Cooked in water, they of course absorb some of it. Looking for weight, not volume conversion. If it varies by type, this is for Red Kidney Beans, but I'm guessing one general number can be good enough for most of them.
Cooking without soaking in a pressure cooker.
I'll try to update this after I finish cooking, but I have 3 pulses soaking right now that are about to be cooked. (red kidney beans, black chickpeas and urad dal)
For dry vs soaked, it seems the kidney beans (and both the black chickpeas and the dal) have pretty nearly exactly doubled in weight. When I cook them, I will include enough water to cover them. I know from experience that they will absorb some of it, but not all of it, but if you don't have enough water in there when you cook them in the pressure cooker, they will scorch.
It is a little hard to calculate exactly the cooked weight of the beans themselves without the cooking water because, at least for the dal and the kidney beans, they get pretty soft and some break up in the water. Unless you cook them with the exact amount of water they will absorb, it's pretty tough to say what is beans and what is water at a certain point.
For lentils and dals that don't need presoaking, I generally cook them with 1:2 dal:water and they absorb all of it and generally turn nearly to mush. For dal that needs to be soaked, I use the same method as the kidney beans -- just barely cover with water (note that this is additional water after soaking, and enough to cover their plumper volume). For those (whole urad dal, whole moong dal) it will absorb most of the water, but there will be some liquid left. I'd guess that it is probably less than half the original weight more in water.
So my best estimate is about 1 dry becomes 2-2.5 cooked, by weight.
I know you're asking about weight, but from a cooking perspective, for any lentils/dals, I always make sure to include twice the volume of water to the volume of the dry dal. For kidney beans i would probably cook them with 4x there volume of the dry beans. For anything where I expect to use some sort of sauce or gravy, I increase that. This is sort of the bare minimum so that they fully hydrate and don't scorch.
I got very curious... My test included 4 sets...
Macro Organic Red Kidney Beans from Woolworths Australia selected. All beans were from the same bag... Water was at 18 C (tap/room temperature), 12 beans selected based on similar total weight.
As expected... the addition of salt or bicarbonate of soda increases the efficiency of absorption of water. Your experience will vary based on brand and freshness of the product.
PS My son and I had great fun measuring, pouring and splashing each other with water ... Part 2 is to see if any of these beans will grow... Does anyone have an opinion on whether "organic" beans should sprout and grow?
