Another FAQ (well, one of mine!) put to rest in my own kitchen
I'm frequently Googling what the dried weight vs. cooked weight is of chickpeas - predominately because I detest the flavour of tinned chickpeas (I find they have a vaguely fishy and, unsurprisingly 'tinny' taint to them, so I won't use them for things like hummus etc.), and also for other people with this affliction. I try to make quantities in recipes convenient(ish) for people to cook them, where I can - so where I use them as an ingredient, if appropriate, I'll go in increments of 240g (as that is the drained weight of a can of a 400g tin of chickpeas), or 120g, so that people haven't got bits and bobs lying around.
So anyway, I usually cook my chickpeas in bulk (due to soaking time), cool and freeze in a big re-sealable bag so I have a ready and convenient supply whenever I want them - as convenient as tinned chickpeas, but taste nicer, and are far cheaper to buy! This time, I decided to weigh them after cooking, so I never have to Google it and wonder whether it's right again...
Obviously, there may be slight variations in weight, down to which chickpeas you use etc., but this is at least a guideline (and one I'll be able to refer back to myself in future!). For reference, I used 'NATCO' brand dried chickpeas, but frankly I'm not that fussy about brands and just pick up whatever is available - I doubt the result would be much different.
In short: for a 500g bag of dried chickpeas, soaked overnight (12-16 hours) in a large pan of plain water (you need triple the volume of water to chickpeas, at least - Ottolenghi recomends adding 1 tbsp bicarbonate of soda per 500g, which reduces cooking time, and then adding 1/2 tbsp to the fresh water when cooking them - be careful though, as it can make them taste soapy. I don't bother), then drained and boiled in fresh, unsalted water for 15 minutes, and then simmered for 1 to 1 and 1/4 hours until tender (skimming off the scum occasionally). For hummus, if you haven't used bicarbonate of soda, you might want to cook for an hour, or even two longer, until they're really, really soft and you can pop them out of their skins easily (for those who want a super-smooth-textured hummus, like whipped butter). Otherwise, stick to the plan - me? I'm happy with a bit of texture to my hummus.
If you forget to soak overnight, put in a large pan with plenty of water, bring to the boil, simmer for two minutes, turn off the heat and leave in the water to cool down for an hour, and then cook.
I got 1,111g of cooked chickpeas (after draining, rinsing, and leaving to cool for a few minutes for the excess water to evaporate). If you want to make hummus with any of your chickpeas, DON'T pour the cooking water down the sink, keep it! (Even if you freeze it! It makes a huge difference to your hummus, instead of using plain water).
If you allow 10-15g for picking out any rogue chickpeas that are grey or dark brown, then call it 1,100g, so your ratio of dried to cooked chickpea weights is 5:11 (1:2.2).
Therefore, if you want to know how many grams of cooked chickpeas a certain weight dried chickpeas will yield (approximately), multiply the dried weight by 2.2 (so 100g dried chickpeas x 2.2 = 220g cooked chickpeas).
And so, if you want to know how many grams of dried chickpeas you will need to give you a certain cooked weight, you need to divide the cooked weight by 2.2 (so for 240g i.e. one tin's worth of cooked chickpeas, you need 240g ÷ 2.2 = 109g dried chickpeas). Obviously, I'd err on the side of caution, and throw in just a few more, just to be certain I had enough!
And on that note, I'm off to make some hummus, and spicy soup with chickpeas in it...