When to Split Pairs in Blackjack
Always split aces and 8s. Never split 10s or 5s. Split 2s, 3s and 7s against a dealer 2 to 7, 6s against 2 to 6, 4s only against a 5 or 6, and 9s against everything except a 7, 10 or ace. That's the whole rule for a 6-deck game where you can double after splitting, which is how House21 plays.
The rest of this guide shows why, with the results of each choice played 150,000 times.
The pair splitting chart
| Your pair | Split against | Otherwise |
|---|---|---|
| A,A | Every card | n/a |
| 10,10 | Never | Stand |
| 9,9 | 2 to 6, 8, 9 | Stand against 7, 10, A |
| 8,8 | Every card | n/a |
| 7,7 | 2 to 7 | Hit |
| 6,6 | 2 to 6 | Hit |
| 5,5 | Never | Play as hard 10: double against 2 to 9 |
| 4,4 | 5 and 6 | Hit |
| 3,3 and 2,2 | 2 to 7 | Hit |
At House21 you can split once per hand, double on either new hand, and split aces receive one card each.
Why splitting works
Splitting does two things. It breaks up a bad total (16 from two 8s is the worst hand in blackjack) and it puts more chips on the table when the dealer is weak. The second effect is why so many splits happen against a 2 to 6: those are the cards the dealer busts from most often.
It also explains when not to split. A 20 from two 10s is already one of the best hands you can hold, and two hands starting from a single 10 are worth less in total.
What each choice is worth
We dealt each pair against each dealer card 150,000 times and played it three ways with House21's rules. Values are the average result as a percentage of your original bet, counting hands where the dealer turns out to have blackjack. Each figure is within about ±0.6 points.
| Hand | Split | Play without splitting | Stand | Best |
|---|---|---|---|---|
| 8,8 vs 7 | +21.1% | -40.9% | -47.8% | Split |
| 8,8 vs 10 | -52.4% | -57.3% | -57.3% | Split |
| 8,8 vs A | -57.8% | -66.3% | -77.1% | Split |
| A,A vs 10 | +9.7% | -13.2% | -57.2% | Split |
| 9,9 vs 6 | +43.6% | +27.6% | +27.4% | Split |
| 9,9 vs 7 | +36.3% | +39.8% | +40.0% | Stand |
| 9,9 vs 9 | -9.7% | -18.3% | -18.5% | Split |
| 10,10 vs 6 | +57.5% | +70.3% | +70.0% | Stand |
| 7,7 vs 7 | -8.8% | -33.1% | -47.8% | Split |
| 7,7 vs 8 | -39.1% | -37.7% | -51.4% | Hit |
| 6,6 vs 2 | -19.9% | -25.1% | -28.9% | Split |
| 6,6 vs 7 | -25.1% | -22.0% | -47.4% | Hit |
| 4,4 vs 4 | +0.8% | +5.1% | -19.9% | Hit |
| 4,4 vs 5 | +9.9% | +8.2% | -15.8% | Split |
| 2,2 vs 7 | +0.2% | -9.0% | -47.3% | Split |
| 3,3 vs 8 | -22.4% | -21.8% | -51.0% | Hit (close) |
| 5,5 vs 6 | +12.5% | +60.0% | -14.5% | Double |
The splits people get wrong
8s against a 10. Splitting still loses money on average, which makes it feel wrong. But not splitting loses more: you're starting from 16 against a 10, a hand that loses over half the time whatever you do. Splitting cuts the loss by about 5 points. Over 100 of these hands at 10 chips, that's around 50 chips saved.
9s against a 7. Stand. Your 18 beats the dealer's most likely total, 17. Against an 8 or 9 the dealer is more likely to reach 18 or 19, so splitting comes out ahead there.
10s against a 6. Splitting 10s looks greedy because the dealer is weak. The numbers say stand: +70% beats +57.5% by a wide margin.
5s, ever. Two 5s are a 10, one of the best doubling hands. Splitting turns it into two weak starts. Against a 6, doubling is worth +60%; splitting about +12.5%.
4s against a 4. Close, but hit. Only against a 5 or 6 does splitting 4s pay, and only because you can double the new hands.
Splits, doubles and your chip stack
A split doubles the chips you have at risk, and doubling one of the split hands adds more. If your stack can't cover it, you're forced into a worse play. That's one reason to keep about 100 bets behind you; see our bankroll guide.
Common questions
Can I split two different 10-value cards, like a king and a queen? Yes, at House21 any two cards of the same value can be split. You shouldn't, though.
Why do split aces only get one card? It's a standard casino rule. Letting players keep hitting split aces would make them even stronger and shave about 0.2% off the house edge, so most casinos don't allow it. A 21 made after a split counts as 21, not blackjack, and pays 1 to 1.
Can I split again if I get another pair? Not at House21. Each hand can be split once.
If a daily mission asks you to win a hand after splitting, do it on a split the chart already recommends, like 8s against a 7. More on missions in XP, levels and daily rewards. The complete chart is in the basic strategy guide.