Adding up rows or columns of numbers is a common operation carried out in all spreadsheet programs. Google Sheets includes a builtin function called SUM for this purpose. With a function in place, the spreadsheet automatically updates when you make changes in the range of cells in the formula. If you change entries or add text to blank cells, the total updates to include the new data.
These instructions use the Google Sheets app for iOS.
How to Write a Function in Google Sheets
A function in Google Sheets and other spreadsheet programs like Microsoft Excel has three parts:
 An equals sign (=). This tells the program that you're entering a function.
 The name of the function. This is usually in allcaps, but that isn't necessary. Some examples are SUM, ROUNDUP, and PRODUCT.
 A set of parentheses: (). If the function includes work on a set of numbers in the spreadsheet, these numbers go in the parentheses to tell the program which data to use in the formula. Some functions, like NOW, which returns the current date and time, have the parentheses but are empty.
How to Write a SUM Function
Using the information above, write a SUM function like this:
=SUM(number_1,number_2, ... number_30)
In this case, the numbers in the parentheses are the individual cells being added. This can be a list, like (A1, B2, C10), or a range, like (A1:B10). The range option is how you add columns and rows.
How to Enter a SUM Function in Google Sheets
Before you begin, enter the information you want to add up into a spreadsheet, then follow these steps:

Click or tap the cell where you want to place the formula.

Tap Enter text or formula to display the keyboard.

Type =sum( to start the formula.

Choose the numbers you want to add together. One way to do this is to tap the cells you want. The cell references appear inside the parentheses in the formula.

To select a range of adjacent cells at once, tap one (for example, the first one in a row or column), then tap and drag the circle to select the numbers you want to add together.
You can include empty cells in a function.

Enter a closing parenthesis to end the function, and then tap the checkmark to run the function.

The function runs, and the sum of the numbers you selected appears in the cell you chose.

If you change any of the values in the cells you selected, the sum updates automatically.
How to Create a Sum Using the Function Button
You can also use a menu to enter a function instead of typing it. Here's how to do it.

Enter the data, then select the cell in which you want the sum to appear.

Click or tap the Function button.
On the desktop version of Google Sheets, Function is on the right side of the formatting bar and looks like the Greek letter sigma (∑).

In the list of function categories, tap Math.
The Function menu on the desktop version of Google Sheets contains a few commonly used formulas. SUM may be on that list.

The functions appear alphabetically. Scroll down, then tap SUM.

In the spreadsheet, enter the range of numbers you want to add together.