How Can I Tell How Much Fuel Is in a Used Canister?

I have a Snow Peak canister stove, and I’ve accumulated a bunch of partially-used fuel canisters. I hate wasting them, but I’m worried about running out of gas on a trip. Is there a good way to estimate the burn time left in a partially used canister? —Clark, Prairie City, OR

If you’re lazy like me, you just guesstimate how much fuel is left in your canisters. It works well enough, though you need some practice to do it reliably. To get a more precise method for determining the remaining fuel amount, I asked Steve Grind, the product manager for MSR stoves. This is what he told us:

There are a couple of reasonably good methods for determining the amount of fuel remaining in the canister. “The most common one is to weigh the canister when it’s full and write that number (in grams or ounces) down onto the canister itself. After the canister has been partially used, it can be re-measured and the weight difference is how much fuel has been consumed. The net weight of fuel in a full canister is printed onto the canister, so you can calculate the percentage used and the percentage remaining.

To get to burn time remaining, you’ll have to know the burn time specific to your stove. We publish burn times per canister for each of our canister stoves. So if you have 60% of your fuel remaining, and the burn time on a full canister (again, specific to your stove) is 80 minutes, you have roughly 48 minutes of burn time remaining. These burn time numbers really need to be taken with a grain of salt, though, because canister output is going to vary dramatically depending on conditions.

A more useful measurement is the number of liters of water boiled per canister. For example, the MSR Reactor boils about 22 liters of water per full 8 oz. IsoPro canister, so a half-full canister will give you about 11 more boiled liters.

The other method of determining the amount of fuel remaining is much easier to do in the backcountry, or in your kitchen if you don’t have an accurate scale at your disposal. The canisters will float upright when placed in water, and the water line will vary depending on how much fuel is remaining in the canister. If you float a full canister and then float an empty canister, you can measure those water lines and mark them on your new canisters, then re-measure after the canister has been partially used. If the water line is halfway between the ‘full’ water line and the ’empty’ water line, the canister is half full.”