A one-ounce American Gold Eagle, Canadian Gold Maple Leaf, and British Gold Britannia each contain one troy ounce of gold.
Still, they do not have identical dimensions or total weights.
That is because fine gold content is only one measurement. Purity, gross weight, diameter, thickness, and mint specifications also help determine the finished coin.
To understand why these differences exist, start with what “one ounce” actually measures.
In many bullion coin listings, one ounce refers to the amount of fine precious metal the coin contains. It does not always describe the coin’s complete weight.
That difference becomes especially important when comparing coins with different purities.
Fine metal content measures the amount of the named precious metal in a coin. Gross weight measures the entire coin, including any alloy metals.
For example, a one-ounce American Gold Eagle contains one troy ounce of pure gold. However, gold accounts for 91.67% of its full composition. The remaining metal consists of silver and copper.
As a result, the coin’s gross weight is higher than its fine gold content.
This is why you should not compare one coin’s fine gold content directly with another coin’s gross weight. They measure different things.
Gold, silver, platinum, and palladium are measured in troy ounces rather than the standard ounces commonly used for food and household goods.
One troy ounce equals approximately 31.1 grams. A standard ounce is lighter at approximately 28.35 grams.
You can find additional conversions in our gold weight guide and silver weight guide.
A one-ounce gold coin and a one-ounce silver coin do not need the same physical volume.
The reason is density.
Density describes how much mass is packed into a given amount of space.
According to the Royal Society of Chemistry, gold has a density of 19.3 grams per cubic centimeter, while silver has a density of 10.5 grams per cubic centimeter.
Gold therefore packs more mass into the same volume.
Because silver is less dense than gold, one troy ounce of silver occupies more volume than one troy ounce of gold.
That additional volume may appear as a wider coin, a thicker coin, or a combination of both. This is why one-ounce silver coins generally appear larger than one-ounce gold coins.
The larger appearance does not mean the silver coin contains more than one troy ounce of its stated metal. The metals simply occupy different amounts of space at equal weights.
Purity describes the proportion of a coin’s total mass made from the named precious metal.
A .9999 fine gold coin is 99.99% gold. A 22-karat American Gold Eagle is 91.67% gold, but it still contains its full stated amount of fine gold.
The U.S. Mint’s specifications list the one-ounce American Gold Eagle at approximately 33.931 grams, or 1.0909 troy ounces in gross weight.
Within that total weight is one troy ounce of pure gold. The additional weight comes from silver and copper included in the coin’s 22-karat composition.
The alloy does not replace part of the stated gold ounce. It is added alongside that ounce.
This makes the American Gold Eagle a useful example of why fine metal content and gross weight should be read separately.
The 2026 Canadian Gold Maple Leaf is 99.99% pure gold and has a published weight of 31.11 grams.
The 2026 British Gold Britannia also contains one troy ounce of pure gold at 999.9 fineness.
Because these coins have very little non-gold material, their gross weights remain close to their fine gold weights. That does not make them inherently better than a 22-karat coin. It simply gives them a different composition.
Even coins made from the same metal, with the same purity and the same fine metal content, do not need identical diameters.
The Royal Canadian Mint lists the 2026 one-ounce Gold Maple Leaf at 30 mm in diameter. The Royal Mint lists the 2026 one-ounce Gold Britannia at 32.69 mm.
Both are high-purity one-ounce gold coins, but their diameters differ.
The same amount of metal can be formed into different proportions.
A mint may produce a coin with a wider diameter and a thinner planchet. Another mint may use a narrower diameter and a thicker planchet.
A coin’s relief also distributes metal across raised and recessed parts of its design. Relief does not add metal to the coin. It affects how the existing metal is arranged across its surface.
This is why diameter alone cannot tell you how much precious metal a coin contains.
Sovereign mints establish specifications for their individual coin programs. These decisions may reflect production equipment, planchet requirements, established program standards, edge designs, security features, and the amount of space needed for the coin’s artwork.
Different specifications do not mean that one mint is applying the one-ounce standard less accurately. The coin’s published fine metal content remains the key measurement.
Dimensions can also change between products or years. Always verify the specifications for the exact coin you are considering.
Physical size can affect how a coin looks and feels, but diameter alone does not establish its precious metal value.
A larger-looking coin does not automatically contain more gold or silver.
If two coins each contain one troy ounce of the same precious metal, their underlying metal content is the same even when their diameters differ. One may simply distribute that metal across a larger surface area.
Gross weight must also be interpreted carefully. A heavier Gold Eagle contains additional alloy metal, not additional gold beyond its stated one troy ounce.
Bullion coin prices include more than metal value. Premiums may also reflect mint recognition, production costs, condition, availability, packaging, and market demand.
For example, the American Gold Eagle, Canadian Gold Maple Leaf, and British Gold Britannia are separate sovereign coin programs with their own designs, specifications, and market demand.
Their physical dimensions are only one part of the comparison.
The word “ounce” should be the start of your specification check, not the end.
Before comparing bullion coins, check:
Official mint specifications are especially useful when you are comparing products from different countries or coin programs.
Make sure the numbers being compared describe the same feature.
Fine gold content should be compared with fine gold content. Gross weight should be compared with gross weight. Diameter should be compared with diameter.
A 33.931-gram Gold Eagle and a 31.11-gram Gold Maple Leaf both contain one troy ounce of gold. Looking only at their gross weights could make the Eagle appear to contain more gold, but the additional weight consists of alloy metals.
“One ounce” tells you how much of the stated precious metal a bullion product is intended to contain. It does not establish one universal diameter, thickness, or gross weight.
Two coins can contain the same fine metal content while using different dimensions. A coin made with added alloy can also weigh more in total without containing more of the stated precious metal.
Once you separate fine metal content from gross weight and physical dimensions, the differences between one-ounce coins become much easier to understand.
Why is a one ounce Gold Eagle heavier than a one ounce Gold Maple Leaf? The Gold Eagle contains one troy ounce of gold plus silver and copper, while the high-purity Maple Leaf contains very little non-gold material.
Do larger bullion coins contain more precious metal? Not necessarily, because two coins can contain the same fine metal weight while using different diameters and thicknesses.
Can two coins have the same weight but different diameters? Yes, one coin may distribute its metal across a wider, thinner planchet while another uses a narrower, thicker planchet.
Does a thicker bullion coin contain more gold or silver? Not automatically, because thickness must be considered together with diameter, composition, and fine metal content.
Why are one ounce gold coins smaller than one ounce silver coins? Gold is denser than silver, so one troy ounce of gold occupies less physical volume.
Does coin size affect its value? Size alone does not determine value, although metal content, recognition, condition, availability, and demand can all affect a coin’s price.