The basic bet
In Over Under 13 you wager on whether your first two cards add up to more or less than thirteen. You pick a side, and many layouts offer a spot for each. If your pick is right, the bet usually pays even money, 1 to 1. If the total is exactly 13, both sides lose.
Aces normally count as one in this bet, not as 11, though rules differ, so the game’s own rules should be checked. Face cards and tens count as ten. The figures below assume aces count as one and that exactly 13 loses.
Why 13 is the heart of the edge
On the face of it the bet looks like a coin flip: over or under, paying 1 to 1. A fair coin flip at even money would have no house edge. The edge here appears because there is a third result, exactly 13, that neither side wins. The first two cards total 13 in about one hand in twelve, and in every one of those hands the house keeps your stake.
The extra complication is that “over” and “under” are not equally likely. Ten-value cards are four of the thirteen ranks, which pulls totals upward. So the two sides carry different edges, and the numbers below show how large the gap is.
The numbers, using an infinite deck
For a clean calculation, treat each card as an independent draw, where ranks one through nine each have probability 1 ÷ 13 and a ten-value card has probability 4 ÷ 13. Counting in units of 1 ÷ 169, all ordered pairs of cards give:
| Result | Weight out of 169 | Probability |
|---|---|---|
| Under 13 (total 2 to 12) | 76 | about 44.97% |
| Exactly 13 | 14 | about 8.28% |
| Over 13 (total 14 to 20) | 79 | about 46.75% |
Check: 76 + 14 + 79 = 169.
How the 14 for exactly 13 is built: the pair 3 and ten has weight 2 × 1 × 4 = 8 (two orders, one three, four ten-value ranks), and each of 4 and 9, 5 and 8, 6 and 7 has weight 2. That gives 8 + 2 + 2 + 2 = 14.
Now the expected value per unit staked at even money:
- Betting under: (76 − 93) ÷ 169 = −17 ÷ 169, an edge of about 10.06%.
- Betting over: (79 − 90) ÷ 169 = −11 ÷ 169, an edge of about 6.51%.
Both sides cost the player, and the “over” side is cheaper than the “under” side in this model. Neither is cheap compared with the main game. In a real shoe, the first card changes what is left for the second, so actual figures differ somewhat and also depend on the number of decks. The general picture of one side costing less than the other should carry over, but check the rules of the game you are actually looking at.
Why an even-money payout can mislead
Because the bet pays 1 to 1, it is easy to feel it resembles the main game’s near-even odds. In the main game, though, a skilled player following basic strategy faces a house edge typically well under one percent, depending on the rules. Here the edge is at least several times that on the best side. The difference comes purely from the losing 13, which never appears as a separate case in the main game’s decisions.
It also means that “just betting the side with the better odds” has a limit. Picking over rather than under lowers the expected cost; it does not remove it. No choice of side makes the bet favourable, and a run of results in one direction says nothing about the next.
Variation in rules
Some tables:
- let you choose whether an ace is high or low;
- pay differently if the total is far from 13;
- apply different rules for the two sides.
Each of these changes the arithmetic, so a number worked out for one version is not safe to apply to another. The general lesson holds for most side bets. Find the full rules, list the outcomes, assign probabilities, and compare with the payout. The break-even approach is the same one used to measure the house edge on any wager.
Practical points
Like every side bet, Over Under 13 is settled before you make a single decision, and your later play cannot change it. Cards dealt in earlier hands do not influence future hands in any way you can rely on. Anyone tempted by the near-even odds should remember that results in the short term are unpredictable and that the long-run average is a loss.
Spend only what you have budgeted for entertainment, and if you notice yourself raising stakes to win back losses, step away. Free, confidential help for gambling problems is available from local support services.