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Tank capacity calculator for CNG, ANG and hydrogen: kg, GGE and range
Set the gas, the cylinder's water volume and the fill pressure. The result shows what the tank holds, what an engine can actually use and how far that goes.
Inputs
Result
Common cylinders at a glance
The table repeats the calculation for typical vessel sizes at 15 °C, with compressed gas counted down to 10 bar and the ANG tank down to 5 bar. It is the same arithmetic the calculator runs live.
| Vessel | Water volume | Pressure | Gas stored | Usable GGE |
|---|---|---|---|---|
| CNG car cylinder | 80 L | 200 bar | 13.4 kg | 5.0 |
| CNG car cylinder | 80 L | 250 bar | 15.9 kg | 6.0 |
| CNG bus cylinder | 320 L | 250 bar | 63.8 kg | 24.0 |
| Hydrogen car tank | 60 L | 700 bar | 2.4 kg | 2.4 |
| Hydrogen bus tank | 320 L | 350 bar | 7.7 kg | 7.4 |
| ANG tank, 150 V/V | 80 L | 35 bar | 8.1 kg | 2.4 |
Reading the result
Cylinders are sold by water volume, the internal volume a vessel would hold if filled with water, because that number does not change with the gas or the pressure. Multiply it by the density of the gas at the fill condition and the result is the stored mass. For methane at 200 bar and 15 °C that density is about 0.168 kilograms per liter; at 250 bar it rises to about 0.199, a gain of only 19 percent for 25 percent more pressure, because the gas compresses less readily as its molecules crowd together.
Hydrogen behaves the other way. At 700 bar it reaches about 0.040 kilograms per liter, well below what the ideal gas law would suggest, which is why a hydrogen storage tank of the same size carries roughly a quarter of the mass a CNG tank does, and a little over half the energy.
Usable gas matters as much as stored gas. Regulators and injectors need a minimum inlet pressure, so the gas still in the cylinder at that pressure never reaches the engine. For compressed tanks that residue is small; for adsorbed natural gas it is larger, because the adsorbent keeps hold of part of the methane at low pressure.
Fill pressure and temperature
A fast fill heats the gas, and a cylinder that reads 250 bar at the end of a fast fill will read less once it cools. Station standards correct for this by filling to a temperature-compensated pressure. Moving the temperature slider shows the effect: the same 200 bar at 40 °C holds a little over a fifth less methane than at 0 °C. More on stations in the guide to CNG fueling stations, and on cylinder classes in the natural gas storage tanks guide.
Questions about the calculator
How much CNG does an 80-liter cylinder hold?
At 200 bar and 15 °C an 80-liter cylinder holds about 13.4 kg of methane, of which roughly 12.9 kg can be used before the pressure falls to 10 bar. That is about 5 gasoline gallon equivalents.
Why does the calculator not use the ideal gas law?
At 200 bar methane is about 25 percent denser than the ideal gas law predicts, and hydrogen at 700 bar is about 30 percent less dense. Using reference densities avoids an error of that size.
What does usable gas mean?
Engines and regulators need a minimum inlet pressure, so the last part of the gas stays in the tank. The calculator subtracts the gas still present at the minimum pressure you set. Compressed tanks are usually run down to about 10 bar; an adsorbed tank is drawn lower, to about 5 bar, and even there the adsorbent still holds about a quarter of its 35-bar charge.
How is ANG calculated?
For adsorbed natural gas the capacity slider gives the volumes of gas stored per volume of tank at 35 bar. Other fill pressures follow a Langmuir uptake curve, which rises steeply at low pressure and flattens above about 30 bar. The gas still adsorbed at the minimum pressure is subtracted, about a quarter of the 35-bar capacity at 5 bar. Temperature is not modeled for ANG, since adsorption capacities are quoted at room temperature.
Does temperature matter?
Yes. A cylinder filled to 200 bar on a hot day holds less gas than one filled to 200 bar on a cold day, which is why fueling standards compensate the fill pressure for temperature.