Why a one-ounce gold coin weighs more than an ounce
A Krugerrand weighs 33.93 grams, and a troy ounce is 31.10. The extra is copper. The coin is 22 carat — 916.67 parts gold per thousand — so it contains exactly one troy ounce of gold and about 2.8 grams of alloy on top. The American Eagle does the same thing with copper and silver. Both were designed to circulate and to survive handling, and pure gold is far too soft for that.
Coins sold as fine bullion take the other approach. The Canadian Maple Leaf, the Austrian Philharmonic and the American Buffalo are 999.9 fine and weigh 31.10 grams — the coin and its gold content are the same thing. Neither approach is better; they simply state the ounce differently, and knowing which you are holding stops the gross weight from misleading you.
The figure that matters is actual gold weight, abbreviated AGW. It is gross weight multiplied by fineness, and it is what a dealer prices from.
Sovereigns and the European standards
The British sovereign is 7.98805 grams at 22 carat, giving 0.2354 troy ounces of gold — a shade under a quarter ounce. It has been struck to that specification since 1817 and still is, which makes it the most traded gold coin in Britain. A half sovereign is exactly half of everything: half the weight, half the gold.
Continental Europe standardised differently. The French 20 franc Napoleon is 6.4516 grams at 900 fine, holding 0.1867 troy ounces, and the Swiss 20 franc Vreneli is the identical coin in different clothes — both were struck to the Latin Monetary Union standard, as were the Belgian and Italian 20 franc pieces. The Austrian 100 corona at 0.9802 ozt and the Mexican 50 pesos at 1.2057 ozt are larger, and both were restruck for bullion long after the dates on their faces, so the year on one of those is not a rarity signal.
When melt is the wrong question
For modern bullion, melt plus a small premium is roughly the whole story, and this calculator gives you the number a dealer is working from. For older coinage it can be badly misleading. Pre-1933 US gold — the $20 Double Eagle and its smaller relatives — frequently sells for well above its metal because the surviving population is limited and collectors want it. Early sovereigns, scarce mint marks and high grades do the same.
The asymmetry is the same one that runs through the rest of this site: a buyer paying melt for a collectable coin has not cheated anyone, and melting is irreversible. Check the date and the mint mark before treating any pre-war coin as bullion.