The odds of being dealt a blackjack hand in any game of blackjack using a single deck is 4.83 percent, or approximately once per 21 hands dealt. A blackjack is any 10-valued card combined with an ace.
There are four 10-value cards in a deck: the 10, jack, queen and king. Therefore, there is a 16-out-of-52, or 30.76 percent, chance of getting a 10-value card. There are four aces in a deck, and so there is a 4-out-of-52, or 7.69 percent, chance of getting an ace. When multiplied together, these numbers yield a 2.41 percent chance of getting blackjack. But since the order of the cards in a blackjack can be either "10 and ace" or "ace and 10," the probability doubles to 4.83 percent.