Method to calculate interpolation step value in Excel

Summary

The following Microsoft Excel formula performs linear interpolation by calculating the interpolation step value

=(end-start)/(ROW(end)-ROW(start))

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.

More information

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

1.       Type the following values in a worksheet:

2.     A1: 9          B1: =(A7-A1)/(ROW(A7)-ROW(A1))

3.     A2: =A1+$B$1

4.     A3:

5.     A4:

6.     A5:

7.     A6:

8.     A7: 11

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

10.   A2: 9.33333

11.   A3: 9.66667

12.   A4: 10.

13.   A5: 10.33333

14.   A6: 10.66667

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