knowing your OUTS....

Poker is played with one deck of cards obviously you know that... that's 52 cards!
here's the basic calculations which run through my head pretty much everyday and all the time.
>>> If there are 52 cards and you have two HOLE CARDS (cards in your hand) = 50
therefore, we can give each card a value of 2% when calculating the percentage chance of getting the card you need.

1) Flush Draw : 9 outs
2) Open-ended straight draw : 8 outs
3) Inside straight draw : 4 outs
4) Straight flush draw : 15 outs
5) Pocket Pair that needs to make a set (trips) : 2 outs
6) Pair that needs to hit kicker (2pair) : 3outs
7) Two live over cards (pair) : 6 outs

After the FLOP you have to chances of getting the card you need [on fourth and fifth street] so your percentage outs increases... OUTS x 2 x 2 
>{this being two cards that need to still open}
>{the second 2 is the percentage value of each card}

And example: Open-ended straight draw
YOU: Ks.10d
Board: {Qs , 4h , Jd}
....to acquire a STRAIGHT you need an Ace or a 9... four of each in the deck = 8 outs
8 x 2 x 2 = 32%  ------- after the flop
8 x 1 x 2 = 16% ------- after the turn