Buying whole fish? Here is a useful 'Yield Guide'....

Yields Guide

Skin-on unless stated (for skinless portions the yield will reduce by approximately 10%).

Whole fish	Cut	Preparation required	Size of whole fish required	Ave yield per fish (g)	Yield per fish %	Portions per fish	Fish req.
Barracuda	Suprêmes	Scaled, filleted, boned & portioned	3kg	1100g	37%	6	2
Barramundi	Suprêmes	Scaled, filleted, boned & portioned	3-4kg	1365g	39%	6-8	2
Brill	Suprêmes	Filleted, trimmed & portioned	3-4kg	1225g	35%	6	2
Brill	Fillets	Filleted	750g	375g	50%	2	5
Brill	Pavés	Headed, trimmed & portioned	2-3kg	1500g	60%	8	2
Cod	Fillets	Filleted	750g	375g	50%	2	5
Cod	Suprêmes	Filleted, boned & portioned	4-6kg	1500g	30%	8	2
Cod	Steaks	Headed, trimmed & portioned	3-4kg	1750g	50%	8	2
Coley	Fillets	Filleted	680g	340g	50%	2	5
Dover Sole	Fillets	Filleted	680g	340g	50%	2	5
Gilt Head Bream	Fillets	Scaled, filleted & boned	850g	375g	44%	2	5
Grey Mullet	Fillets	Scaled & filleted	900g	360g	40%	2	5
Haddock	Fillets	Filleted	750g	375g	50%	2	5
Hake	Steaks	Headed, trimmed & portioned	2-3kg	1250g	50%	7	2
Hake	Suprêmes	Scaled, filleted, boned & portioned	2-3kg	750g	30%	4	3
Halibut	Suprêmes	Filleted, trimmed & portioned	5-7kg	2220g	37%	12	1
Halibut	Steaks	Headed, trimmed & portioned	3-4kg	2310g	66%	12	1
Halibut	Pavés	Headed, trimmed & portioned	5-7kg	3960g	66%	20	1
John Dory	Fillets	Filleted & trimmed	1kg	375g	35%	2	5
Lemon Sole	Fillets	Filleted	650g	375g	57%	2	5
Mackerel	Fillets	Filleted	650g	375g	50%	2	5
Mackerel	Butterfly Fillets	Filleted	340-400g	185g	50%	1	10
Monkfish	Fillets (skinless)	Headed, skinned & filleted	1.8kg	375g	21%	2	5
Monkfish	Steaks (skinless)	Headed, skinned & portioned	4-5kg	1100g	24%	6	2
Plaice	Fillets	Filleted	750g	375g	50%	2	5
Plaice	Pocketed	Pocket filleted	340-400g	185g	50%	1	1
Red Mullet	Fillets	Scaled, filleted & boned	950g	380g	40%	2	5
Salmon	Suprêmes	Scaled, filleted & portioned	3-4kg	2030g	58%	10	1
Salmon	Steaks	Headed, trimmed & portioned	3-4kg	2275g	65%	12	1
Sea Bass	Fillets	Scaled, filleted & boned	850g	375g	44%	2	5
Skate	Wings (skinless)	Cut out & skinned both sides	950g	380g	40%	2	5
Snapper	Suprêmes	Scaled, filleted, boned & portioned	2.5-3kg	770g	28%	4	3
Snapper	Fillets	Scaled, filleted & boned	950g	375g	44%	2	5
Squid	Whole	Clean & trimmed	300g	180g	60%	1	10
Tilapia	Fillets	Scaled, filleted & boned	950g	380g	44%	2	5
Trout	Fillets	Filleted, skinned, trimmed & boned	950g	380g	40%	2	5
Trout	Canoe Filleted	Canoe filleted	290-340g	205g	65%	1	1
Turbot	Fillets	Filleted	750g	375g	50%	2	5
Turbot	Suprêmes	Filleted, trimmed & portioned	3-4kg	1200g	34%	6	2
Turbot	Pavés	Headed, trimmed & portioned	3-4kg	2100g	60%	11	1
Whiting	Fillets	Filleted & boned	950g	380g	40%	2	5
Witch	Fillets	Filleted, skinned & trimmed	1-1.3kg	375g	35%	2	5

Large loins	Cut (all skinless)	Preparation required	Size of loin required	Yield per loin(g)	Yield per loin%	Portions per loin	loins req.
Mahi Mahi	Suprêmes	Trimmed & portioned	2-2.5kg	10 x 170-200g	85%	10	1
Marlin	Suprêmes	Trimmed & portioned	2-2.5kg	10 x 170-200g	85%	10	1
Swordfish	Suprêmes	Trimmed & portioned	2-2.5kg	10 x 170-200g	85%	10	1
Tuna	Suprêmes	Trimmed & portioned	2-2.5kg	10 x 170-200g	85%	10	1

Shellfish	Portion sizes
Lobster	Use a 450-600g whole lobster per person (half a 600g for a starter).  The yield is approximately 35%.
Crab	Use 500-600g of whole crab (300g for a starter).  The yield is approximately 35%.
Scallops (king)	Serve 3-5 per starter (not often served as a main course).
Mussels	Serve 500g per person (300-400g for a generous starter).
Oysters	6 are usually served for a luxurious starter (not often served as a main course).