Gas Volume Calculation

I'd start with the idea that there is a nozzle in the burner which affects the flow out rate and finally your consumption. I guess you can take it from there.

Weigh it.

Maybe you could compare notes with abb12?

A reasonable rule of thumb would be that a small LPG (assuming Propane here) portable stove burner will be somewhere between 8000 and 12000 BTU, using 170 - 250 gm/hr. A large burner could be as much as 100,000 BTU and consume 2000 gm/hr.

Due to the lower specific gravity of LPG, 1 Kg equals about 1.95 litres of liquid.

Find out what size your burners are, use the above figures to extrapolate your gas usage/hr and then divide the gas bottle size by that figure and you should be pretty close. ie 2 x 8000 BTU burners will run flat out for about 2.8 hrs per Kg of liquid. If your burners are rated in MJ - multiply the MJ figure by 948 to get BTU. If your bottle is in pounds divide the pounds by 2.2 to get Kg.

The only reason that two burners use more gas than one is because there are two of them, the gas is still consumed in direct relation with the burner output÷efficiency no matter how many burners you have going.

You can get reasonably accurate gauges to tell you how much liquid is left in the cylinder. The cylinder when "full" will actually be only around 85% full (to allow for expansion and surface evaporation), but the markings on the bottle will tell you how much liquid is actually in there, the liquid expands to roughly 250 times its volume when its pressure is reduced to atmospheric.

Hope this helps.