BijanDiamonds.com Dictionary
Ratio:
The length-to-width ratio is a comparison of the length and the width of a fancy shape diamond (princess, asscher, marquise, emerald, radiant, cushion, oval, pear, heart). This ratio expresses how relatively long or wide a diamond appears when viewed from the top. The length to width ratio certain diamond shape is important to its value and beauty, however, personal preference prevails, and some may prefer a shorter, wider outline; or longer, thinner shape.

Popular diamond quality reports from GIA and other gemological laboratories do not provide length to width ratios. However, they do provide length, width, and depth measurements. Length to width ratios are determined by dividing the length measurement by the width measurement.

Example: Princess cut Diamond
Measurement: 6.25 - 6.05 x 3.77 mm
Measurement: Length - Width x Depth
Length-to-width ratio: Length / Width
          = 6.25 / 6.05
          = 1.03 (This will look perfectly square to the eye)


Different Diamond Shape and Its Preferred Length to Width Ratio

Princess Cut
Ideal Shape is: Length to Width ratio 1:1
Acceptable are: Length to Width ratio 1-1.4:1


Oval Cut
Ideal Shape is: Length to Width ratio 1.5:1
Acceptable are: Length to Width ratio 1.5-1.75:1


Heart Cut
Ideal Shape is: Length to Width ratio 0.9-1.2:1
Or which is most pleasing to the eye


Marquise Cut
Ideal Shape is: Length to Width ratio 2:1
Acceptable are: Length to Width ratio 1.75-2.25:1


Emerald Cut
Ideal Shape is: Length to Width ratio 1.35-1.65:1
Acceptable are: Length to Width ratio 1.25-1.75:1


Trillion Cut
Ideal Shape is: Length to Width ratio 1:1
Acceptable are: Length to Width ratio 1:1


Pear Cut
Ideal Shape is: Length to Width ratio 1.5:1
Acceptable are: Length to Width ratio 1.5-1.7:1


Radiant Cut
Ideal Shape is: For Square Length to Width ratio 1:1
Acceptable are: For Rectangle Length to Width ratio 1.5:1


Asscher Cut
Ideal Shape is: Length to Width ratio 1:1
Acceptable are: Length to Width ratio 1-1.4:1