What is the name of the international standard for gold bar purity and weight?

The international benchmark for large gold-bar purity and weight is London Good Delivery.

The standard is administered through the London Bullion Market Association’s Good Delivery rules. These rules define the characteristics of gold bars accepted for settlement in the wholesale London bullion market, including fineness, fine-gold content, dimensions, markings, appearance, and handling requirements.

A conventional London Good Delivery gold bar is commonly described as a 400-troy-ounce bar, or about 12.4 kilograms. The permitted range is approximately 350 to 430 fine troy ounces, and the minimum acceptable fineness is 995 parts per thousand, equivalent to 99.5 percent gold.

Good Delivery is not a single worldwide law that governs every gold bar. It is a market standard whose acceptance extends internationally because banks, central banks, bullion dealers, exchanges, and recognized vaults use it for wholesale trading. The bar must also carry identifying marks such as a serial number, refiner’s stamp, fineness, and year of manufacture. Smaller investment bars can follow different specifications.