Once you've determined what cut, color, and clarity grade you're looking for in a diamond, it's easy to determine the carat weight of diamond that will fit within your budget.
When diamonds are mined, large gems are discovered much less frequently than small ones, which makes large diamonds much more valuable. In fact, diamond prices rise exponentially with carat weight. So, a 2-carat diamond of a given quality is always worth more than two
1-carat diamonds of the same quality. Since diamonds are measured by carat - weight the measurements of the top of the diamond may be less or more than in this chart depends on how the diamond is cut.
This chart illustrates the average measurements of different carat weights for round and princess cut diamonds. Note that in the setting the diamond will look bigger depends on the measurements of the setting. As you can see a 2-carat diamond does not appear to be twice the size of a 1-carat diamond when viewed from the top.