The standard weight commonly associated with a Good Delivery gold bar is 400 troy ounces.
Good Delivery bars are the large bullion bars accepted for settlement in the London wholesale gold market. The London Bullion Market Association sets requirements covering not only weight, but also fineness, markings, appearance, and the refiner’s accreditation. A bar described as “400 ounces” is therefore a market standard, not a claim that every bar weighs exactly that amount.
Actual permitted gold-bar weights fall within a range of roughly 350 to 430 fine troy ounces, with bars generally close to 400 gross troy ounces. A typical bar weighs about 12.4 kilograms, although its exact gross weight depends on purity and the amount of metal poured.
The troy ounce is heavier than the everyday avoirdupois ounce: one troy ounce equals about 31.1035 grams. The 400-troy-ounce figure applies to the familiar London gold bar; Good Delivery silver bars are much larger by ounce count, typically around 1,000 troy ounces.