I was going to weigh my CO2 tank to see how much gas I have left. I’m thinking I take the TW and add 5# to it to get a full weight and minus the current weight?
That would be equivalent to current weight minus tare weight, so yes. 
Forgive any typos or nonsensical posts for the next month or so (sleepless newborn nights).
I think most 5 pounders are about 12 pounds full with no regulators attached. Way more if your whole kegerator is attached
Just bumping this for a couple reasons…weighed my CO2 tank today (5#) and it weighed 11 pounds. It has a tare weight of 7.7#, so I assume that means I had 3.3 pounds left. Correct?
Secondly, I was hooking up a new regulator today and I forgot to put clamps on the gas line. Went out about 20 minutes later to check something, heard a hissing sound (you know what’s next) and opened my kegerator to find a snow-white CO2 tank.
Thankfully, I only lost a half a pound but I’m pretty low now with only 2.8 pounds left. PSA, check your CO2 tanks!
Correct.
Just yesterday, I finally tracked down the leak that cost me two 20 lb tanks over the past few months. Turns out one of the regulators had a gasket that was only intermittently leaking and the previous times I’d sprayed it down I didn’t see it. :