How many percent of gold is in 18 karat gold

"Caratage" is simply the measurement of purity of gold alloyed with other metals. 24 carat is generally accepted as pure gold with no other metals. Lower caratages contain less gold; 18 carat gold contains 75% percent gold and 25% other metals, often copper or silver.
    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
Gold content(wt.%):91.6
Fineness:916

Purity:18 carat gold
Gold content(wt.%):75.0
Fineness:750

Purity:14 carat gold
Gold content:58.5
Fineness:585

Purity:9 carat gold
Gold content:37.5
Fineness:375