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H₂H₂ mass & SoC calculator
How much hydrogen is really in that tank? At 700 bar the ideal gas law overshoots by almost half; this calculator uses NIST real-gas data instead. Enter any three of pressure, volume, temperature and mass; it solves the fourth and reports the state of charge against the tank's working pressure per SAE J2601.
SOLVE FOR
HYDROGEN MASS
6.993kg
100 % SOC
ρ 40.19 / 40.19 KG/M³ AT 700 BAR, 15 °C · SAE J2601
The ideal gas law would claim 10.25 kg — 46.6 % more than is really in there (Z = 1.466).
+10 °C → −2.3 % mass
Compressibility data: NIST Reference Fluid Thermodynamic and Transport Properties Database (REFPROP). Valid domain: −125…125 °C, 1…1000 bar (abs). SoC per the SAE J2601 definition: density vs the reference density at nominal working pressure and 15 °C. Educational tool, not a substitute for certified metering.
How it works — physics, data sources, scope
Real gas, not ideal. PV = nRT ignores molecular size and repulsion. For hydrogen in this whole domain Z > 1, so the ideal law always claims more hydrogen than is really there: +23 % at 350 bar, +47 % at 700 bar. This calculator uses m = P·V·M / (Z(T,P)·R·T) instead; below ~20 bar the ideal error stays within about one percent.
Data source. Z(T, P) is bilinearly interpolated from the NIST REFPROP table (−125…125 °C, 1…1000 bar) and cross-checked against NIST's independent density table (±0.2 %). Solving for temperature or pressure inverts the same relation by bisection; out-of-domain inputs are reported as such, never silently clamped.
State of charge is a density ratio. Per SAE J2601, SoC = ρ(P, T) / ρ(NWP, 15 °C): 40.2 kg/m³ reference at 700 bar. The working pressure is freely adjustable (1…1000 bar). Because the reference sits at 15 °C, SoC legitimately exceeds 100 % in a cold tank, and a hot tank at full working pressure is far from full.
Gauge vs absolute pressure. Gauges read relative to the atmosphere; the physics needs absolute pressure. The abs/gauge toggle handles the 1.013 bar offset.
FREQUENTLY ASKED QUESTIONS
How much hydrogen fits in a 700 bar tank?
A typical passenger-car tank of 174 L holds about 7.0 kg of hydrogen at 700 bar and 15 °C, calculated with real-gas data from NIST. The ideal gas law would claim over 10 kg: at that pressure it overestimates by about 47 %.
Why is the ideal gas law wrong for hydrogen at high pressure?
Hydrogen at these conditions is far above its Boyle temperature, so molecular repulsion dominates and the compressibility factor is greater than one: Z ≈ 1.47 at 700 bar and 15 °C. The ideal gas law ignores this and always predicts more hydrogen than is really in the tank: about +23 % at 350 bar, +47 % at 700 bar.
When is the ideal gas law still good enough for hydrogen?
Below roughly 20 bar the error stays within about one percent, fine for low-pressure piping estimates. Above ~100 bar it passes the few-percent mark and grows nearly linearly with pressure, so at vehicle refueling pressures a real-gas model is mandatory.
What is the state of charge (SoC) of a hydrogen tank?
SAE J2601 defines SoC as the gas density divided by the reference density at the tank's nominal working pressure and 15 °C: 40.2 kg/m³ for a 700 bar tank, 24.0 kg/m³ at 350 bar. It is a density ratio, not a pressure ratio: a 700 bar tank at 700 bar and 85 °C is only at about 86 % SoC.
Can the state of charge of a hydrogen tank exceed 100 %?
Yes. SoC is referenced to 15 °C, so gas colder than that at working pressure is denser than the reference: a 700 bar tank at −10 °C reads about 106 %. That is why J2601 fueling targets aim for 95–100 % at the expected settled temperature rather than filling to a fixed pressure.