How is the true count calculated in blackjack?

[Bigron]

The true count in blackjack is the final count of cards, which gives a positive or negative value to the player, based on which he can take his playing decisions. Knowing the true count helps blackjack players take the right playing decision as they have an estimate of what cards are remaining in the deck. You do not need to be a mathematician to count cards. There are various systems, both basic and advanced, of card counting learning which can help a blackjack player calculate the true count. How, then, can we calculate the true count of cards in blackjack?