Gas Density Calculator
Weighing an invisible thing: PM over RT turns a gas’s molar mass into grams per litre — and ranks it against the air it lives in.
Gas Density Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Dilute-gas idealisation — real gases near condensation deviate
- Mixtures enter as average molar mass
In short: Air (M 28.97 g/mol) at 1 atm and 273.15 K: ρ = PM/RT = 1.292498 g/L — a cubic metre of winter air weighs about 1.292 kg. Carbon dioxide at the same state: 1.963463 g/L, 1.519363× the air and unapolo-getically sinking; helium: 0.178576 g/L, 0.138116× air and long gone. The same gas at a summer 298.15 K thins to 1.184121 g/L — heat is the cheapest expansion there is.
Formula
ρ = PM/(RT) · air reference 28.97 g/mol · g/L = kg/m³
Gas density is the ideal gas law wearing a scale: replace moles with mass over molar mass and the state equation solves straight for ρ = PM/RT. Two dials only — squeeze with pressure, thin with heat — and the molar mass is the gas’s identity card. The g/L figure and kg/m³ figure share digits: the metric system’s quiet joke again.
Worked Example
- Enter the gas’s molar mass.
- Enter the pressure and temperature it lives at.
- Read the density in g/L (and kg/m³).
- Read the air comparison — sinks or floats, by how much.
Defaults: air 28.97 at 1 atm, 273.15 K → 1.292498 g/L. CO₂: 1.963463 g/L (1.519363× air). He: 0.178576 g/L (0.138116×). Summer air (298.15 K): 1.184121 g/L.
Strengths & Limits Of This Model
Where this engine is strong
- Air comparison computed, not recited
- Vacuum printed honestly as zero
Where it stops
- No humidity correction
- No real-gas compressibility
Practical Use Cases
Balloon and lift work
buoyancy from the ratio
Gas safety
where leaks pool or vanish
Gas flow setup
converting litres to mass
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Gas Density Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why does carbon dioxide pool in low places?
Because at the same P and T its density rides its molar mass: 44.009 against air’s 28.97 is 1.519363 × — it sinks and stays. That is why CO₂ accumulate near floors and in cellars before it announces itself, and why the safety advice for heavy asphyxiants is vertical before horizontal.
Why compare against 28.97 for air?
Because air is a mixture, and 28.97 g/mol is the conventional weighted average of its N₂/O₂/Ar recipe — the reference every ‘heavier/lighter than air’ claim silently assumes. Helium at 0.138116× and hydrogen at 0.069569× of that reference both outrun it; water vapour, lighter still, is why humid air rises.
Why do g/L and kg/m³ share digits?
A gram per litre is a thousandth of a kilogram in a thousandth of a cubic metre — the prefixes cancel exactly as g/L and mg/mL do on the solution pages. One density, two accents, zero arithmetic: the metric system rewarding the attentive.
Why is summer air thinner than winter air?
Heat is the cheapest expansion: at the same pressure, warmer gas spreads out and each litre holds less mass — 1.292498 g/L at 273.15 K against 1.184121 g/L at 298.15 K. The same arithmetic explains hot-air balloons: thin the inside, let the outside do the lifting.
What happens to density in a vacuum?
It dies with the pressure: ρ = PM/RT and P at zero means zero density, whatever the gas’s identity. The page prints the honest zero rather than refusing — an empty chamber is a legitimate state, and the card says so in as many words.
How does the page handle gas mixtures?
Feed it the mixture’s average molar mass — the weighted recipe air’s 28.97 already is. Each component contributes its partial pressure; the densities add; the arithmetic is linear in exactly the way Dalton’s law promises.
Where does the molar mass itself come from?
From the formula and the periodic table — the molecular-weight page prices any formula you can write, and common gases fall out of the ATOMS arithmetic directly (CO₂ = 12.011 + 2×15.999). One parse upstream, one density downstream.
How is this different from the ideal gas page?
Question, not machinery. That page counts MOLES in a state; this page weighs the gas those moles make. ρ = PM/RT is PV = nRT with n swapped for m/M — one substitution of identity. Keep them separate because the questions are separate: how much versus how heavy.