Quick Answer — How a Darts Checkout Works
In standard 501 or 301 double-out darts, you don't just need to reduce your score to exactly zero — your final dart of the leg has to land in a double segment (D1 through D20) or the bullseye (50 points, which counts as "double 25"). Landing on the outer single bull (25 points) does not finish a leg, even if it brings your score to exactly zero.
The maximum possible checkout in three darts is 170: two treble 20s (60 + 60 = 120) followed by the bullseye (50). Scores above 170 can never be finished in three darts, no matter how they're thrown.
This calculator searches for a legal finish the way a player actually plans one: it checks whether the remaining score can be finished in a single dart first, then two darts, then three, always ending the search on a legal double or the bullseye.