Handling decimal item quantities

Question

I am currently working on developing a program that stores Quantity as a decimal with four digit places for the very small minority of products that are sold in part (eg 100ml liquid being sold in variable amounts like 1mg-3.5mg).
I dislike this design as it causes problems for the entire program in deciding how to present the quantity and now some problems where there are products which incorrectly have decimal values.
Can you think of a better way to do this?
I'm looking for an alternative to a four decimal place field in the database.

The project uses SQL Server 2005.

Edit. I made this question more generic in order to obtain a larger potential audience

Answer 1

In .NET, a System.Decimal is going to be your best option as System.Single and System.Double are not good at representing floating point numbers with accurate representational precision.

Can you provide a small, complete example that demonstrates your problem?

Answer 2

Store the quantity in the smallest amount sold. So if something is sold by the mg, store it that way. That way you don't have to store fractions, and deal with rounding.

Just create a looking table for the product so that you can store the value of how it is stored and displayed, as well as any other meta-data about the size.