Chargeable gains (or allowable losses) are calculated as gross proceeds, less direct selling costs, less base cost, less indexation allowance. Indexation allowance is base cost multiplied by the change in the Retail Prices Index movement between the month of purchase and month of sale. Indexation allowance cannot create or increase a loss. Losses may only be set off against chargeable gains of the same or a future accounting period (except certain allowable losses of life assurance companies (see: I minus E basis).