Gold karats, purity and troy ounces
Understand one authoritative fineness factor and why a troy ounce differs from a regular ounce.
What does 21K mean?
On the karat scale, 21 divided by 24 is 0.875, or 87.5% gold by mass. A 20 g piece at that nominal fineness contains 17.5 g of gold before allowance for stones or non-gold components.
Use the purity factor once
When a quote is for pure gold, multiply total metal weight by the chosen fineness once. The reference purity shown beside a selected grade explains that same factor; it is not another reduction. A price already quoted for a particular karat must not be purity-adjusted again.
Weight units matter
A troy ounce is 31.1034768 g; a regular avoirdupois ounce is 28.349523125 g. Precious-metal spot quotes commonly use troy ounces. Match the price unit and weight unit.
Theoretical value is not a dealer offer
Contained-metal value does not include workmanship, dealer margins, taxes, premiums or assay costs. Nominal karat markings also differ from an actual fineness assay. The gold value calculator shows its selected grade and price basis; inspect the market source and timestamps in live mode.