The Google Sheets SUM function is used to add up multiple numbers in google sheets. You can enter the numbers to add manually or you can add an entire range.
SUM can save Google Sheets users a lot of time as you can easily add an entire range of numbers instead of having to enter multiple + signs between each number in a formula.
Read this tutorial to learn what you can do with the SUM function in Google Sheets.
What is the function?
There are two ways to add values in Google Sheets. One involves using the plus sign. This is best for adding two or three numbers. However, this method can easily get taxing when you need to add several numbers, or a range of values.
If you need to add values stored in a range, you should use the SUM function. Its syntax is straightforward but you can use it in a wide variety of cases. It also has other advantages over the manual plus sign method.
Advantages of the function
Can cover large ranges in less time

Can ignore letters in the range

Does not break when a column or row is deleted

SUM Formula Syntax
Adding values of ranges
=SUM(range)
Where
range is where the values you want to add are stored. You can specify the range as either a list of cell references or a reference to an entire range
Adding values of nonadjacent cells
=SUM(number1, number2, …)
Where
number1, number2, … are the numbers you want to add. You can also specify a reference to where the values you want to add are stored:
SUM Example(s)
Get the SUM of a column (Method 1)
Step 1: Click where you want to place the formula

Step 2: Click Functions icon in the toolbar, then select SUM

Step 3: Select the range of the column

Step 4: Press Enter
Check the screencap below:

Get the SUM of a column (Method 2)
Step 1: Identify the range of the column
We want to add values in column A, so we set the range to B7:B9.

Step 2: Type the formula SUM(range)
The formula to type is now
=SUM(B7:B9)
Step 3: Press Enter

Get the SUM of a row (Method 1)
Step 1: Click where you want to place the formula

Step 2: Click Functions icon in the toolbar, then select SUM

Step 3: Select the range of the row

Step 4: Press Enter
Check the screencap below:

Google Sheets sometimes detects a pattern in your sheet and offers to extend the formula to the other cells of the row or column. You can opt to accept the suggestion or not.
Get the SUM of a row (Method 2)
Step 1: Identify the range of the row
We want to add the entries of Row A, so we set the range to B18:D18.

Step 2: Type the formula SUM(range)
The formula to type is now
=sum(B18:D18)
Step 3: Press Enter

Add nonadjacent cells (Method 1)
Step 1: Click where you want to place the formula

Step 2: Click Functions icon in the toolbar, then select SUM

Step 3: Select the range of the row
Hold the Ctrl key (Windows) or ⌘ key (MacOS) as you select the individual cells.

Step 4: Press Enter
Check the screencap below:

Add nonadjacent cells (Method 2)
Step 1: Identify the numbers you want to add
We want to add along a diagonal of the range, which are B30, C29, and D28.

Step 2: Type the formula SUM(number1, number2, …)
The formula to type is now
=SUM(B30,C29,D28)
You can also hold the Ctrl key (Windows) or ⌘ key (MacOS) as you select the cells.
Step 3: Press Enter.

Adding numbers that fulfill a certain condition
There are cases where you need to add only values that fulfill a certain condition. There are two ways of doing so in Google Sheets.
SUM + FILTER
We combine SUM with FILTER:
=SUM(FILTER(range, condition1, condition2, …)
Where
range is where the values you want to add are stored
condition1 and condition2 are the conditions you want to apply
Step 1: Identify the range of the row
We want to add all the numbers along Row 4 that are less than 4. The range is A4:C4 and the condition is A4:C4<4 .

Step 2: Use the formula SUM(FILTER(range,condition1,condition2,...))
The resulting formula is
=SUM(FILTER(A4:C4,A4:C4<4))

Shortcomings of this method
The problem with this method is that it is restricted to only a single row or column. It breaks down if the range is across multiple rows or columns:

The solution is to use the SUMIF and SUMIFS functions in Google Sheets.
Sample Google Sheets spreadsheet to work on