Gold bars can come in many shapes and sizes; but the classic “chocolate-bar shaped” 400 oz gold bar that everyone knows and loves, a little smaller in length and width than a regular house brick and about half the height, is the standard 400 oz London Good Delivery Bar – a gold bar that is traded internationally and held by central banks. It weighs 400 (Troy) ounces (around 12.4 kilos / 27.4 pounds). Unlike the majority of gold bar sizes, the weight is not typically marked on the London Good Delivery 400 oz gold bars; however according to the London Bullion Market Association (LBMA)’s Specifications for Good Delivery Bars  , bars must be stamped with serial number, the assay stamp of the refiner, fineness (to four significant figures i.e. 999.9) and the year of manufacture (in four digits). Since January 2019 the month has now been added to the date requirement.
400 Troy Ounces of fine gold (around 12.5kg) – the good delivery bar. This is the classic, universally-loved “chocolate bar” shaped gold bar. Unexpectedly heavy when first handled, the value of one of these bars at approximate 2023 gold prices ($1,900 per Troy Ounce) would be $760,000.
The majority of the world’s population has only ever seen these famous gold bars in movies or pictures. These gold bricks are required to be at least 99.5% pure gold – and must be kept in high security bullion vaults, usually deep underground. Every detail of their storage and transportation must conform to carefully laid out security procedures. Isn’t it ironic that despite their astonishing color, value and sheer desirability, the majority of gold bricks spend most of their lives unseen and untouched?
London Good Delivery gold bars, along with 100 oz “COMEX Good Delivery” gold bars, are the main ones held by the world’s central banks and traded in the gold market by major bullion dealers.
Another irony is that despite being outside most people’s price range, the 400 oz bars are, ounce for ounce, the cheapest way to buy gold bars owing to transaction fees and the logistics of scale: The premium over spot can be calculated as a percentage markup, with the larger bars commanding a smaller percentage. If you buy smaller gold bars, the premium over spot will typically end up as typically a higher percentage, with the highest markup of all being for the 1 gram gold “chips”.
The London Bullion Market Association produces a list  of refiners past and present whose gold bars were found to meet the Good Delivery requirements.
Weighing Gold Bars
Another astonishing fact – as of 2010, the LBMA still used beam balances to weigh gold, rather than electronic balances. The LBMA balances are incredible feats of engineering precision, and are so accurate that the smallest weights used (0.001 ounce) are handled with tweezers – as perspiration from fingers can affect their accuracy! The scales are housed in cases so that air movement does not move the scales. The LBMA standards are exacting, and it was stated by them that “electronic weighing is unlikely to be as accurate as weighing on a beam balance” – although other nations do use electronic weighing.  An electronic weighing system that meets the exacting standards of the LBMA has been in development for several years. 
400 oz Gold Bar Dimensions
Unlike most of the other gold bars on the market, the 400 oz gold bar is not made to an exact size. This no doubt evolved because the bars are cast from molten gold – and it was therefore easier to simply pour them and weigh them than to try to make them an exact weight. The Good Delivery Bar is approximately 250mm x 70mm x 35mm (around 10 x 3.25 x 1.5 inches), but this figure is not exact. According to Good Delivery specifications the bars are required to have a gold content of between 350 and 430 troy ounces. 
Even though people know gold is heavy, most people are very surprised when they pick one of these gold bricks up for the first time. Its density is so unexpected that they need to ‘recalibrate’ form their expectation of how heavy it looked as though it would be. It’s well known that the person handing it to them should be prepared for the fact that many people will drop the bar without assistance.
400 oz Gold Bar – References: