1. With shellfish including lobster and crab, allow about 0.5kg per person (either one small one or split a larger one)
2. With round fish such as cod, pollack or whiting, allow 6 to 8oz about 200g per portion of filleted fish
3. Flat fish is usually slightly less - around 5-7oz about 180g of fillet
If you cook your fish whole, allow roughly double the weight
Another way of deciding...
1. 1kg of filleted fish would give you approximately six 6oz portions or four 9oz portions
2. 1kg of whole fish will give you two or three portions, depending on the type of fish.
(For us at Fish for Thought, portions would certainly be bigger - it depends how greedy you are!)
Most fish can be filleted and will yield about 50% of the weight in fillet. However if you cook it on the bone (Turbot for example) you will get 5-10% more edible fish.