I thought every one did this but I guess not.
Weigh your EMPTY fuel canisters and write down the weight of each kind. Then weigh all of your partially used cans and write the weight on it with an appropriate pen. Then you will know how much fuel is in them. Before you go on a trip, say with a full can (or not), weigh it before you go and after you return and write it down along with the how much you used it. Soon you will know how much fuel you need for any particular trip and you can start taking partially used cans and save weight.
Jim

You mean to say not everyone does this?!

I guess if you don't own a decent gram scale, you might have a temporary excuse for avoiding this approach, but then you would need a valid excuse for not owning a decent gram scale.

BTW, those large-size canisters (~8 oz of fuel) generally weigh in the vicinity of 5 oz when empty and 13 oz when full. I find I use a bit less than 0.5 oz of fuel per supper, but fuel use varies rather widely from one person to another, and my fuel use figures won't apply to you. Get a gram scale!

Yeah, I had the same reaction as aimless. Must be some sort of gram-weenie OCD ultralight hiker thing.... <grin>

You can also float them in water and mark a water line to know how much fuel is left.