How to Calculate a Parlay
In
summary, 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
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.
NOTE: Don't bother entering anything
into the "Factor" column, because that takes
care of itself and is only there for educational
value.
Step Two: Enter your bet in dollars
where it says "Bet". If you are only interested
in finding out the payout factor, enter
"1" as your bet.
Step Three: Click "Calculate Parlay".
Step Four: If you are planning
on parlaying more than two or three games,
get your head examined.


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 threeteamer
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 threeteam
payout would be 4.86  1 = 3.86.
Simple.