It sounds like OP's question was more along the lines of the mathematical conversion between odds and percent, rather than how to calculate pot odds
If that's what you meant, to convert odds (i.e. a ratio) to a percent, it's simple fraction math. Just add both sides of the ratio together and that's your denominator. For example, odds of 4:1 against is a total of 5 outcomes (4 to win, 1 to lose). A single outcome would be 1/5, which is the same as 20%. So 4:1 is the same as 80% to win (4/5) and 20% to lose (1/5). As another example, let's look at odds of 5:3 against. A single outcome is 1/8. So 5/8 to win is 62.5% and 3/8 to lose is 37.5%.
In terms of the "fastest" way, I guess it depends on your capacity to mentally calculate fractions in your head. There are probably some shortcuts you can take to remember the basics, but (simple) math comes pretty naturally to me so I don't have any problem doing the conversion in my head. Practice makes perfect.