Each leg's decimal odds are multiplied together. Two -110 legs are 1.909 x 1.909 = 3.64, about +264. Every leg has to win, so the chance of hitting falls fast: three coin flips all landing is 12.5%.
The book's cut is built into every leg, and a parlay multiplies it. A single bet at -110 gives up about 4.5% on average; a 4-leg parlay of -110 bets gives up around 17%. Parlays are fun, but the price is worse than betting the legs on their own. Our board shows single bets ranked by expected profit instead.
See today's bets ranked by expected profit