Try setting the Data Type of both Fields to DOUBLE. If you have a Table named Table1 consisting of the Fields [Units]{DOUBLE} and [Subtract]{DOUBLE} with the Data

Units Subtract

5.5157 3.651

23.99875 13.88732

15.99901 8.00034

12.6 7.88745

then the following SQL

SELECT Table1.Units, Table1.Subtract, [Units][Subtract] AS Diff

FROM Table1;

will yield

Units Subtract Diff

5.5157 3.651 1.8647

23.99875 13.88732 10.11143

15.99901 8.00034 7.99867

12.6 7.88745 4.71255
