How the numbers work

Weight

Precious metals are priced by the troy ounce, which is exactly 31.1034768 grams. This is not the ounce on a kitchen scale — the avoirdupois ounce is 28.349523125 g, about 9% lighter. Using the wrong one overstates a valuation by roughly a tenth, and it is the most common unit error in the subject.

A pennyweight is one twentieth of a troy ounce, so 1.555174 g. Many US buyers still weigh in pennyweight, which is why “we pay X per unit” is meaningless until you know which unit. A troy grain is one four-hundred-and-eightieth of a troy ounce.

Purity

Karat is a fraction of 24: 18K is eighteen parts gold in twenty-four, so 750 parts per thousand. Millesimal fineness is the same fraction written as parts per thousand, and it is what most modern jewellery is stamped with.

Where the legal standard and the plain division disagree, this site follows the stamp. 14K is 583.3 by division and is stamped 583 in the US or 585 in Europe; 22K is 916.6 and is stamped 916 or 917. The difference is a fraction of a percent, and the stamped figure is the one on the metal in your hand.

A stamp asserts purity, it does not measure it. Where that distinction matters the pages say so rather than rounding it away.

Melt value

The whole calculation, in one line:

melt = (gross weight − non-metal weight)
       × (fineness ÷ 1000)
       × (spot price per troy ounce ÷ 31.1034768)

Non-metal weight is subtracted before purity is applied, because a stone is not low-purity gold — it is not gold at all. Getting that order wrong inflates the answer.

Payout ranges

The percentages shown beside melt value are ranges published by the trade, presented as ranges and never collapsed into a single figure. They vary with local competition, quantity and purity. They are there so you can place an offer against the market, not to predict what any particular buyer will say.

Ring sizes

The only unambiguous ring measurement is the inner diameter in millimetres. Everything else is a convention laid over it:

• US: size 0 is an inner diameter of 0.458 inch, and each whole size adds 0.032 inch — so diameter in mm is 11.6332 + 0.8128 × size.
• Europe (ISO 8653): the size is the inner circumference in millimetres. Nothing is converted.
• Japan (JIS S 4700): size 1 is 13.0 mm inner diameter and each size adds a third of a millimetre.
• UK and Australia: letters, anchored at the conversion every mainstream chart uses — US 5 is UK J½ — with one full letter per half US size.

Charts from different makers genuinely disagree by up to half a size, because they round these conventions differently. Where a ring is being cut rather than tried on, give the workshop the millimetres.

Diamond and gemstone weight

The metric carat has been exactly 0.2 g since 1907, and divides into 100 points.

Weight is estimated from measurements using the standard trade factors: length × width × depth × a factor for the outline — 0.0061 for a round brilliant, 0.0062 oval, 0.0083 princess, 0.00815 cushion, 0.0080 emerald, 0.00565 marquise, and so on. Full trade practice adds an allowance for girdle thickness and a correction for stones cut off-ratio; this site does not, because both need the stone in your hand. The estimate is good to a few percent for a normally cut stone, and the pages say that rather than implying a scale reading.

Going the other way — from carats to millimetres — additionally assumes normal proportions, because weight alone cannot describe an outline. Two one-carat rounds can differ by several tenths of a millimetre face-up depending on how deep they are cut.

For coloured stones, weight is scaled by specific gravity relative to diamond’s 3.52. This is why a one-carat ruby is visibly smaller than a one-carat diamond, and why coloured stones are sensibly bought by millimetre.

Prices

Spot prices for gold, silver, platinum and palladium are fetched live in your browser each time a page loads. Currency conversion uses European Central Bank daily reference rates. Both requests go directly from your browser to those services; nothing you type into a calculator is sent anywhere at all.

All four metals are fetched in one go, so two figures on the same page can never be minutes apart. If the feed fails, the tools show nothing rather than something out of date, and say the feed is down.

What is deliberately absent

Diamond and gemstone prices. Stone pricing depends on wholesale sheets that are licensed rather than published, on a graded report describing that individual stone, and on a market that moves. A number invented here would look authoritative and be worth nothing. Size, weight and proportion are physics, so those are what this site computes.

The arithmetic is covered by an automated test suite of 42 cases, including the unit conversions, the purity standards, the melt formula, the ring-size definitions and the weight-estimation factors. Jewelers Gallery publishes the method so it can be checked.