How to calculate the volume of air in a compressed container?

This is one of the handiest rules of thumb to know.:

22.4 liters of gas at STP (Standand Temperature and Pressure weighs it’s molecular weight in grams.
For example 22.4 liters of Oxygen weighs 32 grams 22.4 liters of helium weighs 2 grams. 22.4 liters of Nitrogen weights 28 grams.
You have 14 oz ,call it 400 grams of (assume) propane C3H8 ( MW 44) is 22.4 x 400/44 = ~200 liters.

Ideal gas law.

The volume of the compressed gas is equal to the volume of the container. What @LuckyGuy is giving you is the calculation for the mass.

I suspect that the question is “What is the STP volume of gas that I have compressed into this cylinder?”

We know the volume (approximately, for a 14 oz. cylinder) and we know the pressure. Now if we knew the current temperature for the cylinder and gas, we could do the calculation … if anyone has that much interest.

Temperature would probably be around 20–25 degrees Celsius, I totally forgot about that. It is much warmer when actually pumped into the cylinder since the aircon compressor I use warms it up, probably to 40 degrees or so, the cylinder is noticeably warm after. (which makes it a bit more complicated I suppose, but rough is close enough.

It boils down to simple proportions.

A pressure of 1 atmosphere equals 14.7 psi (pounds per square inch).
Therefore 250 psi represents (250)/(14.7) = 17.0 atmospheres.
This is equivalent to compressing 17 times the container’s volume into one volume.

So when expanded back to 1 atmosphere, there are 17*14 oz = 238 oz = 0.249 cu ft of gas in the container.

Since room temperature is about 300K, each degree Celsius of temperature change should amount to only about a 0.3% change in pressure or volume.