Learn Card Counting
A friendly guide to getting started. No math degree required.
What is card counting?
Card counting is a way to track the ratio of high cards to low cards left in the shoe. When more high cards remain (tens, face cards, aces), you have a slight edge over the house. When more low cards remain, the house edge grows.
You don't memorize every card. Instead, you keep a simple running total that tells you whether the remaining deck favors you or the dealer. When the count is in your favor, you bet more. When it's not, you bet the minimum.
The Hi-Lo System
Hi-Lo is the most popular counting system. Every card gets a simple tag: +1, 0, or -1. As cards are dealt, you add each tag to your running count.
Standard Hi-Lo Tags
Low cards (2–6) get +1 because when they leave the deck it's good for you — fewer small cards means more blackjacks and better doubles. High cards (10–A) get -1 because losing them hurts. Middle cards (7–9) are neutral.
Running Count & True Count
The running count (RC) is your cumulative total. Start at 0 and add each card's tag as it's dealt face-up.
But a running count of +6 means something different with 6 decks left vs. 2 decks left. To normalize, divide by the estimated decks remaining to get the true count (TC):
True Count = Running Count / Decks Remaining
Estimate decks remaining by eyeballing the discard tray. If a 6-deck shoe has about 2 decks in the tray, there are roughly 4 decks left. With RC = +8 and 4 decks remaining, your TC is +2.
In standard blackjack, a higher true count means a bigger player advantage. You use the TC to decide how much to bet and when to deviate from basic strategy.
Counting in Spanish 21
Spanish 21 removes all four 10-rank cards from each deck — the literal tens are gone, but jacks, queens, and kings stay. You can still use standard Hi-Lo, but you need a negative starting offset to account for the missing cards.
Standard Hi-Lo Tags (same as regular BJ)
No 10-rank cards in Spanish 21 — they're removed from the deck, but the tags stay the same
Negative Starting Offset
Each removed 10 would have counted as −1. With 4 gone per deck, the shoe is permanently skewed toward low cards. To compensate, start your running count in the negative:
Starting RC = −(Number of Decks × 4)
6 decks → start at −24 | 8 decks → start at −32
The TC interpretation stays the same as standard blackjack — higher true counts favor the player. But because of the large negative offset, the count will spend more time in negative territory, meaning favorable counts are rarer than in a standard shoe.
Alternative: Inverted Hi-Lo
Some players prefer an inverted Hi-Lo system where every tag is flipped (2–6 = −1, faces/aces = +1) and the starting offset is positive (+4 per deck). In this system, lower TCs favor the player instead of higher ones. It's mathematically equivalent, just a different convention.
Inverted Hi-Lo Tags (alternative)
Flipped signs — lower TC favors the player
Playing Deviations: I18 & Fab 4
Basic strategy (BS) is the correct play for every hand without counting. But when the count shifts, the correct play sometimes changes. These count-dependent changes are called playing deviations or index plays.
The Illustrious 18
Don Griffin coined the term “Illustrious 18” for the 18 most valuable playing deviations in standard blackjack. These ~18 index plays capture the vast majority of the EV gain from count-based strategy changes. Classic examples:
- Insurance — take it at TC +3 or higher (the single most valuable deviation)
- 16 vs 10 — stand at TC 0 or higher instead of hitting
- 15 vs 10 — stand at TC +4 or higher
- 10 vs 10 — double at TC +4 or higher
- 12 vs 3 — stand at TC +2 or higher
While the original I18 was defined for standard blackjack, we use “I18” as shorthand for the ~18–20 most profitable index plays for any game variant (Spanish 21, Player's Edge, etc.). The specific hands and thresholds differ by game, but the concept is the same: learn these first for maximum bang-for-your-buck.
The Fab 4 (Surrender Deviations)
The “Fab 4” are the four most important surrender deviations. Basic strategy may say to hit or stand, but at certain counts, surrendering (giving up half your bet) becomes the better play. In standard blackjack:
- 14 vs 10 — surrender at TC +3 or higher
- 15 vs 10 — surrender at TC 0 or higher
- 15 vs 9 — surrender at TC +2 or higher
- 15 vs A — surrender at TC +1 or higher
Like the I18, the Fab 4 were originally defined for standard blackjack. For other variants, the surrender thresholds are generated from simulation data and may differ. They're included in the I18 ranking when applicable.
