Standard 18-carat gold sold in the market contains:

Carat (or karat) expresses the purity of gold as parts out of 24. A 24‑carat piece is pure gold; lower carat values indicate that some parts are other metals. Textbook explanation gives this parts‑based interpretation for caratage (e.g., 22 parts gold + 2 parts alloy) which illustrates the system of ‘‘parts’’ per 24 [1]. Practical sources state that 18‑carat gold therefore contains 18 parts pure gold and 6 parts other metals (18/24 = 75% gold and 25% alloy), commonly copper or silver, so 18K = 75% gold. Hence standard 18‑carat jewellery comprises 18 parts gold and 6 parts other metals.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > QUESTIONS > p. 54