I believe the weight listed on canned beans is for nutritional data for unrinsed and drained beans, by rinsing and draining you are increasing the serving size for 100 calories.
Um does that make sense?
I do have a scale, and so far I have been trusting that...
If it says 1/2 cup of beans is 100 calories, then you'd use 1/2 cup, not 1/3 can. If you're having a problem with whether or not to include the liquid as part of your 1/2 cup, then measure how many cups the whole can is. If the whole can is 1.25 cups (1/2 cup serving size x 2.5 servings), then your 1/2 cup of beans for 100 calories includes the liquid. If the drained beans come out to 1.25 cups, then it is 1/2 cup of drained beans for 100 calories.
If you want to go by the can serving size, measure the serving out BEFORE you rinse it, or else divide the rinsed beans into three so you'll have the right amount - beans plus gooey liquid is more volume and less weight than the beans alone. Say the can had 100g of beans and 20g of water in it but it says one serving is 40g or 1/3 of the can - the serving would actually be 33g beans plus 7g of water, not 40g of beans alone.
I had the same problem trying to figure out the stupid canned beans. Now though, I use dried beans, once a month or so I pre-cook a bunch, portion them out and freeze the ones I don't need right away. Then when I go to cook it's about as easy as canned and tons less sodium. Also I have a much better idea of the accuracy of the calories etc.
Instead, both 2/5 of the can's total beans AND 2/5 of the can's total liquid are 100 calories.
While canned bean (not green beans) typically contains only beans, water and salt, the resulting liquid contains much starch from the beans, hence calories (plus sodium) as well as weight.
As many people drain them, its unfornate they dont break down the calories of canned beans with and without liquid.
