Unless it is food, this is usually based on the value of the goods for a straight ‘Home Use’ type entry (where no special procedure is requested).  Some food entries have duty (or levy) paid according to the net weight of the goods instead.

The value of the goods, the freight and any marine insurance are added together to form a Value for Duty.  Then the correct duty rate is applied to this value.  If the goods are also liable to import VAT then we take afore mentioned Value for Duty figure, add the Import Duty just calculated, and then on top of that we add UK charges (Port handling & release costs, UK delivery charge, customs clearance costs billed by your agent, and also UK insurance on delivery) and these are added to form the Value for VAT.  Then the VAT rate is applied to this figure (as of 2019 the standard Import VAT rate is 20%).  There is discussion within HMRC that post Brexit, Import VAT will be postponed for UK VAT registered businesses (regardless of whether cargo arrives into UK from the EEC or from outside).  This will mean there will be no need to actually pay the import VAT before the cargo is released by HMRC at the port.