3 and a half to 4 cm in width
There is no scientific name for the width of the index finger. The name index finger literally means pointing finger.
foot
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About the width of a finger.
The width of an index finger depends on the size of the person. For instance, an average size person will have an index finger that is roughly one half inch wide.
The unit of length that most nearly matches the width of your finger is the inch.
4cm
Technically its width but if you are talking to someone they should know what you mean by thickness.
For that, use length.
centimeter
about 3/4 of an inch or 2 cm
Half the width of your finger.