Since ratio of pure gold in 14-karat gold and 22-karat gold are not the same, jewellers make their calculations separately. 22-karat represents 91,6% purity. 18-karat gold contains 75% pure gold and 14-karat contains 58,5%. So, while 1000 grams of 14-karat gold has 585 grams of pure gold, 22-karat has 916 grams.
In zakat, this difference is taken into consideration. Calculations are done separately for two groups. Then, total amount is converted into monetary unit and 2,5% of it is given as zakat.