One of the first steps to building a business is understanding the market and understanding the viability of a successful business. Regardless if the goal is to be a multi-billion dollar company or a local joint that can sustain a few employees - understanding the market, and your share of the market is vital.

As Investopedia puts it, "Market share is the percent of total sales in an industry generated by a particular company. Market share is calculated by taking the company's sales over the period and dividing it by the total sales of the industry over the same period. This metric is used to give a general idea of the size of a company in relation to its market and its competitors. The market leader in an industry is the company with the largest market share."
In order to help calculate your market share and your potential to build a large business, it helps to calculate and understand the total addressable market and sensitivity analysis.