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Analyze any two-gas mixture with our free online gas ratio calculator. Enter each gas's volume or moles plus the total pressure to get the simplified volume ratio, mole fractions, and each gas's partial pressure via Dalton's Law — essential for diving gases, lab work, and atmospheric science.
Analyze Gas MixtureInteractive Gas Ratio Calculator
Mixture Analysis
How the Gas Ratio Calculator Works
From two amounts to a full Dalton's Law breakdown in four steps.
Name Your Gases
Label the two components — oxygen and nitrogen for air, helium and oxygen for diving trimix analysis, or any pair you are mixing in the lab.
Enter Amounts
Type each gas's volume (at the same temperature and pressure) or moles. Units cancel out in ratios, so any consistent unit works.
Add Total Pressure
Enter the mixture's total pressure for partial-pressure results. Skip it if you only need ratios and mole fractions.
Read the Analysis
Get the simplified ratio, each gas's mole fraction, and its partial pressure — with a Dalton's Law self-check confirming the parts sum to the whole.
Gas Mixture Formulas: Dalton's Law
John Dalton's 1801 law states that each gas in a mixture behaves as if it alone occupied the container. The math follows directly.
Mole Fraction
Each gas's share of the total moles. Mole fractions always sum to exactly 1.
Dalton's Law of Partial Pressures
Example — air at 1 atm (21% O2, 79% N2): PO2 = 0.21 × 1 = 0.21 atm, PN2 = 0.79 atm. Sum: 1.00 atm ✓
Volume Ratio Equals Mole Ratio
Avogadro's Law: equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. So a 21:79 volume mix is also a 21:79 mole mix.
Simplified Ratio
21 : 79 is already simplest (GCD = 1). A 30:70 mix simplifies to 3:7.
Common Gas Mixtures Reference
Benchmark your results against these well-known mixtures.
| Mixture | Composition | Volume Ratio | Key Partial Pressure |
|---|---|---|---|
| Dry air (sea level) | 78% N2, 21% O2, 1% Ar | 78:21:1 | PO2 = 0.21 atm |
| Nitrox 32 (diving) | 32% O2, 68% N2 | 32:68 = 8:17 | PO2 = 0.32 atm |
| Nitrox 36 (diving) | 36% O2, 64% N2 | 36:64 = 9:16 | PO2 = 0.36 atm |
| Heliox 21/79 | 21% O2, 79% He | 21:79 | PO2 = 0.21 atm |
| Exhaled breath | ~16% O2, ~4% CO2, 78% N2 | 16:4:78 | PCO2 ≈ 0.04 atm |
| Natural gas | ~95% CH4, ~5% others | 95:5 = 19:1 | PCH4 ≈ 0.95 atm |
Why Gas Ratios Matter
Scuba and Technical Diving
Every breathing gas is a ratio problem. Nitrox blends raise the oxygen fraction to extend bottom time; trimix adds helium to reduce narcosis at depth. Divers compute partial pressures at depth (P × depth pressure) to stay within oxygen toxicity and narcosis limits.
Medicine and Anesthesia
Anesthetic gas mixtures (e.g., sevoflurane in oxygen) are specified by volume percent — a direct gas ratio. Ventilators blend oxygen and air to hit prescribed FiO2 values like 0.40 or 0.60.
Industrial Processes
Welding shielding gases (argon/CO2 blends like 75/25), modified-atmosphere food packaging, and semiconductor fabrication all depend on precise gas ratios. A drift of a few percent can scrap a production batch.
Atmospheric Science
Climate models track trace-gas ratios — CO2 at ~420 ppm is a ratio of 0.00042:1 against air. Tiny ratios, planetary consequences.
Background reading: Dalton's Law and partial pressure on Wikipedia.
Gas Ratios in Industry and Research
Beyond diving and medicine, precise gas ratios underpin entire industries.
Welding and Metal Fabrication
MIG welding uses argon/CO2 blends — a common 75/25 mix shields the arc while the CO2 stabilizes it. Change the ratio and weld penetration, spatter, and bead shape all shift. Welders buy pre-mixed cylinders precisely because the ratio is the process.
Semiconductor Manufacturing
Chip fabrication runs on gas ratios: silane/nitrogen for nitride deposition, argon/oxygen for plasma etching. A 1% drift in etch-gas ratio can move critical dimensions out of spec across a whole wafer batch worth hundreds of thousands of dollars.
Modified Atmosphere Packaging
Your salad bag is a gas-ratio product: typically ~70% nitrogen, ~30% CO2, near-zero oxygen. The ratio suppresses microbes while avoiding anaerobic risks. Packaging engineers validate the mix with — you guessed it — partial-pressure math.
Calibration Gases
Analytical labs buy certified mixtures like "1000 ppm CO in nitrogen" — a ratio of 0.001:1 — to calibrate detectors. The certificate's uncertainty (often ±1–2%) is a ratio tolerance, and the entire measurement chain inherits it.
Gas Ratio FAQs
In a gas mixture, each gas exerts the pressure it would exert alone — its partial pressure — and the total equals the sum of the parts. Mathematically: Pi = Xi × Ptotal, where Xi is the mole fraction. Try air: 0.21 + 0.79 = 1.00 atm.
Yes, when compared at the same temperature and pressure — that is Avogadro's Law. 21 L of O2 mixed with 79 L of N2 (same T, P) is a 21:79 mole ratio too. This is why the calculator treats volume and mole inputs identically.
Convert percent to a fraction (divide by 100), then multiply by total pressure. For 32% oxygen at 1 atm: 0.32 × 1 = 0.32 atm. At 3 atm depth: 0.32 × 3 = 0.96 atm.
A mole fraction is one gas's moles divided by total moles — its share of the whole. Since the shares partition the entire mixture, they must add to exactly 1 (or 100%). The calculator's Dalton check verifies this.
Directly it handles two, but you can chain it: compute the ratio of gas A to (B+C combined) first, then split B and C in a second run. For air's three main components, run O2 vs (N2+Ar), then N2 vs Ar.
Because partial pressures scale with depth. Oxygen above ~1.4 atm risks seizures; nitrogen above ~3–4 atm causes narcosis. Choosing the right O2/N2/He ratio for a planned depth is literally life-support engineering.
The ratio itself (n1:n2) does not change with temperature — but partial pressures do, since Ptotal rises with T at fixed volume. Enter the total pressure at the actual conditions for correct partial pressures.
Mole fraction counts molecules; mass fraction counts kilograms. They differ because molecules have different masses — 21% O2 by volume is about 23% by mass (O2 is heavier than N2). Gas laws use mole fractions, not mass fractions.
Worked Example: Nitrox 32 at Depth
A diver breathes Nitrox 32 (32% O2, 68% N2) at 30 m depth, where ambient pressure is 4 atm. What are the partial pressures?
Step 1: Mole fractions — XO2 = 0.32, XN2 = 0.68.
Step 2: PO2 = 0.32 × 4 atm = 1.28 atm (under the 1.4 limit ✓).
Step 3: PN2 = 0.68 × 4 atm = 2.72 atm.
Step 4: Check — 1.28 + 2.72 = 4.00 atm ✓.
Enter 32 and 68 with total pressure 4 atm in the calculator to reproduce this exactly.