There is more than one way to compute a money-line parlay, but I'll stick to the most popular way since its the only one I understand - and it's still kind of convoluted.
First, some background. You all know that, in a perfect world, the money-line is an attempt by the bookmaker to arrange things so that there will be an equal amount of possible payouts on both sides of any game. The only way he can do this is if he changes the money line to reflect the odds of each team winning as accurately as possible.
Imagine that a bookmaker thinks that the Yankees at home with X pitching will beat Minnesota on the road with Y pitching about 70% of the time. Well, in a perfect world, that should mean that about 70% of the money will be riding X, while the other 30% will be hoping for an upset. Thus, the bookie will make X about a -235 favorite, because that way the money on both sides will be equal.
Now a parlay, as you know, is simply a payout based on the probability of two or more things happening. For baseball, what the bookie does is apply a factor to each game and then he can mix and match whatever games his players want and easily calculate the payouts.
The parlay factor is calculated differently for favorites and underdogs.
First we'll deal with favorites: For favorites, the parlay factor is calculated as: (money line + 100) / money line.
If you look closely, this is just the reciprocal of the chance of success. That is, if you liked X and his 70% chance of success (according to conventional wisdom and diligent research), then X's parlay factor would be 1/.70 or 1.43. If you liked another pitcher ("Z") in another game too (at -172, about a .632 chance), then Z's parlay factor would be 1/.632 or 1.58.
For underdogs, things are a bit easier.
The parlay factor is: (money line + 100) / 100. That is, if you also like Cincinnati with Q starting at +115, then Cincy's parlay factor would be (115+100)/100 = 2.15.
So now you've got three games that you'd like to parlay. In a perfect world, the payout for the parlay would be the three parlay factors multiplied out, or 1.43x1.58x2.15 = 4.86.
However, this isn't a perfect world, and if a three-teamer with a couple of good pitchers like X and Z in the mix paid out at 4.86, then there would be a lot of broke bookies. This just in: Bookies don't like going broke.
Therefore, the parlay payout equals the parlay factors multiplied out, minus one. So your three-team payout would be 4.86 - 1 = 3.86.
In summation, here are the steps to calculate a money line parlay:
Step 1: Apply a parlay factor to each team you want to bet on. For favorites, the parlay factor is: (money line + 100) / money line. For dogs, the parlay factor is: (money line + 100)/ 100.
Step 2: Multiply the parlay factors.
Step 3: Subtract one.
Step 4: Forget everything and use this handy Baseball Parlay Calculator
Parlay Calculator Instructions
Step One: Enter the lines of your
wagers into the column marked "Lines". Make sure you remember to put a minus
("-") sign in front of the favorites. Don't bother entering anything into
the "Factor" column, because that takes care of itself and is only there for