How much gold the words actually promise
In the United States, the plated descriptions are not loose marketing words. The Federal Trade Commission’s Guides for the Jewelry, Precious Metals, and Pewter Industries define them in 16 CFR § 23.3 and § 23.4. “Gold electroplate” means gold of at least 10 karat applied with a minimum thickness throughout equivalent to 0.175 microns of fine gold. “Heavy gold electroplate” raises that to 2.5 microns. “Vermeil” is sterling silver coated or plated with at least 10 karat gold, again at a minimum thickness equivalent to 2.5 microns of fine gold.
The useful detail is the phrase “equivalent to … of fine gold”. The rule does not set the thickness of a 10 karat or 14 karat layer, whose density would depend on the alloy; it sets the thickness the layer would have if it were pure gold. That makes the arithmetic exact. Thickness times area gives a volume of fine gold, and the density of pure gold, 19.32 grams per cubic centimetre, turns that volume into a weight.
The arithmetic, worked through
One micron over one square millimetre is one thousandth of a cubic millimetre, or one millionth of a cubic centimetre. At 19.32 grams per cubic centimetre, a square centimetre of fine gold one micron thick weighs 1.932 milligrams. Gold electroplate, at 0.175 microns, therefore carries at least 0.338 milligrams of fine gold per square centimetre. Heavy gold electroplate and vermeil, at 2.5 microns, carry at least 4.83 milligrams per square centimetre.
A plain ring band is a convenient shape because its surface is pure geometry: the inside wall, the outside wall and the two edges. A band 18 mm across the inside, 4 mm wide and 1.5 mm thick has a surface of about 674 square millimetres. Described as heavy gold electroplate, it must carry at least about 32.5 milligrams of fine gold; as gold electroplate, about 2.3 milligrams. For any other shape, the calculator takes the plated area directly.
Why the result is a floor, not a measurement
The rule sets a minimum, and a minimum throughout: every significant surface must reach it. A plater may lay down more, so the real figure on a given piece can be higher. Wear works the other way. A ring rubbed against a desk for years has less on its high points than it left the factory with. The calculator gives the quantity the description commits to on the day it was sold, which is the one figure that can be stated without measuring the piece.
Measuring the actual thickness is possible, but not at home. X-ray fluorescence instruments can estimate plating thickness without damaging the piece, and a cross-section under a microscope measures it directly at the cost of cutting it. Neither tells you anything the description did not already promise as a minimum, unless the plating turns out to be thinner than the rule allows.
“Gold plated” on its own, “gold tone” and “gold flashed”
Many pieces carry none of the defined terms. A listing that says only “gold plated”, or “gold tone”, or a piece described as gold flashed or washed, commits to no thickness at all. The rule mentions a gold wash only to require disclosure where a base metal layer sits over the primary gold coating; it sets no minimum for the wash itself. “Gold tone” describes a colour, not a material.
For those descriptions the calculator shows “not stated” rather than a number. That is not the same as zero. There may be gold on the surface; the description simply makes no claim about how much, and inventing a figure would be worse than giving none.
What the gold is worth as metal
At live prices the calculator also shows the melt value of that minimum weight, because people ask. The answer is small: a few milligrams of gold. That figure is arithmetic, not a price anyone pays. Recovering a layer measured in microns from a base metal ring costs more than the gold returns, which is why refiners decline plated scrap. Vermeil is the exception worth knowing: its core is sterling silver, which has a melt value of its own.