Traditionally, profitability is defined as the share of profits in revenue. Accordingly, in order to calculate the profitability of the store, you need to determine three things: the revenue for a certain period, all expenses during the same period (including the cost of goods sold) and received in absolute terms a profit.
Probably with a calculation of the revenue problems will not arise. Usually the main currency of the store is over the counter. Less likely to accept cashless payment from customers in most major stores that cater to the b2b sector). If you use both payment methods, a sum of revenue on them.
Make a table in which, line by line, write down all related activity costs. To calculate ROI you must calculate all the costs method "on shipment". This method means that all costs undertaken in the period are evenly distributed to all incoming in a period of months. For example, in the current quarter, the store had repaired computer equipment for 3000 rubles. For a correct account should include the cost of repairs to each of the 3 months for 1000.
To calculate the profitability of the store, fold all expenses for the selected period and subtract their sum from the proceeds. The total value is the profit derived from the operation of the store in a given period of time. Dividing the absolute value of profits in revenues and multiplying the result by 100%, you will receive a return.