How much pure gold is inside 18kt gold alloy?

18-kt gold is 18/24 or 75% pure gold.

18kt gold is 75% pure gold, which means that in one piece of gold alloy you have 75% of pure gold inside.

More about purity and fineness ofgold you can read on this link.

What is carat and how to calculate purity of gold?

The carat represents unit of purity of gold alloys and the symbol of carat is written as kt. Also beside carat unit is known as Karat with symbol K.

Gold is usually mixed with other metals for easier manipulation in a production line. Gold mixture with other metals is called gold alloy and there is good to know in which percentage the gold is present inside per value of weight.

Most wanted purity of gold on the selling market is 24 carat and it means that piece of gold has 99.99% purity inside. To see other marks for gold purity check this site.