CO2 Partial Pressure Calculator and Formula

A CO2 partial pressure calculator estimates the pressure contributed by carbon dioxide in a gas mixture using its mole fraction and the mixture’s total pressure. Enter the CO2 fraction and total pressure; the calculator returns CO2 partial pressure in the same pressure unit.

Formula

For an ideal gas mixture, Dalton’s law gives the partial pressure of a component as its mole fraction multiplied by the total pressure:

pCO2 = xCO2 × P

• pCO2 is the partial pressure of carbon dioxide, in atmospheres (atm) in the first output.
• xCO2 is the dimensionless mole fraction of CO2 in the gas mixture. Enter a fraction from 0 to 1, not a percentage.
• P is the total pressure of the gas mixture, in atm.

The second output converts atmospheres to kilopascals: 1 atm = 101.325 kPa, so pCO2 in kPa = pCO2 in atm × 101.325. The conversion is exact by definition. If the composition is given as a percentage, divide the percentage by 100 before entering it; for ppm by volume in an ideal gas, divide ppm by 1,000,000.

Worked Example

Using the calculator’s default values, the CO2 mole fraction is 0.0004 and total pressure is 1 atm.

1. Apply Dalton’s law: pCO2 = xCO2 × P.
2. Substitute the values: pCO2 = 0.0004 × 1 atm = 0.0004 atm.
3. Convert to kilopascals: 0.0004 atm × 101.325 kPa/atm = 0.040530 kPa.

The calculator displays 0.0004 atm and 0.0405 kPa after rounding to four decimal places.

Assumptions

• The gas mixture is treated as ideal, so each component’s partial pressure is its mole fraction times total pressure.
• The entered composition is a mole fraction for the same gas mixture and pressure being evaluated.
• Pressures are absolute pressures, not gauge pressures. Convert gauge pressure to absolute pressure before using the formula.
• The calculation does not adjust for water vapor, chemical reaction, dissolution, or non-ideal gas behavior unless those are represented in the input composition or handled separately.

When to Use a CO2 Partial Pressure Calculator

Partial pressure describes how much of a mixture’s total pressure is attributable to one gas component under Dalton’s law. It is useful in gas-law exercises, atmospheric calculations, and laboratory planning where gas composition and total pressure are known. The result is a pressure, not a concentration in liquid, a mass of CO2, or a measure of total CO2 in a sample.

For example, a gas containing 400 ppm CO2 at 1 atm has a mole fraction of 400 ÷ 1,000,000 = 0.0004. Its CO2 partial pressure is 0.0004 atm, or 40.53 Pa. The calculator’s kPa output is 0.0405 kPa; converting kPa to pascals means multiplying by 1,000. At a different total pressure, the partial pressure changes even if the CO2 fraction stays the same. At 2 atm and the same fraction, it would be 0.0008 atm.

Use the calculator when the CO2 proportion and total gas pressure are both available. It cannot infer the CO2 fraction from total pressure alone. If you have a CO2 concentration reported in ppm, percent, or another unit, first make sure it refers to the gas phase and convert it to a mole fraction.

How to Enter Pressure and Gas Composition

• Convert percentages: 5% CO2 is 0.05, not 5. The input must remain between 0 and 1.
• Convert ppm: 1,000 ppm is 0.001 as a fraction; 10,000 ppm is 0.01.
• Use absolute pressure: Dalton’s law uses absolute pressure measured relative to a vacuum. Gauge pressure excludes atmospheric pressure, so it is not interchangeable. For example, a gauge reading must have local atmospheric pressure added to obtain absolute pressure.
• Keep units consistent: the first output uses atm because total pressure is entered in atm. The second is a conversion to kPa, not a separate physical estimate.

For another pressure unit, convert the total pressure to atm before entering it, or convert the returned value using a reliable unit conversion. Common equivalents include 1 atm = 101.325 kPa and 1 atm = 760 mmHg under the conventional definition. Do not mix a pressure in kPa with a value intended to be atm.

What Partial Pressure Does—and Does Not—Mean

Partial pressure is the pressure a component would exert if it alone occupied the same volume at the same temperature. In an ideal mixture, the sum of all component partial pressures equals total pressure. Thus, if the mixture contains CO2, nitrogen, oxygen, and other gases, their mole fractions add to 1 and their partial pressures add to the mixture’s total pressure.

A gas-phase CO2 partial pressure is not the same as dissolved CO2 concentration in water or another liquid. Relating the two requires additional information, such as temperature, solvent properties, and an appropriate gas–liquid equilibrium model. A Henry’s law calculation, for example, depends on the form of Henry’s constant used and the chosen units and conditions. Do not interpret this calculator’s output as a dissolved concentration.

Likewise, a reading described as “CO2” may represent different quantities depending on the method: mole fraction, partial pressure, mass concentration, or total inorganic carbon. Check the definition and reporting units before applying this formula.

Common Errors and Limits

• Entering 400 instead of 0.0004: 400 ppm must be converted to a fraction. Entering 400 violates the allowed 0–1 range.
• Using gauge pressure: a gauge value is not absolute pressure. Convert it first, particularly when ambient pressure is a substantial part of the measurement.
• Confusing gas and liquid values: a gas-phase partial pressure alone does not state how much CO2 is dissolved.
• Assuming the mixture is perfectly ideal: real-gas behavior can matter at high pressures or under conditions where intermolecular effects are significant. In those cases, fugacity or a suitable real-gas model may be needed.
• Ignoring water vapor: if a gas sample is humid, the reported dry-gas composition and the wet-gas composition are not the same. Use the fraction corresponding to the sample basis and total pressure.

The calculation is direct, but its accuracy cannot exceed the accuracy and relevance of the inputs. Rounding a fraction too early can noticeably affect very small partial pressures. Keep adequate significant figures during conversion and round only the final result to match the precision of the original measurement.

Frequently Asked Questions

How do I calculate CO2 partial pressure?
Multiply the CO2 mole fraction by the mixture’s absolute total pressure: pCO2 = xCO2 × P. Make sure the fraction is expressed from 0 to 1.

What is the partial pressure of 400 ppm CO2 at 1 atm?
400 ppm is a mole fraction of 0.0004. At 1 atm, pCO2 = 0.0004 atm, which is about 40.5 Pa.

Can I use a percentage instead of a mole fraction?
Convert the percentage to a fraction first. For example, 2% becomes 0.02. Entering 2 would mean 200% and is outside the calculator’s valid input range.

Is CO2 partial pressure the same as dissolved CO2?
No. Partial pressure describes CO2 in the gas phase. Estimating dissolved CO2 requires a gas–liquid equilibrium relationship and condition-dependent information such as temperature.

Should I enter gauge or absolute pressure?
Use absolute pressure. Gauge pressure is measured relative to local atmospheric pressure and must be converted to absolute pressure before applying Dalton’s law.

Does the formula work at any pressure?
It is the standard ideal-gas calculation and is usually appropriate for ordinary gas-mixture calculations. At sufficiently high pressures or under strongly non-ideal conditions, a real-gas model may be more appropriate.

Related

• Van der Waals Gas Pressure Calculator: Real Gas Pressure
• The Ideal Gas Law Calculator: Pressure From PV = nRT
• Molar Volume Calculation: Ideal Gas Vm = RT ÷ P

in x | CO2 mole fraction | - | 0.0004 | 0 | 1
in p | Total gas pressure | atm | 1 | 0
out pc | CO2 partial pressure | atm | x * p | 4
out pk | CO2 partial pressure | kPa | pc * 101.325 | 4