An 18 carat gold represents 75% gold and other metals. This can be calculated by dividing the carats by 24. That is, 18/24=0.75 value of gold
To get the percentage, multiply it by 100. That is 0.75×100=75% gold.
There are some other values of gold that is sold they include; 0.999 0.995 0.990 or 22 carats, 14 carats etc depending on what you buy as gold.
List of carats of Gold and the gold content
Purity: 22 carat gold
Purity:18 carat gold
Purity:14 carat gold
Purity:9 carat gold