Which dice should I keep?

Tap your dice to match what you rolled, pick what you’re going for, and see the exact expected-value-optimal keep decision — precomputed for all 252 possible rolls, not estimated live.

What “optimal” means here: this solves one category at a time. It is exact — a full dynamic-programming solution over every roll and reroll, not a heuristic — but it maximises the category you pick and ignores the rest of your sheet, so it will happily wreck a scorecard chasing the box you asked about. Playing the game well means weighing all thirteen open boxes against the upper bonus and the Yahtzee bonus at once, which is a far larger solve and a different tool: play a full game for that one.