This calculation is for a Britannia specified as containing one troy ounce of fine gold. The fine-ounce denomination already describes pure-metal content, so fineness is not multiplied again. The pound value uses daily reference FX. A Royal Mint or dealer invoice can differ because of a product premium, delivery and other stated charges. Verify the specific coin and year before applying this basis.
Fine-metal formula
Quantity × 31.1034768 g × 1 × fine-gram reference
Fine-gram reference = USD/troy ounce ÷ 31.1034768 × daily reference FX. Fineness is never applied twice.
Fine ounce is the calculation input
One fine troy ounce is exactly 31.1034768 grams of gold for the reference conversion. Treat it differently from an alloy’s gross weight, which needs a fineness adjustment.
Coin variants and retail invoices
Check the documentation for fractional denominations and older products. This page values the one-fine-ounce specification only. It does not assert that every Britannia variant has identical alloy weight or retail terms.