Betting maths and calculators
Why can a 90% chance bet still be bad value?
Direct answer
A bet with 90% true win probability is negative value if its decimal payout is LESS than 1/0.90 = 1.1111 (before any other costs). A likely winner can be overpriced.
How to use this answer
This answer is based on the mathematical principle or published rule identified in the source below. If the question involves a live betting market, confirm its settlement definition before applying it to a specific bet.
If the offered payout is too small relative to the price paid, the expected return can be negative despite a high win probability.
Use the precise stake, decimal odds, market terms and outcome probabilities when calculating the result. Expected value measures an average over hypothetical repetitions, not a guaranteed profit.