The common size ratio for each line on the financial statement is calculated as follows: Common Size Ratio. Item of Interest Reference Item For example, if the item of interest is inventory and it is referenced to total assets (as it normally would be the common size ratio would be. Finance Common-Size Statements Common Size Financial Statements Common size ratios are used to compare financial statements of different-size companies, or of the same company over different periods. By expressing the items in proportion to some size-related measure, standardized financial statements can be created, revealing trends and providing insight into how. A common-size analysis is especially useful when comparing companies of different sizes. It often is insightful to compare a firm to the best performing firm in its industry (benchmarking). A firm also can be compared to its industry as a whole.