MS Excel VBA - Bilinear interpolation function

Bilinear interpolation (interpolating within a 2-dimensional table) can be done with regular MS Excel functions. But it will become a pretty long formula, that is hard to read and/or copy across. You can also implement a user defined function interpolateXY. The VBA code of this function is found at the end of this page.

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Calculation explanation of above example


Visualisation of the data table above
Calculation
X = 451 lies between X = 400 and X = 500. X-fraction = (451 - 400)/(500 - 400) = 0.51
Y = 75 lies between Y = 60 and Y = 80. Y-fraction = (75 - 60)/(80 - 60) = 0.75

The 4 data point values to interpolate between are (X,Y):
     1. (400,60) = 149
     2. (500,60) = 163
     3. (400,80) = 169
     4. (500,80) = 187

Interpolation X:
     1.&2. → 149 + 0.51 × (163 - 149) = 156.14
     3.&4. → 169 + 0.51 × (187 - 169) = 178.18

Interpolation Y:
     156.14 + 0.75 × (178.18 - 156.14) = 172.67 which is the final result.

VBA code to be put in a module


In MS Excel, press Alt+F11 to open the Visual Basic Editor (Windows). Via top menu: Insert → Module. Copy the code below into the module.