The rules in brief
Blackjack is played against the dealer, not against other players. Each hand starts with two cards. Cards 2 to 10 count at face value, face cards count 10, and an ace counts 1 or 11, whichever suits the hand. The goal is to finish closer to 21 than the dealer without going over. If you go over 21 you bust and lose immediately, even if the dealer would later have busted too.
The choices after the deal are:
- Hit: take another card.
- Stand: keep the current total.
- Double down: double the bet, take exactly one more card, then stand.
- Split: when the first two cards match in value, turn them into two separate hands with a second bet (rules vary on what is allowed afterward).
The dealer has no choices. A fixed rule says when to hit and when to stand, commonly standing on 17 or more. Whether the dealer hits a soft 17 (a 17 that includes an ace counted as 11) differs by table, and so do several other details.
Why decisions matter here
In many casino games the player makes no meaningful choice. Blackjack is different because the player’s decisions change the outcome probabilities. That means the house edge for blackjack is not one fixed number: it depends on the rules and on how well the player plays. Basic strategy is the set of decisions that gives the lowest edge for a given set of rules.
How a basic strategy table is built
A basic strategy table has the player’s hand along one axis and the dealer’s visible card (the upcard) along the other. Each cell says which action has the highest expected value. Building it is a calculation, not a rule of thumb.
For one cell, say player 16 against a dealer 10, the process is:
- List every card that could come next, and the probability of each. For a single deck with no other cards removed, the probabilities come directly from how many of each rank remain.
- For the “stand” option, work out the probabilities of the dealer finishing on 17, 18, 19, 20, 21 or busting, starting from a 10 showing. Compare the player’s 16 against each. A standing 16 only wins when the dealer busts.
- For the “hit” option, work out each possible new total, then the best continuation from there, then the result against the dealer’s possible finals.
- Convert each result into expected value per unit bet: a win is +1, a loss is -1, a push is 0.
- Choose the action with the higher figure.
For 16 against 10, both options have a negative expectation, which is the uncomfortable truth about many blackjack hands. Hitting loses slightly less on average than standing, so the table says to hit (with some exceptions that depend on the exact composition of the hand and rules). The aim of the table is to lose as little as possible on average in spots where winning on average is not possible.
What drives the numbers: the dealer’s bust rate
The main variable in any cell is how likely the dealer is to bust. A dealer showing a 6 busts roughly 42% of the time under common rules, while a dealer showing a 10 busts roughly 21% of the time. Those figures are approximate and shift slightly with the number of decks and the rules.
That is why the table tells players to stand on stiff totals like 12 to 16 against low upcards (2 to 6): the dealer is more likely to be forced to hit a weak hand and bust. Against high upcards (7 to ace), the dealer is more likely to make a strong hand, so the player has to accept more risk to improve.
Doubling follows the same logic. Doubling is attractive when the player starts with a good chance of winning (such as 11 against a low upcard), because the increased stake is placed on a favourable situation. It would be a mistake to double just because a hand feels promising, since the table’s recommendations come from the math, not hunches.
Why rule changes alter the table and the edge
Small rule changes shift the expected values and can flip individual decisions:
- Blackjack payout: a natural (an ace plus a ten-value card in the opening two cards) happens about 4.8% of the time with a single deck, since 2 x 4 x 16 / (52 x 51) = 128/2652. Paying 3 to 2 versus 6 to 5 on a 10 unit bet means 15 units versus 12 units. A 0.3 unit difference per unit bet, multiplied by about 4.8%, is roughly 1.5 percentage points of extra edge for the house under 6 to 5.
- Dealer hitting soft 17: this slightly increases the house edge, because the dealer gets more chances to improve.
- Number of decks: more decks reduce the effect of removing cards, which changes some close decisions.
- Doubling and splitting restrictions: the fewer the options, the higher the edge.
Each of these requires recalculating the table, which is why published strategy charts are labelled with the rules they assume.
What basic strategy can and cannot do
Playing perfect basic strategy reduces the edge to its lowest level under the given rules, which in many common games is a small fraction of a percent to around one percent. It does not turn the game into one with positive expectation. It also does nothing to the randomness of a single session: even with perfect play, the swings can be large. The idea of a law of large numbers only applies across many hands.
Two related points are worth keeping in mind. First, each hand’s expected value is a weighted average of possible results, which is the same calculation described in the article on weighted averages. Second, a hand that “should have won” but did not was not a strategy error. A decision is judged by its expectation, not by what the next card turned out to be, and mixing up the two is one of the most common cognitive biases in card games.
As with all gambling, outcomes are uncertain and money can be lost. This article explains the structure of the game; it is not a promise of any result.