Setting conditional values for a cross reference chart

Good afternoon! First post here...
I have a Conversion chart where a tree diameter equals a specific "Unit" for a tree density survey. I have conducted a survey of ~200 trees, and need whatever diameter I entered to have an alternate column that formulates the units those measurements equal for each cell. So, for example, if the diameter is 12, the next cell should interpret that as 1.6, but if it is 14, the next cell should say 2.2. I need conversions for 12-30" diameters for each cell entered. How would I create this formula for an entire column?
Thank you!

Hi

12 ->1.6 ? or 1.9
14 -> 2.2
16 -> 2.5

If is this then I suppose you want
=A2/(2*PI())

where A2 is the value you want convert. You can copy the formula down to convert A3, A4, ....