Startups often misunderstand the essence of the market. They often measure it in the number of customers, although the number of customers is only a property of the market. 

The market is the money that customers spend to buy what is sold on the market - goods or services. And understanding how to count this money determines how well a startup understands to whom it sells its product. 

The market can be calculated in different ways. The ideal approach is to determine how much people actually spend on solving their problems with similar products this is the so-called bottom-up approach. Alternatively, you can estimate it by assessing the likelihood of purchasing all known services at a known price the top-down approach.

In fact, it does not matter how to calculate the value, it is important that in different methods of calculation the values are similar and do not differ by an order of magnitude or more. 

The market is usually counted for a year and its value is necessarily expressed in money.