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Diamond Weight Estimate for Round Brillinat Cut

The most common formula is that for round brilliant estimation. This one should be memorized since it is used the most often.

Round Brilliant Formula: (Narrowest Diameter + Widest Diameter) divided by 2 squared x Depth x .0061 x girdle thickness adjustment * (GTA)

*Refer to previous page for GTA multipliers.

A round brilliant will almost always be slightly out of round; to obtain average diameter the narrowest and the widest measurements must be located. It’s a good idea to write them down to avoid errors. It is possible to get more than one depth measurement if the table is not parallel with the girdle but for estimation purposes one measurement will do.

1) Example of formula in use:

Narrowest measurement: 6.05 mm
Widest measurement: 6.11 mm
Depth measurement: 3.63 mm
Girdle: *Medium

6.05 + 6.11= 12.16

12.16 divided by 2= 6.08 (average diameter)

6.08 x 6.08 x 3.63 x .0061= .8185/ Rounded up to .82 carat.

2) Example of formula in use:

Narrowest measurement: 7.02 mm
Widest measurement: 7.17 mm
Depth measurement: 4.47 mm
Girdle: *Medium

*Since the girdle is medium the GTA is not needed in the formula. (GTA = 1)

7.02 + 7.17= 14.19

14.19 divided by 2= 7.095/ Round up to 7.10 mm (average diameter)

7.10 x 7.10 x 4.47 x .0061= 1.374 Rounded down to 1.37 carat.

Narrowest measurement: 6.05 mm
Widest measurement: 6.11 mm
Depth measurement: 3.63 mm
Girdle Thickness Slightly Thick

6.05 + 6.11= 12.16

12.16 divided by 2= 6.08 (average diameter)

6.08 x 6.08 x 3.63 x .0061 x *1.02 (GTA)= .8349 Rounded down to .83 carat.

*Referring back to the GTA table you can see that a Slightly Thick girdle requires a multiplier of 1.02 for the slight additional weight of such a girdle.

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