The central idea
Accumulator prices multiply because every selection must win for the full return. The combined price grows as more legs are added.
Combined odds = odds₁ × odds₂ × … × oddsₙ
Worked example
Odds of 1.50, 2.00 and 1.80 combine to 5.40. A 25-unit stake targets 135 units back. The reciprocal price is 18.52%, assuming the selections can be treated as independent.
Limits of the calculation
Selections from the same event may be correlated. Multiplying their standalone probabilities does not create a valid true joint probability when independence fails.