Excel equations subject to data

I have run into an Excel brick wall and need some inspiration. I have a spreadsheet where pressures could be added in a variety of units and i want to calculate the outputs in mBar. I have tried nesting IF statements such as:

=IF(ISBLANK(K22),"",SUM(K22*1013.25)),IF(ISBLANK(M22),"",SUM(M22*33.8639)),IF(ISBLANK(O22),"",SUM(O22*1.33322))

but this returns an error.

Ideally what i want is the user to be able to enter data in any cell (relative to the measurement) such as ATM in K22 and output mBar in Q22?

The other way i suppose i could do it is leave the input cell blank and just calculate a range of outputs but that isnt as neat and i open to user error, such as Bar or mBar, inHg or mmHg etc..