Velvet Box Society — Gemstone Size Chart (mm → Carat)

How it works: Carats are computed from millimeters using the GIA colored-stone method (Length × Width × Depth × Specific Gravity × Shape-Factor). Defaults reflect typical L:W ratios and depths for each shape; adjust as needed for your cutting.

Shape

L:W ratio (auto per shape)

Depth % of width (auto per shape)

Start mm (length)

End mm (length)

Weight correction (WCF) %

Step (mm)

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  Reset Defaults

Notes: Estimates assume evenly proportioned stones. Use WCF (±%) for unusual profiles (very shallow/steep crowns, heavy pavilions, thick/thin girdles). For hearts, deeper clefts often require a small negative WCF. Cabochons use cab shape-factors and shallower depth defaults. Always treat results as approximate.

SHAPE CARAT TO MM SIZE CONVERSION CHART
​(ALSO CZ WT TO MM SIZE)

This chart is for purposes of estimating the mm size and/or carat weights for the more popular gemstone shapes.  Actual sizes and weights will vary depending on the individual cut and material of each stone.  As a general rule, Cubic Zirconia will weigh approximately 70% more than its Diamond weight material.

(mm.)
CARAT DIA WT	ROUND	OVAL	OCTAGON	PEAR	MARQUISE	HEART	SQUARE	TRILLIANT
0.010	1.25
0.020	1.50
0.025	1.75						1.5 x 1.5
0.040	2.00						1.75 x 1.75
0.050	2.25				3 x 1.5		2 x 2
0.070	2.50				3.5 x 2.5		2.25 x 2.25
0.080	2.75				4 x 2		2.5 x 2.5
0.110	3.00					3 x 3	2.75 x 2.75	3 x 3
0.150	3.25			4 x 3	5 x 2.5		3 x 3
0.180	3.50			5 x 4		3.5 x 3.5	3.25 x 3.25
0.210	3.75						3.5 x 3.5
0.250	4.00	5 x 3		5 x 3	6 x 3	4 x 4	3.75 x 3.75	4 x 4
0.300	4.25		5 x 3				4 x 4
0.360	4.50	5.5 x 3.5		5.5 x 3		4.5 x 4.5	4.25 x 4.25	4.5 x 4.5
0.400	4.75		5.5 x 3.5		7 x 3.5		4.5 x 4.5	5 x 5
0.500	5.00	6 x 4		6 x 4		5 x 5	4.75 x 4.75	5.5 x 5.5
0.550	5.25		6 x 4		8 x 4		5 x 5	6 x 6
0.750	5.75	7 x 5	6.5 x 4.5	7 x 5	9 x 4.5	6 x 6	5.5 x 5.5	6.5 x 6.5
1.000	6.50	7.5 x 5.5	7 x 5	8 x 5	10 x 5	6.5 x 6.5	6 x 6	7 x 7
1.250	6.75	8 x 6	7.5 x 5.5	9 x 6			6.5 x 6.5	7.5 x 7.5
1.500	7.00	8.5 x 6.5	8 x 6	9.6 x 6.5	11 x 5.5	7 x 7	7 x 7	8 x 8
2.000	8.00	9 x 7	8.5 x 6.5	10 x 7	12 x 6	8 x 8	7.5 x 7.5	8.5 x 8.5
2.500	8.50	9.5 x 7.5	9 x 6.5	11 x 7	13 x 6.5	8.5 x 8.5	8 x 8	9 x 9
3.000	9.00	10 x 8	9 x 7	12 x 8	14 x 7	9 x 9	8.5 x 8.5	10 x 10
3.500	9.50	11 x 9	10 x 8	13 x 8	15 x 7.5	9.5 x 9.5	9 x 9	10.5 x 10.5
4.000	10.00	11.5 x 9.5	11 x 9	14 x 9	16 x 8	10 x 10	9.5 x 9.5	11 x 11
5.000	11.00	12 x 10	12 x 10	15 x 10	18 x 9	11 x 11	10 x 10	11.5 x 11.5
6.000	12.00	13 x 10		16 x 10	19 x 10	12 x 12	10.5 x 10.5	12 x 12
7.000	12.50	14 x 10	13 x 10	17 x 10
8.000	13.00	14 x 12	13.5 x 10.5	17 x 12	20 x 10		11 x 11	12.5 x 12.5
9.000	13.50		14 x 10	18 x 11	21 x 10	13 x 13
10.000	14.00	16 x 12		18 x 13	22 x 11	14 x 14		13 x 13
12.000	15.00	18 x 13	14 x 12	19 x 12	24 x 12	15 x 15		14 x 14
15.000	16.00		16 x 12	23 x 14	24 x 14	15.5 x 15.5		15 x 15
18.000	17.00	20 x 15		24 x 15	25 x 13	16 x 16		16 x 16
20.000	18.00		18 x 13	24 x 16	26 x 13	17 x 17		17 x 17
30.000	20.00	22 x 16	20 x 15	25 x 18	25 x 18	20 x 20		21 x 21
40.000	22.00	23 x 19	22 x 16	27 x 18		21 x 21		22 x 22
60.000	23.00	25 X 18	24 x 17	28 x 19		22 x 22		23 x 23