Determine what games you will be betting on. For example, take a four-team parlay with two favorites of -140 and -220, and two underdogs of +250 and +180.
Convert the odds of all favorite bets into a decimal number. Do this by taking 100 and dividing it by the line of the favorite. Add 1 to the result. Do this for each favorite line and write down each number.
For example: 100/140 = .71. Add 1 to equal 1.71. And: 100/220 = .45. Add 1 to equal 1.45.
Divide the line of each underdog by 100 and add 1 to the result of each. Write down each of these underdog decimal values.
For example: 250/100 = 2.5. Add 1 to equal 3.5. And: 180/100 = 1.8. Add 1 to equal 2.8.
Multiple all of the underdog and favorite lines together to get the total odds value of your parlay.
For example, with your above odds: 2.8 x 3.5 x 1.71 x 1.45 = 24.29.
So, your odds will equal 24.29:1. A $10 winning bet would pay off 24.29 x $10 = $242.90.