Read the weight as a pair
Stud sizes are given in carat total weight (ct tw), the weight of both earrings together. A 1 ct tw pair is two diamonds of about half a carat each, so halve the number to get the weight in each ear.
Carat is weight, not size. On the ear what you see is the diameter of the stone, which grows more slowly than the weight: doubling the carats adds about a quarter to the width.
Round stud size chart
| Pair (ct tw) | Each stone | Approx. diameter | Shop this size |
|---|---|---|---|
| 1/2 ct tw | 0.25 ct | about 4.1 mm | Ask the concierge |
| 5/8 ct tw | 0.315 ct | about 4.4 mm | |
| 3/4 ct tw | 0.375 ct | about 4.7 mm | |
| 1 ct tw | 0.5 ct | about 5.1 mm | |
| 1 1/2 ct tw | 0.75 ct | about 5.9 mm | |
| 2 ct tw | 1 ct | about 6.5 mm | |
| 2 1/2 ct tw | 1.25 ct | about 7.0 mm | |
| 3 ct tw | 1.5 ct | about 7.4 mm | |
| 4 ct tw | 2 ct | about 8.1 mm |
How these sizes are worked out
Jewelers estimate the weight of a round brilliant from its measurements with a standard formula: diameter squared, times depth, times 0.0061 (all in millimeters). We solved it for the diameter, with the depth at 61% of the diameter, a typical proportion for a well-cut round.
That makes every figure above an estimate, rounded to a tenth of a millimeter. Two diamonds of the same weight can differ by a few tenths depending on their cut, and fancy shapes (oval, pear, emerald) spread the same weight differently, so this chart is for rounds only.
Questions
- What does ct tw mean on stud earrings?
- Carat total weight: the weight of both diamonds in the pair together. A 1 ct tw pair is two diamonds of about 1/2 carat each.
- How big is a 1 carat diamond stud?
- A 1 ct tw pair has two round diamonds of about 1/2 carat each, roughly 5.1 mm across. A pair with a full carat in each ear is 2 ct tw, about 6.5 mm per stone.
- Are the millimeter sizes exact?
- No. They are estimates from the standard round-brilliant weight formula. Two diamonds of the same weight can differ by a few tenths of a millimeter depending on how they are cut.