This tutorial explains how to calculate the dot product in Excel.

**What is the Dot Product?**

Given vector *a* = [a_{1}, a_{2}, a_{3}] and vector *b* = [b_{1}, b_{2}, b_{3}], the **dot product** of vector a and vector b, denoted as **a · b**, is given by:

**a · b** = a_{1} * b_{1} + a_{2} * b_{2} + a_{3} * b_{3}

For example, if *a* = [2, 5, 6] and *b* = [4, 3, 2], then the dot product of *a* and *b* would be equal to:

**a · b = **2*4 + 5*3 + 6*2

**a · b = **8 + 15 + 12

**a · b = **35

In essence, the **dot product **is the sum of the products of the corresponding entries in two vectors.

**How to Find the Dot Product in Excel**

To find the dot product of two vectors in Excel, we can use the followings steps:

**1. Enter the data**. Enter the data values for each vector in their own columns. For example, enter the data values for vector *a* = [2, 5, 6] into column A and the data values for vector *b* = [4, 3, 2] into column B:

**2. Calculate the dot product.** To calculate the dot product, we can use the Excel function **SUMPRODUCT()**, which uses the following syntax:

**SUMPRODUCT(array1, [array2], …)**

**array**– the first array or range to multiply, then add.**array2**– the second array or range to multiply, then add.

In this example, we can type the following into cell **D1** to calculate the dot product between vector *a* and vector* b*:

**=SUMPRODUCT(A1:A3, B1:B3)**

This produces the value **35**, which matches the answer we got by hand.

Note that we can use **SUMPRODUCT() **to find the dot product for any length of vectors. For example, suppose vector *a *and *b *were both of length 20. Then we could enter the following formula in cell **D1 **to calculate their dot product:

**=SUMPRODUCT(A1:A20, B1:B20)**

**Potential Errors in Calculating the Dot Product**

The function **SUMPRODUCT() **will return a **#VALUE!** error if the vectors do not have equal length.

For example, if vector *a *has length 20 and vector *b *has length 19, then the formula **=SUMPRODUCT(A1:A20, B1:B19)** will return an error.

The two vectors need to have the same length in order for the dot product to be calculated.