Pokémon
Direct Answer
Buy singles when one specific card is the goal. Most people spend more chasing than they expect, especially once a miss turns into another pack and then another.
If you still want to rip, the calculator is the fastest way to see whether your pack budget still makes sense before the chase turns expensive.
What to do next: pick the card you want most and run the calculator.
What usually matters most
If you would be annoyed after missing on a small pack run, buying the single is usually the cleaner move.
If you only care about one or two cards, every extra pack tends to feel worse than it looked at the start.
Calculator shortcut
Use the calculator to compare three things fast: the card price, the point where packs usually become a bad deal, and the chance you still miss after 36 packs.
That is usually enough to tell whether opening packs is just for fun or whether buying the card makes more sense.
Best next step
Pick the chase card you actually want, then check before you rip.
Pick a card and run the calculator