How big is a 3 carat heart shaped diamond?

By HeyGema™ · Updated November 5, 2026

In short

A 3 carat heart shaped diamond measures about 9.4 x 9.4 mm. It faces up 17% smaller than a round diamond of the same weight. On a size 6 finger it covers 57% of the width. For the same weight, a round diamond is 9.3 mm.

9.4 x 9.4 mm3 carat heart
0.37 x 0.37 inthe same in inches
-17%face-up size vs a round
57%of a size 6 finger's width

Carat to mm calculator

ShapeGemCarat

About 9.4 x 9.4 mm

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

3 ct heart

9.4 x 9.4 mm

3 ct round

9.3 mm

Side by side

3 ct round inside 3 ct heart

A 3 carat heart diamond beside a 3 carat round, to the same scale.

3 carat heart diamond size

A well-cut 3 carat heart shaped diamond measures about 9.4 x 9.4 mm (0.37 x 0.37 inches) at a length-to-width ratio of 1.00:1. It faces up 17% smaller than a round diamond of the same weight. It weighs 0.60 grams.

3 carat heart on a finger

9.4 mm stone on a 16.5 mm finger: it covers 57% of the widthA 3 carat heart diamond on a size 6 finger.

4: Finger width: 14.9 mm; Heart covers: 63%

5: Finger width: 15.7 mm; Heart covers: 60%

6: Finger width: 16.5 mm; Heart covers: 57%

7: Finger width: 17.3 mm; Heart covers: 54%

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

9: Finger width: 18.9 mm; Heart covers: 49%

US ring size	Finger width	Heart covers
4	14.9 mm	63%
5	15.7 mm	60%
6	16.5 mm	57%
7	17.3 mm	54%
8	18.1 mm	52%
9	18.9 mm	49%

Heart sizes near 3 carat

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

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

3 (this page): Length x width: 9.4 x 9.4 mm; Round of the same weight: 9.3 mm

3.5 carat: Length x width: 9.9 x 9.9 mm; Round of the same weight: 9.8 mm

4 carat: Length x width: 10.3 x 10.3 mm; Round of the same weight: 10.2 mm

Carat	Length x width	Round of the same weight
2.25 carat	8.5 x 8.5 mm	8.5 mm
2.5 carat	8.8 x 8.8 mm	8.8 mm
3 (this page)	9.4 x 9.4 mm	9.3 mm
3.5 carat	9.9 x 9.9 mm	9.8 mm
4 carat	10.3 x 10.3 mm	10.2 mm

3 carat in other shapes

Every row is a well-cut diamond of 3 carat. The last column is the face-up size against the heart.

Round: Length x width: 9.3 mm; Against heart: +21%

Oval: Length x width: 11.5 x 7.9 mm; Against heart: +28%

Cushion: Length x width: 8.1 x 8.1 mm; Against heart: +4%

Princess: Length x width: 7.9 x 7.9 mm; Against heart: +11%

Emerald cut: Length x width: 10.1 x 7.2 mm; Against heart: +25%

Asscher: Length x width: 7.9 x 7.9 mm; Against heart: +2%

Radiant: Length x width: 9.4 x 7.2 mm; Against heart: +18%

Pear: Length x width: 12.3 x 7.9 mm; Against heart: +17%

Marquise: Length x width: 15.0 x 7.5 mm; Against heart: +40%

Trillion: Length x width: 10.1 x 10.1 mm; Against heart: +4%

Baguette: Length x width: 11.5 x 5.8 mm; Against heart: +18%

Shape	Length x width	Against heart
Round	9.3 mm	+21%
Oval	11.5 x 7.9 mm	+28%
Cushion	8.1 x 8.1 mm	+4%
Princess	7.9 x 7.9 mm	+11%
Emerald cut	10.1 x 7.2 mm	+25%
Asscher	7.9 x 7.9 mm	+2%
Radiant	9.4 x 7.2 mm	+18%
Pear	12.3 x 7.9 mm	+17%
Marquise	15.0 x 7.5 mm	+40%
Trillion	10.1 x 10.1 mm	+4%
Baguette	11.5 x 5.8 mm	+18%

Questions

A 3 carat heart shaped diamond is about 9.4 x 9.4 mm at the usual 1.00:1 ratio.

No. Face-up, a 3 carat heart is 17% smaller than a 3 carat round (9.3 mm).

There is no rule. The stone is 63% 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.