You can check out above graph to see how the market cap of Polkadot has changed over time. The market cap value depends on the value of Polkadot on that particular day and number of Polkadot in supply at that day.

How is Polkadot Market Cap calculated?

The Polkadot Market cap is calculated using a simple formula. You can find it below:

Polkadot Market Capitalization in USD = Number of DOT in supply X Price of one Polkadot in USD

Since the price of Polkadot is highly volatile you can see the market cap going up and down a lot. Sometimes price and market cap are not linear to each other due to increase in the supply of coins in the market and stable or decrease in the price of the coin. So you can say that it's not simple to predict the market cap of Polkadot or predict the price of Polkadot.