A one-in-ten chance is not certain after ten tries, so how many do you need?

Players, and many designers, expect a one-in-ten drop to show up within ten tries. It does not, and the gap is bigger than most people think. This entry works out exactly how likely it is, and how many tries it takes to be nearly sure.

Why this works

Each try starts fresh and has no memory of the ones before, so chances do not add up. What adds up is the chance of missing every time, and success is whatever is left once you take that away from certainty.

When not to use it

Skip it when tries are not independent: with a boost after misses, a shared pool of luck, or limited items that run out, the plain formula gives the wrong answer. Work out those cases with the method that matches how the luck actually behaves.

In the full entry

• What to build, step by step
• Starting values and the reason for each
• 3 common mistakes and the fix
• 28 lines of example code with 3 checks that pass
• How your AI shows you that it works

Say this to your AI

Use the Spielerando entry "A one-in-ten chance is not certain after ten tries, so how many do you need?" (read_entry chance-of-at-least-one-in-n-tries) in my game. Build it step by step, and show me how you checked that it works.

Or skip this and connect the MCP: your AI then finds all of this by itself.Connect the MCP