A 1 carat diamond typically measures about 6.4 to 6.5 millimeters in diameter if it is a round brilliant cut, which is the most common shape. This size is roughly equivalent to the diameter of a standard pencil eraser or about 1/4 of an inch across

. More specifically:

• A well-proportioned 1 carat round diamond usually has a diameter around 6.5 mm with a depth of about 3.9 mm

• The face-up area (the visible surface from the top) of a 1 carat round diamond is approximately 33 mm²

• Different diamond shapes with the same 1 carat weight can have varying dimensions. For example, a princess cut 1 carat diamond might be about 5.5 mm by 5.5 mm, while an oval 1 carat diamond can be around 7.5 mm by 5.5 mm

It is important to note that carat is a measure of weight (1 carat = 0.2 grams), not size, so two diamonds of the same carat weight can look different in size depending on their cut, shape, and proportions

. In summary, a 1 carat diamond is about 6.4 to 6.5 mm in diameter if round, roughly the size of a pencil eraser, but the exact dimensions can vary by shape and cut quality.