How the gram price is derived
Although British shops quote per gram, the global market quotes gold in US dollars per troy ounce. One troy ounce equals exactly 31.1034768 grams, so the pure gold (24K) gram price is the ounce price divided by 31.1034768, converted to GBP at the live exchange rate. Lower purities are then a simple multiplication: 22K/916 is the 24K gram price × 0.9167, and 18K is × 0.75.
London is the centre of the global gold trade: the LBMA gold price, fixed twice daily, is the world's main benchmark, and the spot market quotes in USD per troy ounce. The GBP price on this page is that benchmark converted at the live GBP/USD rate.