Converting American style odds to European odds and vice versa
When converting American odds to European you have to divide the underdog’s price (meaning the positive odds) by 100 and add 1 or divide the favourite’s price (meaning the negative odds) into 100 (after you drop the minus sign) and add 1. To convert decimal odds of less than 2.0 to American you have to subtract 1 and divide into -100. For odds greater than 2.0, subtract 1 and multiply by 100.
The above mentioned in practice for a match between Portugal – Spain would be like this:
To reverse this and convert the European odds to American:
Portugal win is offered at 1.40 which is under 2.00. It would be 100 / (1.40 – 1) = 250. The odds over 2.00 for Spain to win convert as (5.00 – 1) x 100 = 400
Converting British odds to European odds and vice versa
British to European conversion is a bit simpler. You will just have to divide the two sides of British odds to get a fraction and add 1. Portugals victory in British odds is 2/5 which is 0.4 + 1 = 1.40. As simple as that but when reversing this conversion some minor round off errors might happen as some odds cannot be translated exactly into the British fractions.
Converting British odds to American and vice versa
The American odds with a + sign simply show how much money you would make for every $100 you bet, excluding your original stake of $100. To convert into the fractional odds you simply have to divide the expected profit by 100. For example (150/100 = 1.5/1) and then multiple both sides with 10 which gives you 15/10. To see the decimal odds just add the two fractional odds together (1.5 + 1 = 2.5).