Lots of numbers here, isn't it confusing? Many people come back home to a camping corner full of half used canisters not knowing how many days supply is left. Easy answer to that? Buy a lab scale (might set you back $70-120) and weigh the canister. Step 2: Subtract the canister weight (bottom row) and divide by 33 grams of fuel per day (approximately) to get number of day of fuel left.