XL: Method to Calculate Interpolation Step Value

The following Microsoft Excel formula performs linear interpolation by calculating the interpolation step value: where "end" is the cell address of the larger number, and "start" is the cell address of the smaller number.

Interpolation is a method used to determine a present or future value factor when the exact factor does not appear in either a present or future value table. Interpolation assumes that the change between two values is linear and that the margin of error is insignificant.

To create a sample linear interpolation formula, follow these steps:

1. Enter the following values in a worksheet:

         A1: 9          B1: =(A7-A1)/(ROW(A7)-ROW(A1))
         A2: =A1+$B$1
         A3:
         A4:
         A5:
         A6:
         A7: 11

2. Select cells A2:A6. On the Edit menu, click Fill Down. The formula is filled down, and the following values are displayed in cells A2:A6:

         A2: 9.33333
         A3: 9.66667
         A4: 10.
         A5: 10.33333
         A6: 10.66667

NOTE: You must enter the reference to the step value in cell B1 as an absolute reference (with dollar signs).