Market cap = # of currently existing coins X current price.
Market cap (capitalization) is a way to measure the relative overall value of a cryptocurrency. To calculate a cryptocurrency’s market cap, the price of the coin is multiplied by the number of coins in circulation, or “circulating supply”. Circulating supply is an estimation of how many coins are circulating in the market, which might be slightly different than the total supply, which is the total amount of coins currently in existence. Since prices can differ from market to market, the value of a cryptocurrency is calculated by the weighted average price of the overall volume in circulation. Therefore, it is primarily the market cap and not the price, that is used in making informed investment decisions.
For example, if there are 100 coins of a certain cryptocurrency in circulation, and each coin is being traded for $10, then its market cap is $1000. At the time of writing, there are 17,222,940 BTC in circulation, the price of Bitcoin is $4,328.64, hence the market cap is $74,376,408,910.