List the things that all have to go right. Say how sure you are of each. The answer is usually not what the room believes.
Chance everything holds
People average their assumptions. Reality multiplies them. Six beliefs each judged 85% likely feel like an 85% plan. But all six have to hold, so the real figure is 0.85 to the power of six — about 38%. Nobody in the room felt 38% confident, and nobody was lying.
This is why plans fail without any single assumption looking unreasonable. It is not pessimism, and it is not a modelling trick. It is the arithmetic that the meeting skipped.
Two honest caveats. This calculator treats every item as a hard gate — all-or-nothing. In reality most assumptions degrade rather than kill: adoption reaches 55%, not zero. It also treats them as independent, when several often rise and fall together. Both simplifications make this number lower than a careful analysis would give.
Which is the point at which a calculator stops being enough.
The full instrument separates the assumptions that kill a plan from the ones that only hurt it, holds your confidence as a range set by your evidence, and tells you which belief is worth paying to check. Free, no account.
Open the free tool