# How to calculate the length of a triangle side

Calculating the size of one triangle side using Excel or any other spreadsheet software can be helpful in various scenarios, particularly when dealing with **geometric** or trigonometric calculations. This information can be helpful in understanding the shape and properties of the triangle, such as its perimeter, area, or angles.

There can be many ways for calculating the triangle side in Excel, but we’ll discuss two easy and simple ways to calculate the length of a triangle side. One method is by using **Pythagoras’** theorem for a **right-angled** triangle. Another approach is to use the **Law of Cosines** for **oblique** triangles.

## Case 1 – Right-angled triangle

For right-angled triangles, we can easily calculate the size of the third side if the lengths of the other two sides are known. The data below contains the known information about the triangle.

## Step 1 – Select the cell where you want the result

- Go to the location where you want to calculate the size of the triangle side.
- Click on the cell to select it.

## Step 2 – Use the Pythagoras theorem to find the length

- Create an appropriate formula using the Pythagoras theorem.
- In this case, the used formula is
**=SQRT(B2^2 + B3^2).** - Enter the formula in the cell.
- Press Enter.

## Case 2 – Oblique triangle

For oblique triangles(other than right-angled), we have to use the Law of Cosines. We need the lengths of the two other sides and also the angle between them. Consider the dataset above that contains the details of the triangle.

## Step 1 – Convert the known angle into radians

- Select a cell where you want to use the “Radians” formula.
- Write the formula using the
**RADIANS**function, and the cell that contains the angle value. - Press Enter.

## Step 2 – Use the law of cosines to find the size of the side

- Click on the cell where you want to apply the formula
- Create an appropriate formula according to the Law of Cosines.
- In this case, the used formula is
**=SQRT(B2^2 + B3^2 – 2*B2*B3*COS(B5)),** - In this formula, B2 is the Length A, B3 is the Length B, whereas B5 is “Angle in Radians”.
- Type the formula in the cell.
- Press Enter.