The ideal approximation is:

For real gas behavior, ρ=PM/(ZRT). At hundreds of bar, Z cannot safely be assumed to equal 1 if accuracy matters.

Enter absolute pressure, temperature and storage volume, or use 30/200/350/700 bar presets. Compare the real-gas estimate with the ideal result and inspect density, mass and relative density to air.

Compressed-hydrogen density rises with pressure, but not linearly at high pressure because molecular interactions create non-ideal behavior. This is why a 700-bar tank does not simply contain seven times the mass density of a 100-bar tank at the same temperature.

A quoted hydrogen density is incomplete without pressure and temperature. Low-pressure reference values are useful for comparison, while vehicle/fueling storage conditions require high-pressure property models.

Hydrogen has a very low molar mass and is much less dense than air at comparable ordinary conditions. Buoyancy, leakage and dispersion behavior still depend on temperature, momentum, confinement and ventilation.

Once gas density at tank conditions is known, stored hydrogen mass is m=ρV. Tank “volume” here is internal gas volume; total system mass includes the pressure vessel and balance-of-plant hardware.

At 350–700 bar, intermolecular effects are large enough that reference-quality equations of state are preferred. NIST hydrogen property research provides standardized formulations for high-pressure fueling and metrology.

This calculator estimates thermodynamic properties. It does not size pressure vessels, relief systems, ventilation or hazardous-area controls. High-pressure hydrogen systems require applicable codes, certified components and qualified engineering.

Hydrogen peroxide is a liquid chemical mixture with concentration-dependent density. “Hydrogen column density” is an astronomy quantity. Neither belongs in a compressed H₂ gas-density calculation simply because the word hydrogen appears in the query.

For a fixed tank volume, increasing storage pressure increases hydrogen mass, but the relationship becomes increasingly non-linear. Comparing the ideal-gas and real-gas outputs at 350 or 700 bar is a useful way to see why a compressibility correction matters.

Vehicle storage commonly uses nominal 350-bar or 700-bar systems, while industrial storage can use many other pressures. Actual density also changes with gas temperature during filling and cooling. A single “density at 700 bar” number without temperature is incomplete.

Do not use gauge pressure where absolute is required, do not assume ideal-gas behavior at 700 bar, and do not treat tank geometric volume as hydrogen volume at standard conditions. Compressed-gas inventory requires the actual storage state.