Compare the offered odds with your estimated probability to see whether the bet is positive (+EV) or negative.
Expected value measures what you'd win or lose, on average, if you made the same bet many times over. It works out as EV = p × (decimal odds − 1) − (1 − p), where p is your estimated probability as a decimal.
If your estimated probability beats the implied probability of the odds, EV is positive and the bet has value over the long run. If it's lower, EV is negative. The calculator shows both so you can compare.
EV says nothing about your next bet: it says what repeating that same bet many times over would return on average. A +EV bet can lose, and some of them will. What it measures is the price, not the outcome.
A worked example: odds of +120 (2.20 decimal) imply 45.45%. If your estimate is 48%, EV is 0.48 × 1.20 − 0.52 = +0.056 — that is, +5.60% of your stake; on $50, about $2.80 per bet on average.
And now the part that matters: if your real estimate were 45% instead of 48%, the same odds would give 0.45 × 1.20 − 0.55 = −0.010, a negative EV. Three points of error flip the sign. Break-even sits exactly at the implied probability of the odds — 45.45% here — so the question isn't whether your number beats it, but whether it beats it by more than your own error.
Informational tool only. EV is only as good as your probability estimate. Not investment advice.