XL: Method to Calculate Interpolation Step Value

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This article was previously published under Q95479

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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:

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

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).