How to Use a Linear Interpolation Calculator in Excel: Step-by-Step Tutorial with Examples

Posted on by Gemma

Linear interpolation is a powerful mathematical technique used to estimate unknown values that fall between known data points. If you’re working with data in Excel, understanding linear interpolation calculator excel how to build and use a linear interpolation calculator can save you time and increase your analytical accuracy. This tutorial walks you through everything you need to know — with clear steps, visuals, and real-world examples.

🔍 What is Linear Interpolation?

In its simplest form, linear interpolation assumes that the change between two known points is linear (i.e., follows a straight line). It is used to estimate an unknown value y at a known point x, between two other data points (x₁, y₁) and (x₂, y₂).

The linear interpolation formula is:

y=y1+(x−x1)(x2−x1)×(y2−y1)y = y₁ + \frac{(x – x₁)}{(x₂ – x₁)} \times (y₂ – y₁)

This can be applied directly in Excel with just a few formulas.

🧰 What You’ll Need

  • Microsoft Excel (any modern version)

  • Two known data points

  • A target x-value where you want to estimate y

🧪 Example Scenario: Temperature Estimation

Imagine you’re monitoring a machine, and you only have readings at 2 PM and 4 PM. You need to estimate the temperature at 3 PM.

Data:

Time (Hour) Temperature (°C)
2 50
4 70

Goal: Estimate temperature at 3 PM (i.e., x = 3).

🧮 Step-by-Step: Build the Linear Interpolation Calculator in Excel

✅ Step 1: Set Up Your Spreadsheet

Create a table like this:

A B
x₁ (Time 1) 2
y₁ (Temp 1) 50
x₂ (Time 2) 4
y₂ (Temp 2) 70
x (Target Time) 3
y (Interpolated) (leave blank)

✅ Step 2: Insert the Interpolation Formula

In the cell where you want the interpolated value (say, B6), enter:

excel
= B2 + ((B5 - B1) / (B3 - B1)) * (B4 - B2)

✅ Explanation:

  • B1 = x₁ (2)

  • B2 = y₁ (50)

  • B3 = x₂ (4)

  • B4 = y₂ (70)

  • B5 = x (3)

This Excel formula is a direct translation of the interpolation formula.

✅ Step 3: Press Enter

The result in B6 will be:

bash
= 50 + ((3 - 2) / (4 - 2)) * (70 - 50)
= 50 + (1/2) * 20
= 50 + 10 = 60

So, the estimated temperature at 3 PM is 60°C.

🔁 Optional: Make a Reusable Interpolation Calculator

Want to use this frequently? Build a dynamic calculator:

1. Label Columns:

A B C
x₁ y₁
x₂ y₂
x (target) Interpolated y

2. Add Formula in C3:

excel
= B1 + ((A3 - A1)/(A2 - A1)) * (B2 - B1)

Now, whenever you change x₁, x₂, y₁, y₂, or your target x, the interpolated value will update automatically.

💡 Tips for Using Linear Interpolation in Excel

  • Data must be linear: This method assumes straight-line change. For nonlinear data, use polynomial or spline interpolation instead.

  • Avoid divide-by-zero: Ensure x₂ ≠ x₁, or Excel will throw a #DIV/0! error.

  • Format cells: Use number formatting (decimals, units) for easier reading.

  • Visualize it: Add a line chart to plot the known points and interpolated value.

🧠 Bonus: Visualizing Interpolation in Excel

To see the interpolation in action:

  1. Plot x₁ and x₂ on a scatter plot.

  2. Add the interpolated x, y as a third point.

  3. Use a line to connect the data.

  4. Style the interpolated point with a different color or shape.

This can make your reports more intuitive.

📌 Final Thoughts

Excel isn’t just for budgets — it’s a secret powerhouse for mathematical modeling. By mastering linear interpolation with simple formulas, you can build custom calculators tailored to scientific research, business forecasting, or even day-to-day analytics.

Next time you’re missing a data point between two knowns, let Excel bridge the gap with just one formula.