Is there a magic formula for converting lbs of something to water gallons? I do not have to be accurate to the Nth degree, just in the ballpark.

Examples

How many water gallons is 600 lbs of R-22 refrigerant?

How many water gallons is 20 lbs of propane (blowtorch tanks)?

Etc. etc.

I need to do this for reporting requirements. The form specifies flammable gas and non-flammable gas amounts in "water gallon capacities" so I need to know how much I have in "water gallon capacity" to find out if I'm required to report it or not.

Thanks for any help