Help calculating pot odds in general

Alright, so I've been reading about pot odds for the past few days, and want to clarify that I am actually understanding the concept, and the steps involved in calculating the pot odds, so that if I am doing it wrong it can quickly be stopped and I can be guided in the proper direction.

So through the articles written here on flopturnriver I was able to find the percentages of completing a drawing hand.
1) With turn and river to come and 1-9 outs, then, (# of outs * 4) = approx. percentage of drawing one of your outs. And anything higher than 9 outs I know the percentage.
2) With only River to come, then, (# of outs * 2) + 1 = approx percentage of drawing one of your outs.

Now let's just for arguments sake say that with only 2 people in the pot after the flop I hold a nut flush draw. That means I have 9 outs and approximately a 36% chance of making my nut flush hand by the river. If the pot has $20 in it already, and I'm facing a bet of $18 do I have the proper odds to call?

Well here's where my question lies. After the $18 bet the pot is $38 and I'm needing to contribute another $18. Well when factoring pot odds do I use the total pot (including my call of the bet) or the pot (without my call) to determine whether a call is justified?

So essentially would I do this:

WIth $38 in the pot and a bet of $18 dollars to me I'm contributing 47.3% of the pot ($18 / $38 = .473). And the call would not be right because I'm contributing more that 36% of the pot.

OR

With $38 in the pot and a bet of $18 to me the pot will be $56 dollars after I call so I'm essentially looking at putting in $18 for a chance to win a $56 pot. So, I'm contributing 32% ( $18 / $56 = .32 )... and the call is correct to make.

Which is the proper way? Am I even close? And also when explaining please use percentages and not ratios. I just feel more comfortable with them if that's at all possible. Thank you and sorry if it was drawn out for a seemingly standard play. I just want to make sure I'm not on the wrong path and that I understand all I can.

Hi and welcome to FTR, it's good to have you here! :)

Your first method is the correct way to calculate it. If the pot has $20 in it on the flop BEFORE opp's bet and opp bets $18, you need to call $18 to see a pot of $38, so you're getting 2.11 to 1 odds and therefore need to be 1/(2.11+1) = 32% to win to make this a good call ASSUMING you won't need to decide whether to call another bet on the turn (eg. the opp betting $18 is AI on the flop).

In this case you would call with your nut flush draw since you are roughly 35-36% to win but only need to be 32% to win according to the pot odds.

Hope that helped!

Okay thank you. So even though I was right to calculate the odds before my call of the bet is entered into the pot I went about it the wrong way and got the wrong percentage. So the steps to do this correctly would be to divide the pot by the bet size, which will give you a x : y ration. Then take 1/ (x + y) and that will give the percentage you must be higher than to call. I understand it I believe. Is there anywhere you know of with a quiz of some sort over pot odds that I can calculate and get the answers to, just to make sure I'm doing everything right?

Hi and welcome to FTR from me too :)

I would recommend HoH (Harrington on Hold'em Vol. 1) as a good book to get you started. It's about tournament poker, but still ... the basics are explained pretty well (including pot odds) and there's always a quiz section at the end of each chapter. I'm not sure if there are any online quizzes somewhere.

The general concept is pretty easy: If the pot is $20 and it takes you $5 to call (4:1) then you need at least 20% chance to win. Because you'll loose the $5 call 4 times and the 5th time you'll win the pot (over a large sample size of course).