Fick Cardiac Output Calculator

Q: Why does the calculator ask for my patient's sex and age group?

Sex and age set the assumed oxygen use (VO₂ index). The tool uses four values: Male under 70: 125 mL/min/m² Male 70 or older: 110 mL/min/m² Female under 70: 110 mL/min/m² Female 70 or older: 100 mL/min/m² Older bodies and female bodies use a bit less oxygen per square meter, so their index is lower.

Q: Why did I get an error saying SaO2 must be greater than SvO2?

Arterial blood always holds more oxygen than mixed venous blood. If SvO₂ is equal or higher, the math breaks (you would divide by zero or a negative number). Check that you did not swap the two numbers, and make sure the venous sample came from the pulmonary artery.

Q: 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.

Q: What does a low cardiac index mean?

Normal cardiac index is about 2.5–4.0 L/min/m². 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.

Q: 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. Low hemoglobin alone can raise the number, since the body pumps faster to carry the same oxygen.

Q: Which BSA formula does this tool use?

It uses the Haycock formula: BSA = 0.024265 × height(cm) 0.3964 × weight(kg) 0.5378 Other formulas (Du Bois, Mosteller) give slightly different answers, so your cardiac index may shift a little if you compare tools.

Q: 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.

Q: 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.

Q: What does the SvO2 sensitivity chart show me?

It plots cardiac output across a range of SvO₂ values, with your patient marked as a dot. The curve gets steep at high SvO₂. That shows you how much your answer would change if the venous sample were off by a point or two.

Q: 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.

Fick Cardiac Output Calculator

Updated Jul 26, 2026 By Jehan Wadia

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.

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. If you already know the cardiac output and only need the size-adjusted figure, the Cardiac Index Calculator does that step on its own.

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. If you need to convert a chart weight by hand, the Kg to Lbs Calculator can help.

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 — see the standalone BSA Calculator if you want to compare BSA formulas. The Cm to Inches Calculator is handy for quick height conversions.

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. For general fitness context on heart rate, see the Max Heart Rate Calculator and the Heart Rate Zone Calculator.

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 1 gram of hemoglobin can hold (1.36 mL), times 10 to fix the units.

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

BSA here comes from the Haycock formula: BSA = 0.024265 × height(cm)^0.3964 × weight(kg)^0.5378.

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/min
Cardiac index: about 2.5–4.0 L/min/m²
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. 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 and pairs with the MAP Calculator for mean arterial pressure. Related bedside tools include the QTc Calculator, the CHA2DS2-VASc Calculator, and the ASCVD Risk Calculator for longer-term cardiovascular risk. Fluid and renal status often matter too, so clinicians may reach for the Urine Output Calculator, the GFR Calculator, or the Anion Gap Calculator alongside these numbers.

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. Body size checks such as the BMI Calculator or the Adjusted Body Weight Calculator can flag when the assumption is shaky.
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)

\text{BSA} = 0.024265 \times H_{\text{cm}}^{0.3964} \times W_{\text{kg}}^{0.5378}

Estimated Oxygen Consumption (VO2)

\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

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

Why does the calculator ask for my patient's sex and age group?

Sex and age set the assumed oxygen use (VO₂ index). The tool uses four values:

Male under 70: 125 mL/min/m²
Male 70 or older: 110 mL/min/m²
Female under 70: 110 mL/min/m²
Female 70 or older: 100 mL/min/m²

Older bodies and female bodies use a bit less oxygen per square meter, so their index is lower.

Why did I get an error saying SaO2 must be greater than SvO2?

Arterial blood always holds more oxygen than mixed venous blood. If SvO₂ is equal or higher, the math breaks (you would divide by zero or a negative number). Check that you did not swap the two numbers, and make sure the venous sample came from the pulmonary artery.

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. 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. Low hemoglobin alone can raise the number, since the body pumps faster to carry the same oxygen.

Which BSA formula does this tool use?

It uses the Haycock formula:

BSA = 0.024265 × height(cm)^0.3964 × weight(kg)^0.5378

Other formulas (Du Bois, Mosteller) give slightly different answers, so your cardiac index may shift a little if you compare tools.

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.

What does the SvO2 sensitivity chart show me?

It plots cardiac output across a range of SvO₂ values, with your patient marked as a dot. The curve gets steep at high SvO₂. That shows you how much your answer would change if the venous sample were off by a point or two.

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%.

Do I need to enter the oxygen values as decimals or percents?

Enter them as percents, just like the blood gas report shows. So type 98, not 0.98. The tool divides by 100 for you.

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.

Does this calculator work for children?

The VO₂ index values here are set for adults, and the normal ranges shown are adult ranges. Children have higher oxygen use per square meter and different targets. Use pediatric-specific tables and values instead.

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.

Results

Rate this calculator

Introduction

How to use our Fick Cardiac Output Calculator

What Is the Fick Cardiac Output Method?

The Fick Equation

Direct vs. Indirect Fick

Cardiac Index and Stroke Volume

Normal Ranges in Adults

Where It Is Used

Things That Can Throw Off the Result

Formulas used

Frequently asked questions

Why does the calculator ask for my patient's sex and age group?

Why did I get an error saying SaO2 must be greater than SvO2?

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

What does a low cardiac index mean?

Why is my cardiac output high?

Which BSA formula does this tool use?

Is estimated Fick as accurate as thermodilution?

When is the Fick method least accurate?

What does the SvO2 sensitivity chart show me?

Can I switch units after I enter a value?

Where does the 13.6 constant come from?

Do I need to enter the oxygen values as decimals or percents?

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

Does this calculator work for children?

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

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

What is the difference between cardiac output and cardiac index?

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

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