Share of wallet (SOW) is a survey method used in performance management that helps managers understand the amount of business a company gets from specific customers.
Another common definition is the following: Share of wallet is the percentage ("share") of a customer's expenses ("of wallet") for a product that goes to the firm selling the product. Different firms fight over the share they have of a customer's wallet, all trying to get as much as possible. Typically, these different firms don't sell the same but rather ancillary or complementary product.
Share of wallet is commonly used in the finance and banking sectors to describe share-of-customer. Increasing share-of-customer is a key consideration increasing customer lifetime value. The reason is that retaining and growing customers is cheaper than acquiring new customers.
Forte Consultancy says: "The percentage of a customer’s spend that is with a given company over a given amount of time. For a gas retailer, for example, it’s the number of times a given customer fills up their car’s gas tank one month at their own pumps divided by the total number of times the same customer fills up their car’s gas tank that entire month. So a customer who fills up his or her car’s gas tank four times a month with three of those fills at one gas retailer is giving that gas retailer 75% share of their wallet."