Partial Pressure Calculator

Compute partial pressures in a gas mixture using Dalton's Law: \(P_i = x_i \cdot P_{total}\). Add up to 8 gas components, adjust mole fractions, and instantly see partial pressures. The interactive pie chart visualizes the fractional composition.

atm (or any pressure unit)
Use any consistent unit: atm, kPa, bar, mmHg. Results will be in the same unit.
Gas Mixture Components
Composition & Partial Pressures
Privacy first: All calculations run locally in your browser. No data is sent to any server.
? Air (N₂+O₂+Ar+CO₂)
? Scuba: Trimix 21/35
? Medical O₂/CO₂ mix
? Simple: N₂ 80% / O₂ 20%

Dalton's Law of Partial Pressures

In a mixture of non‑reacting ideal gases, the total pressure exerted is equal to the sum of the partial pressures of individual components. Mathematically, \(P_{total} = P_1 + P_2 + ... + P_n\). The partial pressure of a gas is the pressure that gas would exert if it alone occupied the entire volume at the same temperature. For component \(i\): \(P_i = x_i \cdot P_{total}\), where \(x_i\) is the mole fraction (moles of component i divided by total moles).

\[ x_i = \frac{n_i}{n_{total}}, \quad P_i = x_i \, P_{total} \]

Discovered by John Dalton (1766–1844), this law is fundamental in gas chemistry, respiratory physiology, scuba diving (to prevent decompression sickness), and industrial gas blending. Our calculator lets you explore how changing composition affects partial pressures in real time.

Why an Interactive Partial Pressure Tool?

  • ⚛️ Educational clarity: Visual pie chart reinforces mole fraction → partial pressure relationship.
  • ? Real‑world mixtures: Air composition, nitrox, heliox for diving, anaesthetic gas blends.
  • ? Engineering & safety: Calculate oxygen partial pressure in hyperbaric chambers, purge gas systems.
  • ? Instant feedback: Alter any mole fraction, see pie chart and partial pressures update immediately.

Step‑by‑Step Calculation

  1. Define total pressure (default 1 atm).
  2. Add gas components, assign each a name (optional) and mole fraction (0 ≤ xᵢ ≤ 1).
  3. The sum of mole fractions must equal 1 (within ±0.005). If not, the tool will warn you.
  4. Partial pressure of each gas = mole fraction × total pressure.
  5. The pie chart displays the fractional composition, and the results table shows each gas partial pressure.

Case Study: Scuba Diving & Trimix 21/35

Trimix 21/35 contains 21% oxygen, 35% helium, and the balance nitrogen (44%). At a depth of 30 meters (absolute pressure ≈ 4 atm), the partial pressure of oxygen becomes \(0.21 \times 4 = 0.84\) atm (safe range: 0.16–1.6 atm). Meanwhile, nitrogen partial pressure is \(0.44 \times 4 = 1.76\) atm, reducing narcotic effects compared to air. Our preset "Scuba: Trimix" instantly shows how mole fractions affect partial pressures at any total pressure. This tool helps dive planners avoid hypoxia and oxygen toxicity.

Common Applications

  • Respiratory medicine: Alveolar gas equation – oxygen partial pressure determines gas exchange.
  • Gas chromatography: Carrier gas mixtures, detector response based on partial pressures.
  • Chemical reactors: Partial pressures drive reaction rates via Le Chatelier's principle.
  • Deep‑sea diving: Dalton's law prevents decompression sickness (the bends) by controlling inert gas partial pressures.

Frequently Asked Questions

Any consistent pressure unit (atm, kPa, bar, psi, mmHg). All partial pressures will be expressed in the same unit. The calculator does not convert automatically, so be consistent.

Mole fractions represent fractional composition of the gas mixture. By definition they must add up to 1. If the sum deviates, the mixture is not fully specified. The tool will show a warning and disable results until the sum is corrected.

Yes – Dalton's law states \(P_{total} = \sum P_i\). Our tool primarily computes partials from total pressure and mole fractions, but you can manually sum partials if you have individual pressures. A future version will include reverse mode.

Dalton's law is an ideal gas approximation. At low to moderate pressures (typical for many engineering and lab conditions) it works very well. For high‑pressure or polar gases, corrections like fugacity may be required.
References: LibreTexts – Dalton's Law, Respiratory physiology applications, Wikipedia: Partial Pressure. Reviewed by GetZenQuery tech  team, June 2026.