Calculating the Odds.

Is there a quick way to calculate odds to compare to pot odds based on number of outs I have?

At the moment, i'm doing 47/4 for four outs on the flop, but this can be tricky to work out quickly in my head.

Do you just calculate odds for each number of outs then learn by rote? Or is there a quicker way to calculate on the spot?

Thanks,

G.