Unit wise calculation

HelloI am working on a billing system in wpf for a kirana shop.

If I have unit in piece (pc) then amount will be calculated easily, like amount = qty*unitRate.

 

For eg following products have opening balance:

1. Product having a tootpaste (in pc)

2. Product having oil (In litre)

3. Product having flour (in kg)

 

My requirement is, how to make calculation in such a way that all the amount of above products? shall be calculated? at the time of sale.

 

Kindly suggest the best possible way, do I have to write formula for unit conversion for each volumne or something else would be required.

Please do suggest something to overcome this.  

 

What type of database structure do I have to prepare for this? 

 

Thanks Devendra