The three formats and what they mean
Decimal (2.50) is the total you collect for each unit staked, your stake included. Fractional (3/2) is the net profit on the amount staked. American odds use a sign: +150 is what you win staking 100, and −200 is what you must stake to win 100. Different notation, identical information.
American to decimal
If the number is positive, divide by 100 and add 1: +150 becomes 2.50. If it is negative, divide 100 by the absolute value and add 1: −200 becomes 1.50. With those two lines of arithmetic you can compare any American line against a European one without depending on the bookmaker.
Fractional to decimal
Divide the numerator by the denominator and add 1: 3/2 is 2.50 and 1/4 is 1.25. Fractional odds survive in the British market and in racing, and they are the ones that cause most confusion when they appear mixed with decimals inside a comparison page.
The step almost nobody takes: implied probability
Divide 1 by the decimal price and you have the probability the bookmaker assigns to the event. A price of 2.50 implies 40 per cent. Only then can you compare your own estimate against the price: if you believe it happens more often than that, there is value; if not, there is none, however pretty the number looks.
Adding the probabilities to see the margin
Add up the implied probabilities of every outcome in a match and the total goes past 100. That excess is the bookmaker's margin. Comparing margins between operators is the quickest way to know which one is charging you more for exactly the same bet.
Doing it by hand or letting a calculator do it
The arithmetic is one line long, but repeating it for ten matches a day makes no sense. We keep public calculators for expected value and implied probability, and our predictions already publish the minimum price, which is the same conversion run backwards: the price below which the bet stops being worth taking.