Method to calculate interpolation step value in Excel


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.

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

  1. Type the following values in a worksheet:

    A1: 9        B1: =(A7-A1)/(ROW(A7)-ROW(A1))
    A2: =A1+$B$1
    A7: 11
  2. 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:

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


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