Relation between length and font-size of a string and width of a textbox-Collection of common programming errors

I’ve a field in a database, this field has a maximum length, and I want to set a textbox to the appropriate width. I’m creating the textboxes in runtime. How can I calculate the value of width property?

For example, if I have a field nvarchar(5) named IDClient, and the font-Size is 13, I want to create a texbox with a width enough to write 5 chars.