If we talk about **linear interpolation** in Excel, we first need to understand what this term is. **Linear interpolation **is a method from mathematics that serves us to **estimate values between two data points**, if the data points are familiar.

In Excel, we can use linear interpolation with functions and formulas. In the example below, we will show how to do it.

## Linear Interpolation in Excel

For our example, we will have **two data points: (xy, y1) and (x2,y2).** Our job will be to find the **y-value** corresponding to a certain **x-value**, that is located **between x1 and x2**.

To explain better, we will have the following table:

By looking at the table below, we could assume that following the logic of linear interpolation, the **y2 number, located in cell A5 will be 20** since the **progression of x is 2**, and the **progression of y is 10**. To confirm this by formula, and generally to find the way to calculate it automatically, we will use the following formula in **cell B5**:

1 |
=FORECAST(A5, B2:B3, A2:A3) |

**FORECAST formula has three parameters: x, known_ys, and known_xs**

In our case, **x is number 4, known_ys are 10 and 30, and known_xs are numbers 2 and 6**.

This is the result we will end up with:

Instead of the **FORECAST formula**, we could also use **INTERCEPT**. Our formula in **cell B6** will be:

1 |
=(A5-A2)*(B3-B2)/(A3-A2)+B2 |

To break down this formula:

**A5-A2 calculates**the difference between the desired x-value and the lower known x-value.**B3-B2 calculates**the difference between the upper known y-value and the lower known y-value.**A3-A2 calculates**the difference between the upper known x-value and the lower known x-value.

This formula then **multiplies the first two differences** that we have and **divides this result by the third difference** in order to find the slope of the line connecting our two data points.

In the end, the formula adds the result to the **lower known y-value** to find the **interpolated y-value** for the **given x-value**.

We will get the **same result** using this formula, as we did with **FORECAST**:

Just remember- linear interpolation **demands a straight-line relationship** between the data points. If the data points are not linear, then we should use other interpolation methods.