Maybe it sounds like a funny question, but as I suspected, the answer is not 15. The 15 ounces (425 grams) includes the weight of the liquid in which the beans were canned.
I often cook my own beans instead of buying canned, which can leave me at a little bit of a loss when I encounter a recipe that calls for a "can of beans." Normally when this happens I just eyeball the amount- in a soup, stew, or curry the exact amount doesn't make a huge difference. But this weekend I'm baking black bean cookies, and baking cookies requires a lot more precision than simmering a stew.
Since I had no idea exactly how many cooked beans are in a drained can of beans, I had to go out and buy a can for the cookie recipe instead of cooking my own. Can in hand, it seemed like a good opportunity to figure out, once and for all ,how many cooked beans are in a can.
That fifteen ounce can of beans in your pantry actually contains about 9 ounces of cooked beans:
Or, if you're a metric freak like me, that's about 260 grams.
If you don't need to be super precise, and can afford to measure by volume, one drained can of black beans is almost exactly 1 1/2 cups of cooked beans.
Next time I have a big bowl of cooked black beans and need to measure a "can" for a soup or stew recipe, I'm going to grab a measuring cup and scoop out a cup and a half of beans.
I haven't yet tested the weight and volume of any other type of beans. I'd assume approximately the same measures for kidney beans, cannellini beans, and black-eyed peas, since they have a similar form factor and size. I'll definitely repeat this experiment with garbanzo beans and perhaps lentils, since these have different form factor and size. It's worth the effort, because I can say with some confidence that a "15 ounce can" of any bean or lentil is most definitely not equivalent to 15 ounces of cooked beans.
A can of beans is 1 1/2 cups, or 9 ounces, or 260 grams. I'll sleep better tonight having cracked the mystery of the 15 ounce can. As for everyone else, hope this was a little bit useful!