Expected Value (EV)

Expected value is the average profit you'd realize per unit wagered if you placed the same bet an infinite number of times. The formula:

EV = (True Probability × Profit if Win) - (Probability of Loss × Amount Risked)

A bet has positive EV (+EV) when the price you're getting is better than the true probability of the outcome. For example, if a coin is truly 50/50 and someone offers you 2:1 odds on heads, your EV per $1 bet is:

(0.5 × $2) - (0.5 × $1) = $1 - $0.50 = +$0.50

Long-run, you'd net $0.50 per dollar wagered.

In practice, "true probability" is impossible to know. The best approximation we have is the no-vig consensus across many independent sportsbooks. If Kalshi prices an outcome at 40% but the book consensus says 45%, Kalshi is offering +EV on the YES side (you're paying for 40% but getting something worth 45%).

This is the heart of value betting and the only theoretically sound way to make sustained profits in betting markets. SportsBookISH surfaces +EV opportunities by comparing Kalshi's price to the de-vigged book median — the standard industry proxy for fair price.