Close to it — and the law says how close
A US gold stamp is a claim, and the National Gold and Silver Marking Act, 15 U.S.C. § 295, says how far below that claim the metal may fall. It sets two tests that must both be met. First, the gold or alloy itself, assayed with any solder left out, may fall short of the marked fineness by no more than three parts per thousand. Second, the entire quantity of gold and alloy in the article, solder included, may fall short by no more than three parts per thousand for a watchcase or flatware, and seven parts per thousand for anything else.
Applied to 14 karat jewellery, that means a stamp claiming 583.3 parts per thousand gold is lawful if the alloy tests at 580.3 or above, and the whole article including its solder joints at 576.3 or above. Below either figure, the stamp overstates the gold.
Why 14K is 583.3, not 585
A karat is one twenty-fourth part, so 14 karat is exactly 14/24 gold: 583.33 parts per thousand. The figure 585 is a millesimal standard used in many hallmarking systems, set slightly above 14/24. They are close but not the same claim. The calculator converts a karat stamp with the exact definition, so a 14K stamp is held to 583.3 and a 585 stamp to 585.
The same applies throughout: 10K is 416.7, 18K is 750 exactly, and 22K is 916.7. A stamp with letters after the karat, such as KP, is read at the karat it states.
Why solder gets its own allowance
Most gold jewellery is assembled, and the joints are made with solder that melts at a lower temperature than the parts it joins. Solder of a lower fineness than the parts is a long-standing reason an assembled article can test below the alloy it was made from. The Act deals with this by testing twice: once on the gold without solder, held to the tighter three parts per thousand, and once across the whole article, where the wider allowance for ordinary articles accommodates the joints.
Watchcases and flatware are held to three parts per thousand in both tests. The table on this page shows both columns so the difference is visible at a glance.
What a tolerance cannot tell you
These figures are the lowest a stamp may lawfully mean. They do not show that any particular piece meets them; only an assay measures that. Acid tests and electronic testers give a band rather than a fineness to the part per thousand, and X-ray fluorescence reads the surface, which on plated or worn pieces may not represent the whole. A piece can also carry a stamp that has nothing to do with what it is made of.
The Act applies to articles marked for sale in the United States. Other systems work differently: a UK hallmark is struck under the Hallmarking Act 1973 by an assay office that has tested the metal before marking it, and the Hallmarking Convention’s Common Control Mark is applied only after testing to the Convention’s methods. The tolerance arithmetic on this page is for US stamps only.
Using the weight box
If you enter the weight of the gold parts, the calculator multiplies it by the lowest lawful whole-article fineness to give the least fine gold the stamp allows. It is a floor, not an estimate of what is there, and it excludes stones and non-gold components, which should be left out of the weight. Melt value at the live price is a separate step on the scrap gold calculator, and what anyone would pay for the piece is a separate question again.