frac{\dot{V}O_2}{13.6 \times Hb \times \dfrac{SaO_2 - SvO_2}{100}}
Cardiac Index 4
CI = \frac{CO}{\text{BSA}}
Stroke Volume
SV = \frac{CO}{HR} \times 1000
Unit conversions for inputs
W_{\text{kg}} = W_{\text{lb}} \times 0.453592, \quad H_{\text{cm}} = (ft \times 12 + in) \times 2.54, \quad Hb_{\text{g/dL}} = \frac{Hb_{\text{g/L}}}{10}

Frequently asked questions

Can I use a central line SvO2 instead of a PA catheter sample?

It is not the same. A central line gives ScvO₂, which usually runs a few points higher than true mixed venous SvO₂. If you enter ScvO₂, your cardiac output will read too high. Use a pulmonary artery sample when you can.

What does a low cardiac index mean?

Normal cardiac index is about 2.5–4.0 L/min/m².4 Under 2.2 L/min/m² is a red flag and can point to cardiogenic shock or bad heart failure. The heart is not moving enough blood for the body's size. Treat this as a signal to act, not a diagnosis on its own.

Why is my cardiac output high?

High output (over 8 L/min) shows up with sepsis, fever, anemia, an overactive thyroid, pregnancy, or a shunt.1 Low hemoglobin alone can raise the number, since the body pumps faster to carry the same oxygen.

Is estimated Fick as accurate as thermodilution?

No. Thermodilution and direct Fick (with measured VO₂) are more accurate. Estimated Fick guesses the VO₂ from body size, sex, and age, so errors of 10–25% are common. It is most useful as a cross-check or when thermodilution is unreliable, such as in tricuspid regurgitation or low-output states.

When is the Fick method least accurate?

When cardiac output is high. The gap between SaO₂ and SvO₂ gets small, so a 1% measuring error causes a big swing in the answer. It is also shaky in patients who are sedated, on a ventilator, very obese, shivering, or febrile, because their real VO₂ is far from the assumed value.

Can I switch units after I enter a value?

Yes. Tap the unit button next to weight, height, or hemoglobin and the number converts for you. For example, 70 kg becomes 154.3 lbs. Results recalculate right away.

Where does the 13.6 constant come from?

One gram of hemoglobin carries about 1.36 mL of oxygen. Multiply by 10 to change g/dL into g/L of blood, and you get 13.6. Some labs use 1.34 or 1.39 instead, which shifts the answer by 1–2%.1

Why is stroke volume low when my cardiac output is normal?

Stroke volume is cardiac output divided by heart rate. A fast heart rate can keep output normal while each beat moves less blood. That pattern is common in heart failure, blood loss, or after fluid loss.

What if my hemoglobin lab value is a few days old?

Use the newest value you have. Hemoglobin sits right in the bottom of the formula, so an old number after bleeding or a transfusion will push the result the wrong way. A drop from 14 to 10 g/dL raises the calculated output by about 40%.

Why do the SaO2 and SvO2 samples need to be drawn at the same time?

The Fick equation assumes a steady state. If the samples are minutes apart, the heart rate, breathing, or drips may have changed in between. That mismatch adds error you cannot see in the answer.

What is the difference between cardiac output and cardiac index?

Cardiac output is the raw flow in L/min. Cardiac index divides that by body surface area, so it fits the patient's size. A 5 L/min output is fine for a small adult but low for a large one, and cardiac index catches that.

Why do some fields show an orange warning instead of an error?

An orange note means the value is possible but unusual, so double-check it. An error in red means the tool cannot run the math. For example, a hemoglobin of 3 g/dL gets a warning; a hemoglobin of 0 gets an error.

Sources

  1. King J, Lowery DR. Physiology, Cardiac Output. StatPearls. StatPearls Publishing, NCBI Bookshelf. Accessed September 3, 2026.
  2. Flint B, Das JM, Hall CA. Body Surface Area. StatPearls. StatPearls Publishing, NCBI Bookshelf. Accessed September 3, 2026.
  3. Cullen G, et al. Resting oxygen consumption estimates in scleroderma can lead to underestimation of cardiac output. Pulmonary Circulation. 2026. doi:10.1002/pul2.70280. Accessed September 3, 2026.
  4. Patel N, Durland J, Awosika AO, Makaryus AN. Physiology, Cardiac Index. StatPearls. StatPearls Publishing, NCBI Bookshelf. 2024. Accessed September 3, 2026.

Related Health calculators

All Health calculators
Health

ASCVD Risk Calculator

Estimate 10-year, lifetime, and 30-year ASCVD risk using PCE and PREVENT equations. Compare statin, BP med, smoking, and aspirin scenarios with full math.

Open Health

Cholesterol Calculator

Estimate LDL via Friedewald, plus non-HDL, VLDL, and cholesterol ratios. Get 10-year ASCVD risk from age, blood pressure, smoking. mg/dL or mmol/L.

Open Health

Cholesterol Ratio Calculator

Calculate your cholesterol ratio instantly. Enter total cholesterol and HDL values to assess heart disease risk. Free, fast, and easy-to-use online tool.

Open Health

CVD Risk Calculator

Estimate 10-year ASCVD, heart failure, and total CVD risk with PREVENT or Pooled Cohort equations. View therapy projections and step-by-step math.

Open Health

LDL Calculator

Calculate LDL cholesterol from total cholesterol, HDL, and triglycerides. Choose Friedewald, Modified Friedewald, or Martin-Hopkins with mg/dL or mmol/L.

Open Health

MAP Calculator

Compute MAP from SBP and DBP using classic or heart-rate-adjusted formulas. Solve for missing BP, find pulse pressure. Adult and pediatric interpretation.

Open Health

QTc Calculator

Free QTc calculator to compute corrected QT interval using Bazett, Fridericia, and other formulas. Enter heart rate and QT interval for instant results.

Open Health

Total Cholesterol Calculator

Calculate total cholesterol from LDL, HDL, and triglycerides with the Friedewald equation. Get non-HDL, VLDL, TC/HDL and TG/HDL ratios in mg/dL or mmol/L.

Open