1 to 5 volts = 20.8 to 32 in. Hg, need formula.

There is no correlation between voltage and pressure, so I am assuming that you are asking how to convert the voltage output of a barometric pressure transducer to pressure in inches of mercury? That information should be available on the product spec available from the transducer manufacturer. Assuming that the tarnsducer you are interested in provides an output voltage of 0-5V based on pressure in the range 20.8 to 32 inches Hg, then assumning linear performance (which is typical) then pressure in inches of mercury is related to voltage output of the transducer by:

P = 20.8 +2.24 V