When is a BBQ gas cylinder empty?

I am having a BBQ tomorrow (regardless of the weather) and I don't want to get caught with my gas running out. Is there any way of checking?

I don't want to return a cylinder that is half full and I son't want to make en eejit out of myself if the gas runs out.

Anyone know how much an empty cylinder weighs?

Just found this on calor site:
How full is a Calor gas cylinder in a full condition, in percentage terms?
All cylinders are 80-87% full as space needs to be left for the expansion of liquid. The weight on the front of the cylinder is the total weight of gas in the bottle.
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Doesn't answer the question though of what weight they are when empty - I can usually tell, I can lift when they are empty or nearly empty - when full my DH has to move it!