What is a troy ounce? Gold, silver and commodity price units explained

A troy ounce is a unit of mass equal to exactly 31.1034768 grams. Gold, silver, platinum and palladium are priced per troy ounce, so a gold price of $4,286.74 means $4,286.74 for 31.1 grams of metal, not for the 28.35-gram ounce used for food and postage. The two ounces differ by 9.7%, and mixing them up is the most common unit bug in finance apps.

If you are pulling metal prices into code, the gold price API guide covers the requests. This page is about the units underneath them: where the troy ounce comes from, how to convert it, and how the rest of a commodities screen is quoted, from barrels to MMBtu to cents per pound.

Troy ounce vs ounce: the 2.75 grams that matter

Both ounces are built from the same smallest unit, the grain, fixed at exactly 64.79891 milligrams since the international agreement on the pound in 1959. A troy ounce is 480 grains; an avoirdupois ounce is 437.5 grains. Divide one by the other and you get the 9.7% that separates a correct gram price from a wrong one.

That last point trips people up in the other direction. The troy system has 12 ounces to the pound and the avoirdupois system 16, so a troy pound is 373.24 grams and an ordinary pound 453.59. Nobody prices metal in troy pounds today, but historical records still carry the unit, and a converter that assumes "a pound is a pound" will get them wrong.

Why gold is weighed in troy ounces

The name is usually traced to Troyes, a market town in Champagne whose medieval trade fairs drew merchants from across Europe. Its weights spread to England, where the troy ounce became the standard for coinage and bullion, and it survived every later metric reform because the precious-metals market never switched. Refiners, mints, vaults and dealers still count in it, so a spot quote, a futures contract and most bar certificates agree without conversion.

The unit is even written into currency codes. ISO 4217, the standard behind USD and EUR, defines XAU as one troy ounce of gold and XAG as one troy ounce of silver, with XPT and XPD doing the same for platinum and palladium. A quote such as XAUUSD carries its unit in its name: US dollars for one troy ounce of gold.

Convert troy ounces to grams and kilograms

Silver works the same way with a smaller number. At the 25 September close of $64.335 an ounce, a gram of silver is $2.07 and a kilogram $2,068.42. To price jewelry, multiply the per-gram figure by the fineness as well: 0.75 for 18 karat gold, 0.925 for sterling silver.

Commodity price units, symbol by symbol

Precious metals are the tidy case. Everything else on a commodities screen comes in its own quantity, and some of it in cents rather than dollars. The table sets the market convention next to what the TickerLayer commodities docs state for each symbol.

Copper is the one to watch. US futures quote it per pound and the London market per metric tonne, and a tonne is 2,204.62 pounds, so the same metal can appear as a single-digit number in one feed and a five-digit number in another. Crude and gas have their own conventions and pitfalls, covered in the oil price API guide.

Cents, not dollars: sugar and coffee per pound

SUGARUSD is quoted in US cents per pound, the convention of the world raw-sugar futures market, and TickerLayer documents it that way. A value of 18.542 is 18.542 cents, or $0.18542, for one pound of raw sugar.

{
  "symbol": "SUGARUSD",
  "bid": 18.537,
  "ask": 18.547,
  "bid_size": 37,
  "ask_size": 36,
  "last_price": 18.542,1
  "last_timestamp": 1790590261059,4
  "prev_close": 18.5,
  "change": 0.042,2
  "change_percent": 0.2273
}

Coffee is conventionally quoted the same way, so a coffee futures price of 350 would mean $3.50 a pound. TickerLayer does not document a unit for COFFEEUSD, and the safe reaction to an undocumented unit is to refuse to convert it rather than to guess. A small lookup table does that for you:

# Units the TickerLayer docs define for each symbol. XAU and XAG are one troy
# ounce by ISO 4217; sugar and grains are documented in US cents.
DOCUMENTED = {
    "XAUUSD": (1.0, "troy ounce"),
    "XAGUSD": (1.0, "troy ounce"),
    "SUGARUSD": (0.01, "pound"),
    "CORNUSD": (0.01, "bushel"),
    "WHEATUSD": (0.01, "bushel"),
    "SOYBEANUSD": (0.01, "bushel"),
}


def usd_price(symbol: str, value: float) -> tuple[float, str]:
    """Convert a quoted value to US dollars per unit, or refuse."""
    if symbol not in DOCUMENTED:
        raise ValueError(f"{symbol}: no documented unit, refusing to guess")
    factor, unit = DOCUMENTED[symbol]
    return value * factor, unit


for symbol, value in [("SUGARUSD", 18.542), ("XAUUSD", 4286.74), ("COFFEEUSD", 350.0)]:
    try:
        price, unit = usd_price(symbol, value)
        print(f"{symbol:<9} {value:>8} -> {price:.6g} USD per {unit}")
    except ValueError as err:
        print(err)

SUGARUSD    18.542 -> 0.18542 USD per pound
XAUUSD     4286.74 -> 4286.74 USD per troy ounce
COFFEEUSD: no documented unit, refusing to guess

How unit mistakes show up in real systems

• The 9.7% gram errorDividing by the kitchen ounce. The numbers look plausible and are wrong by 9.7%, usually until a customer weighs a coin.
• The hundredfold chartSugar or grains in cents on the same axis as dollar quotes. One line hugs zero or leaves the chart entirely.
• The copper mix-upA per-pound price compared with a per-tonne price: a factor of 2,204.62 that no amount of smoothing hides.
• The fund-share confusionA gold fund share holds a fraction of an ounce, so its price is not the metal price. Compare returns, not levels.

Units also matter over time, not just across symbols. A futures-based price series can step when one contract gives way to the next, which is a different kind of discontinuity, covered in contango and backwardation. For current prices, 15 minutes delayed, the gold symbol page and the sugar symbol page show each reference with its documented unit.