How to calculate gold prices in local currencies

A local gold price has two moving inputs: the USD gold price and the USD-to-local exchange rate. The core formula is local_per_gram = usd_per_troy_ounce / 31.1035 * usd_to_local_rate. Keep the gold and FX observation times separate so a fresh response cannot disguise stale market data.

The conversion formula

Gold is commonly quoted as USD per troy ounce. A local display usually needs a currency amount per gram. Convert the weight first, then apply the USD exchange rate:

const GRAMS_PER_TROY_OZ = 31.1035;

function localGoldPerGram(usdPerTroyOz, usdToLocalRate) {
  return (usdPerTroyOz / GRAMS_PER_TROY_OZ) * usdToLocalRate;
}

The weight constant is fixed. The two market inputs are not: both the gold quote and the FX rate have their own observation time and freshness rules.

Option 1: convert gold in one request

The quickest implementation is goldprice.dev `/v1/convert`. It combines live gold and FX data server-side and does not require an API key. This example requests the value of one gram of gold in Indonesian rupiah:

const params = new URLSearchParams({
  from: "XAU",
  to: "IDR",
  amount: "1",
  unit: "gram",
});

const response = await fetch(
  `https://api.goldprice.dev/v1/convert?${params}`,
);

if (!response.ok) {
  throw new Error(`goldprice.dev returned ${response.status}`);
}

const conversion = await response.json();
console.log(conversion.result, conversion.to, conversion.timestamp);

Choose this path when your interface only needs the final value. Its single timestamp follows the gold spot leg and does not independently evidence FX freshness; use the two-leg path when either input's provenance matters.

Option 2: keep the gold and FX legs separate

Fetch both APIs when you need to retain each source timestamp, cache the inputs independently, or explain which leg is stale. goldprice.dev spot supplies USD gold per troy ounce; exchangerate.dev supplies the USD-to-local rate.

const GRAMS_PER_TROY_OZ = 31.1035;

async function getGoldInCurrencies(currencies) {
  const symbols = currencies.join(",");
  const [spotResponse, fxResponse] = await Promise.all([
    fetch("https://api.goldprice.dev/v1/spot/XAU-USD-SPOT"),
    fetch(`https://api.exchangerate.dev/v1/latest/USD?symbols=${symbols}`),
  ]);

  if (!spotResponse.ok || !fxResponse.ok) {
    throw new Error("A market-data request failed");
  }

  const spot = await spotResponse.json();
  const fx = await fxResponse.json();
  const usdPerGram = Number(spot.price) / GRAMS_PER_TROY_OZ;

  return {
    goldComputedAt: spot.computed_at,
    fxDataUpdatedAt: fx.data_updated_at,
    fxResponseTimestamp: fx.timestamp,
    fxSource: fx.source,
    fxMarketSession: fx.market_session,
    prices: Object.fromEntries(
      currencies.map((currency) => [
        currency,
        usdPerGram * Number(fx.rates[currency]),
      ]),
    ),
  };
}

const result = await getGoldInCurrencies(["EUR", "IDR", "JPY"]);

Handle stale or missing legs explicitly

A combined price should not survive if either input is missing, zero, non-numeric, or outside your product's accepted freshness window. Keep the last valid value only if the interface labels it with its original observation times.

• Check both HTTP responses before doing arithmetic.
• Reject non-finite or non-positive prices and rates.
• Set separate freshness thresholds for gold and FX.
• Treat market_session: weekend as context, not as proof that a carried rate is current.
• Log the currency pair and both underlying observation times with conversion errors.

Round for display, not during calculation

Keep full precision through the multiplication and let the display locale decide the final number of decimal places. Intl.NumberFormat handles currency symbols and grouping without changing the underlying value:

function formatPrice(amount, currency, locale) {
  return new Intl.NumberFormat(locale, {
    style: "currency",
    currency,
  }).format(amount);
}

console.log(formatPrice(result.prices.IDR, "IDR", "id-ID"));
console.log(formatPrice(result.prices.JPY, "JPY", "ja-JP"));

Use the result as an indicative value

A calculated market value is not the retail price of a coin or bar. Dealer premiums, taxes, fabrication, payment fees, and executable FX spreads are outside this formula. Label the output as indicative unless your application adds a real dealer quote and its commercial terms.