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# XL: Method to Calculate Interpolation Step Value

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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.

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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).

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## Applies to:

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Keywords: KB95479, kbhowto

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Article Info
 Article ID : 95479 Revision : 3 Created on : 9/19/2011 Published on : 9/19/2011 Exists online : False Views : 416