How big is a 2.5 carat trillion cut diamond?

By HeyGema™ · Updated November 5, 2026

In short

A 2.5 carat trillion cut diamond measures about 9.5 x 9.5 mm. It faces up 14% smaller than a round diamond of the same weight. On a size 6 finger it covers 58% of the width. For the same weight, a round diamond is 8.8 mm.

9.5 x 9.5 mm2.5 carat trillion
0.37 x 0.37 inthe same in inches
-14%face-up size vs a round
58%of a size 6 finger's width

Carat to mm calculator

ShapeGemCarat

About 9.5 x 9.5 mm

An estimate for a well-cut stone. A deep or shallow cut faces up smaller or bigger.

2.5 ct trillion

9.5 x 9.5 mm

2.5 ct round

8.8 mm

Side by side

2.5 ct round inside 2.5 ct trillion

A 2.5 carat trillion diamond beside a 2.5 carat round, to the same scale.

2.5 carat trillion diamond size

A well-cut 2.5 carat trillion cut diamond measures about 9.5 x 9.5 mm (0.37 x 0.37 inches) at a length-to-width ratio of 1.00:1. It faces up 14% smaller than a round diamond of the same weight. It weighs 0.50 grams.

2.5 carat trillion on a finger

9.5 mm stone on a 16.5 mm finger: it covers 58% of the widthA 2.5 carat trillion diamond on a size 6 finger.

4: Finger width: 14.9 mm; Trillion covers: 64%

5: Finger width: 15.7 mm; Trillion covers: 61%

6: Finger width: 16.5 mm; Trillion covers: 58%

7: Finger width: 17.3 mm; Trillion covers: 55%

8: Finger width: 18.1 mm; Trillion covers: 52%

9: Finger width: 18.9 mm; Trillion covers: 50%

US ring size	Finger width	Trillion covers
4	14.9 mm	64%
5	15.7 mm	61%
6	16.5 mm	58%
7	17.3 mm	55%
8	18.1 mm	52%
9	18.9 mm	50%

Trillion sizes near 2.5 carat

2 carat: Length x width: 8.8 x 8.8 mm; Round of the same weight: 8.1 mm

2.25 carat: Length x width: 9.2 x 9.2 mm; Round of the same weight: 8.5 mm

2.5 (this page): Length x width: 9.5 x 9.5 mm; Round of the same weight: 8.8 mm

3 carat: Length x width: 10.1 x 10.1 mm; Round of the same weight: 9.3 mm

3.5 carat: Length x width: 10.6 x 10.6 mm; Round of the same weight: 9.8 mm

Carat	Length x width	Round of the same weight
2 carat	8.8 x 8.8 mm	8.1 mm
2.25 carat	9.2 x 9.2 mm	8.5 mm
2.5 (this page)	9.5 x 9.5 mm	8.8 mm
3 carat	10.1 x 10.1 mm	9.3 mm
3.5 carat	10.6 x 10.6 mm	9.8 mm

2.5 carat in other shapes

Every row is a well-cut diamond of 2.5 carat. The last column is the face-up size against the trillion.

Round: Length x width: 8.8 mm; Against trillion: +16%

Oval: Length x width: 10.9 x 7.5 mm; Against trillion: +22%

Cushion: Length x width: 7.6 x 7.6 mm; Against trillion: -1%

Princess: Length x width: 7.5 x 7.5 mm; Against trillion: +6%

Emerald cut: Length x width: 9.5 x 6.8 mm; Against trillion: +20%

Asscher: Length x width: 7.5 x 7.5 mm; Against trillion: -2%

Radiant: Length x width: 8.8 x 6.8 mm; Against trillion: +13%

Pear: Length x width: 11.5 x 7.5 mm; Against trillion: +12%

Marquise: Length x width: 14.1 x 7.1 mm; Against trillion: +34%

Heart: Length x width: 8.8 x 8.8 mm; Against trillion: -4%

Baguette: Length x width: 10.9 x 5.4 mm; Against trillion: +13%

Shape	Length x width	Against trillion
Round	8.8 mm	+16%
Oval	10.9 x 7.5 mm	+22%
Cushion	7.6 x 7.6 mm	-1%
Princess	7.5 x 7.5 mm	+6%
Emerald cut	9.5 x 6.8 mm	+20%
Asscher	7.5 x 7.5 mm	-2%
Radiant	8.8 x 6.8 mm	+13%
Pear	11.5 x 7.5 mm	+12%
Marquise	14.1 x 7.1 mm	+34%
Heart	8.8 x 8.8 mm	-4%
Baguette	10.9 x 5.4 mm	+13%

Questions

A 2.5 carat trillion cut diamond is about 9.5 x 9.5 mm at the usual 1.00:1 ratio.

No. Face-up, a 2.5 carat trillion is 14% smaller than a 2.5 carat round (8.8 mm).

There is no rule. The stone is 64% of the width of a size 4 finger and 52% of a size 8. Compare it with a ring you have tried on in the finger chart above.

Yes. A lab grown diamond has the same density as a mined one, so it measures the same for the same weight.