It's only the petals, though the whole plant is edible. You might try the leaves as a salad but I expect they don't taste wonderful. This website has some useful info:
http://www.herbexpert.co.uk/cooking-with-calendula.html

Using the flowers to colour rice orange is a good idea and much cheaper than using saffron.

Thanks Bertie. Have been meaning to ask you a question I previously asked on the site, only got one reply, but I don't think they understood my question. I have redcurrant and blackcurrant bushes. One of each. This year for some unknown reason, the blackcurrant bush produced red. The reply I had was that this couldn't happen unless they had been grafted. They haven't, and I've checked them to see if they've spread, but 'nope' there are still only 1 of each and I can get beween them, hold the spreading branches up, and they are about 4 ft away from each other, so would be glad if you came up with the answer. I've actually got a recent pic uploaded of how red they were. :o))