How to Do Linear Interpolation Excel VBA (with Easy Steps)

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Sometimes, you may deal with a dataset that follows a linear regression. You want to know an unknown value from a known value of that dataset. In this case, you need to do a linear interpolation. It helps you to find out the unknown data point. There are several functions to perform the interpolation. But whenever you need to create a custom function, you need the help of VBA Macros. It can create a function that will do a linear interpolation. In this article, we’re going to show you some easy steps to do linear interpolation with VBA in Excel. So, let’s get started.

Here’s the final output that you will get by the end of the article.

Overview Image

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What Is Linear Interpolation in Excel?

Interpolation is a method of finding a value from known values. In linear interpolation, you might get a new data point from two known data points.

Suppose you have a point W(a,b) and V(c,d). Now, for an unknown point of U(x,y), you want to know the y value for the known value of x. There is a very common mathematical equation for this scenario. The equation is:

y=c + ((d - c) * (x - a) / (b - a))

From the above equation, you can find the unknown y value.

4 Easy Steps to Calculate Linear Interpolation with VBA in Excel

There are several functions from which you can do linear interpolation in Excel. Also, you can create a function with the help of VBA from where you will be able to find the unknown value.

Here, we have used the Microsoft 365 version. You may use any other version at your convenience.

Step 01: Create a Dataset

  • In the beginning, you need to create a dataset. In our case, we have taken a dataset of Solubility in Different Temperatures. We all know that solubility changes with respect to temperature.

Sample dataset

Now, we want to know the solubility of a fixed temperature which is not provided in the dataset.

Read More: How to Interpolate Between Two Values in Excel (6 Ways)

Step 02: Open Visual Basic Editor

  • We need to create a function that will do our job for this interpolation. For creating a User Defined Function in Excel, firstly, navigate to the Developer tab and move to Visual Basic.

Opening Developer tab for linear interpolation Excel VBA

  • Apparently, Visual Basic Editor opens. Choose Insert tab >> Module >> Module1 from there.

Visual Basic Editor for linear interpolation Excel VBA

Note: you can also open the Visual Basic Editor with ALT + F11 key.

Read More: How to Do VLOOKUP and Interpolate in Excel (6 Ways)

Similar Readings

Step 03: Insert Function with VBA Macros

  • In Module 1, write up the following code.
Function Lin(x, w, v)
a = Application.WorksheetFunction.Index(w, Application.WorksheetFunction.Match(x, w, 1))
b = Application.WorksheetFunction.Index(w, Application.WorksheetFunction.Match(x, w, 1) + 1)
c = Application.WorksheetFunction.Index(v, Application.WorksheetFunction.Match(x, w, 1))
d = Application.WorksheetFunction.Index(v, Application.WorksheetFunction.Match(x, w, 1) + 1)
Lin = c + ((d - c) * (x - a) / (b - a))
End Function

VBA code for creating function for linear interpolation Excel VBA

Code Explanation:

We have declared a function named Lin here. We called out the WorksheetFunction Index and Match. The VBA Match function finds the relative position of the largest value of w and v which is the range of data you have in your dataset. The VBA Index function will return the largest value obtained by the Match function. Then, we used the same formula for the y value of the point w, but here, we insert +1 for the value after the largest value. Similarly, we set c and d, which are the coordinates of point v. We wrote up the interpolation equation and named it the Lin function.

Read More: How to Perform Bilinear Interpolation in Excel (with Easy Steps)

Step 04: Use the Function for Interpolation

  • Eventually, run the code with the F5 key and it will create the function.
  • Sequentially, move to cell C13 and insert the below formula.

Here, C12 is the known x value. B5:B10 is the range of the w value, and C5:C10 is the range of the v value.

The custom Lin function will do linear interpolation from the given data.

  • Eventually, press ENTER.

You will get the following output, like the image below.

Using Custom Function for linear interpolation Excel VBA

Moreover, if you change the known value of the Temperature, the Solubility changes accordingly. See the below image for a better understanding.

Changing Y value with the Change of X

Read More: How to Do 2D Interpolation in Excel (2 Suitable Ways)

Practice Section

We have provided a practice section on each sheet on the right side for your practice. Please do it by yourself.

Practice Section


That’s all about today’s session. And these are some easy steps for linear interpolation with VBA in Excel. Please let us know in the comments section if you have any questions or suggestions. For a better understanding please download the practice sheet. Visit our website ExcelDemy, a one-stop Excel solution provider, to find out about diverse kinds of excel methods. Thanks for your patience in reading this article.

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Fahim Shahriyar Dipto

Fahim Shahriyar Dipto

Hello! Welcome to my Excel blog! I am a big fan of MS Excel. I am learning new and exciting things in Excel and writing the process here. I think this will be helpful for you to get used to Excel. Keep visiting our website for new and updated Excel methods.

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