How do I convert a diamond’s millimetre size to carat weight when checking specs?

ZKZ Editorial TeamMain answer · Product & gemmology desk

Millimetres describe size and carat describes weight, so one does not fix the other. Shape, depth and proportions change the weight of a stone of the same diameter. We did not verify any formula that converts size to weight reliably, so we give none. Specify both measurements and check against the report.

What we verified

Fact GIA says carat measures weight, not size, that a metric carat is defined as 200 milligrams, and that each carat is subdivided into 100 points.

Fact GIA says its laboratory-grown diamond reports are issued for loose stones of 0.15 carats or more, with the girdle laser inscribed with the term Laboratory-Grown and a report number.

How to check an order

1. Specify shape, carat weight range, length and width in millimetres, and depth if it matters.
2. Ask for the report number and check it on the laboratory site.
3. Weigh a sample on a calibrated scale, or have a third party do it.
4. Measure with callipers and compare with the report.
5. Agree in writing what tolerance is acceptable and what happens outside it.

Our advice Do not rely on a size-to-weight chart from the internet for a purchase order. Use the measured values and the report.

It depends Two stones of the same weight can have different face-up sizes, and the same diameter can mean different weights.

Writing the spec clearly

Our advice In the purchase order, list shape, minimum and maximum weight, length, width, colour and clarity range, and the laboratory that will issue the report. Add what happens when a stone falls outside the range: replacement, discount or rejection. A clear spec prevents most disputes about weight, because both sides can check the same numbers on a scale and on the report.

Our advice When a supplier quotes a weight from a size, ask whether the figure was measured or estimated. For a first order, weigh a few stones yourself on a calibrated scale and keep the readings with the order records.

Uncertain We did not verify a conversion formula or typical weight for any given diameter, or accepted weighing tolerances.

Related reading: How should diamond weight be stated: decimal or fraction? · Eternity Band Carat Weight: What You Get at Each Size

Sources

• GIA 4Cs, Nine things about diamond carat weight
• GIA, Laboratory-grown diamond services

Fact = stated by the cited source. Our advice = editorial recommendation. Uncertain / it depends = limits of what can be said in general.