Custom Calculator for website

We need a custom calculator to add to our existing website. This is a very simple calculator. The user will have 2 options. Choose from a dropdown and the correct answer will be displayed or they can enter a dimension and the answer will be calculated. See image example attached. This calculator and app will be used my machine operators to determine what size hole to drill into a part based on their needs. So if a user picks from the dropdown the answer will be displayed according to predefined answers. This should be simple. Each item will have a value to be displayed if selected. If the user enters a number manually into the field, the answer shown needs to be 3% larger than the number entered. Example: if the user enters .2525 the answer would be (.2525 x .03 + .2525) .260