When to split pairs in blackjack

Always split aces and eights. Never split tens or fives. Split 2s, 3s and 7s against a 2–7, 6s against a 2–6, 9s against everything except a 7, a ten and an ace, and 4s only against a 5 or a 6. These rules are for our table, where you may double after splitting.

• Any two cards of the same value make a pair: 8♠ 8♥, and also Q♦ 10♣.
• A split costs a second bet equal to the first. Each card becomes a separate hand and gets a new second card.
• If the new card makes another pair, you may split again, up to four hands in all.
• Aces are split only once, and each ace gets just one card. An ace and a ten after a split is 21, paid 1 to 1, not a blackjack.
• You may double on any split hand except split aces.

Doubling after a split is the rule that makes the small pairs worth splitting. Without it, our calculation says, 2s and 3s would not be split against a 2 or 3, 6s not against a 2, and 4s never — and the house would gain 0.14%.

A pair of aces is a soft 12, a weak hand. Split it and you have two hands that each start at 11, where any ten-value card makes 21. Against a dealer’s 6, splitting aces earns +68.2 per 100 debens of your original bet, while hitting the soft 12 earns +18.8. Even against an ace the split comes out ahead: +12.5 against −2.1.

Eights are the opposite story. 8,8 is 16, the worst total in the game: it busts on any card from 6 up and rarely wins by standing. Two hands that start with 8 are mediocre rather than good, and against a 9, a ten or an ace you still lose after splitting — just less:

Splitting eights against a ten puts twice the money on the table in a bad spot, and it feels wrong. The numbers say the second bet is the price of escaping 16, and it is worth paying.

Two tens are 20, the second-best total there is. Against a 6, standing on 20 earns +70.3 per 100; splitting earns +56.7. The split looks tempting against a weak card, but you would swap a hand that wins most of the time for two hands that start at 10 — and 20 is what each of them hopes to reach. Over all dealer cards, the habit of splitting tens costs 3.2% of everything you bet.

Two fives are 10, a doubling hand. Against a 6, doubling earns +59.9 per 100, hitting +29.9, splitting only +13.6: each 5 is a poor start that often turns into a stiff 12–16. Play 5,5 as hard 10.

18 is good but not great: it loses to 19 and 20. Against small cards the dealer busts often, and two hands that start with 9 collect that twice. Against a 7 the dealer’s most likely total is 17, so 18 already wins — stand. Against an 8 or a 9, 18 is only level or behind, and splitting does better. Against a ten or an ace the dealer is strong enough that a second bet only adds to the loss, so keep the 18.

These pairs make weak totals — 4, 6, 8, 12 and 14 — and the right play depends on the dealer’s card and on being allowed to double after the split.

• 2,2 and 3,3 — split against 2–7, hit against 8 and up. Against a 7, splitting 2s earns +0.8 per 100, hitting −8.9. The closest cell is 3,3 against a 2: split −13.4, hit −14.2.
• 4,4 — split only against a 5 or 6, where a 4 plus a 6 or 7 gives you a 10 or 11 to double. Against a 6: split +16.6, hit +12.4. Against a 4 the split already loses to a plain hit: +0.9 against +4.8.
• 6,6 — split against 2–6. Against a 2: split −19.8, hit −25.3, stand −28.8. Against a 7 and up, hit the 12.
• 7,7 — split against 2–7. Against a 7: split −4.9, hit −33.2. Against an 8, hitting 14 is slightly better: −37.8 against −38.9.

Pairs come up in about one hand in seven, and you split in 2.8% of all rounds. That sounds minor, but the simulator shows what happens if you never split: the house edge goes from 0.43% to 0.99%. Splitting adds more than half a percent to your results, and wrong splits take it away again — splitting tens against a 4–6 alone costs 0.36% of everything you bet.

Each split hand is played on its own, with the full chart. An 8 that receives a 3 is 11: double it against anything but an ace. A 9 that receives a 2 is 11 too. If an 8 gets another 8, split again, as long as you have fewer than four hands. Only split aces stop after one card.

Keep enough debens beside the bet. A split costs one more bet, and a double after it another. With a 1,000 bet at the Scribes’ Table, splitting to four hands and doubling all of them would put 8,000 on the table — rare, but the chart will ask for it now and then.