The only hard hands you need to consider doubling down on are 9, 10, and 11.
With any 11 you double down against every dealer card except an ace if the dealer stands on soft 17. If the dealer hits on soft 17 you double on every hard 11.
When you have a hard ten total, double against any dealer card except an ace or ten valued card. This is the same for both when the dealer hits or stands on a soft 17.
With a hard 9 double against any dealer 3, 4, 5, and 6.
Your soft hands create a different set of hands to double.
Soft 13 and 14 double against dealer 5 or 6.
Soft 15 and 16 double against dealer 4, 5, and 6
Soft 17 and 18 double against dealer 3, 4, 5, and 6.
If the dealer hits a soft 17 you also double with a soft 18 against a dealer 2 and a soft 19 against a dealer 6.
This may seem like a lot of hands to remember, but it shouldn’t take too long to learn them.
Start by deciding to only play dealer hits on soft 17 or stands on soft 17 tables. That way you only need to memorize one variation to start. You don’t have to worry about most other variations because you’ll double if allowed on all tables.
Here’s an example. If you plan to play on tables where the dealer hits a soft 17 follow this plan to help remember what to do.
It’s fairly easy to remember no hard hands except 9, 10, and 11 should be doubled, so let’s start there. Add another rule by doubling all hard 11’s and all 10’s except against 10’s and aces.
The dealer up cards of 3, 4, 5, and 6 are weak and cover three different situations. As you double a hard 9, soft 17 and soft 18 against all four weak cards, it’s the next rule to learn.
This only leaves soft 15 and 16 and soft 13 and 14. I try to visualize moving down a ladder from soft 17 and 18 to soft 15 and 16. As I move down the ladder one dealer card goes away. Moving down one more rung and another card goes away.
Don’t try memorizing all of the steps to start. Focus on the first few. Once you have them down add a couple more. It helps to play while you’re learning as it will start ingraining it in your memory faster than just trying to memorize them.