Full Indexes (FI)
Beyond the I18, there are 100+ possible deviations for every hand/dealer combination. Full Indexes (FI) includes all of them. In practice, the I18 captures most of the advantage — the remaining deviations add only marginal EV but are much harder to memorize. Most players stick with BS + I18 and see ~90% of the total deviation benefit.
Optimal Betting: Kelly Criterion & Bet Spread
Knowing when you have an edge is only half the game. The other half is sizing your bets correctly — big enough to capitalize on your advantage, but not so big that a losing streak wipes you out.
The Kelly Criterion
The Kelly Criterion is a formula that determines the mathematically optimal bet size to maximize long-run bankroll growth. For blackjack:
Optimal Bet = (Edge / Variance) × Bankroll
At each true count, the simulation provides your edge (advantage) and variance. The Kelly formula tells you exactly how much to bet. A higher edge means a bigger bet; higher variance means a smaller bet relative to your edge.
Fractional Kelly
Full Kelly (1.0) maximizes growth rate but comes with stomach-churning swings. In practice, most players use a fractional Kelly:
- Full Kelly (1.0) — maximum growth, maximum volatility. Expect 50%+ drawdowns.
- Half Kelly (0.50) — 75% of the growth rate with much lower variance. A popular choice.
- Quarter Kelly (0.25) — very conservative, smooth ride, still captures meaningful EV.
The Kelly multiple slider in the EV calculator lets you dial this in and instantly see how it affects your hourly EV, risk of ruin, and bet spread.
Bet Spread & Max Spread
Your bet spread is the ratio between your largest and smallest bet. Max spread is the cap on that ratio — the maximum multiple of your minimum bet you're willing to place.
Max Bet = Min Bet × Max Spread
e.g. $5 min × 40x spread = $200 max bet
A wider spread captures more EV from favorable counts, but it's more noticeable to casino surveillance. Common spreads:
- 1-8 — conservative, low heat
- 1-20 — moderate, good balance of EV and cover
- 1-40+ — aggressive, maximum EV but very conspicuous
Wong Out (Back-counting)
Wonging out (named after Stanford Wong) means leaving the table or sitting out hands when the count becomes unfavorable. Instead of betting the minimum into a negative edge, you simply don't play.
The Wong out TC threshold is the count at which you stop playing. For standard blackjack with Hi-Lo, a common threshold is TC -1 or lower (sit out when the count drops below -1). For Spanish 21 with inverted Hi-Lo, the equivalent is TC 14 or higher.
Wonging out dramatically improves your overall advantage because you avoid playing hands where the house has a larger-than-normal edge. The tradeoff is fewer hands per hour and potentially more scrutiny from the casino.
Risk of Ruin
Risk of ruin (RoR) is the probability of losing your entire bankroll before recovering. It depends on your edge, bet size, and bankroll:
RoR = e(-2 × EV × Bankroll / σ²)
- < 5% — well-bankrolled, safe for serious play
- 5–10% — moderate risk, acceptable for many players
- > 10% — underbankrolled for this spread; reduce bet size or increase bankroll
To reduce RoR: increase your bankroll, reduce your Kelly multiple, narrow your bet spread, or wong out more aggressively. The EV calculator shows your RoR in real time as you adjust these parameters.
Next Steps
- Memorize basic strategy. The strategy chart tells you the mathematically correct play for every possible hand. Study it until the decisions are automatic — this is your foundation.View Strategy Chart →
- Practice counting. Use the practice mode to build speed and accuracy with the running count and true count. You can peek at the strategy chart from there whenever you need a refresher.Start Practicing →
- Learn deviations. Once basic strategy feels automatic, learn the Illustrious 18 (I18) — the top count-dependent plays that capture most of the counting advantage. Switch the practice mode to include deviations.
- Calculate your edge. Use the EV calculator to figure out your expected hourly win, optimal bet spread, and risk of ruin for your bankroll.EV Calculator →