Adding up rows or columns of numbers is one of the most common operations carried out in all spreadsheet programs. Google Sheets includes a builtin function called SUM for this purpose.
With a function in place, the spreadsheet will automatically update 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  or other spreadsheet programs like Microsoft Excel  has three parts, in this order:
 An "equals" sign (=). This tells the program that you're entering a function.
 The name of the function you're using. This is usually in allcaps, but that isn't necessary. Some examples are SUM, ROUNDUP, and PRODUCT.
 A set of parentheses: (). If your function includes work on a set of numbers in your spreadsheet, they 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, still has the parentheses, but they're empty.
How to Write a SUM Function
Using the information above, you write a SUM function like this:
=SUM(number_1,number_2, ... number_30)
In this case, the numbers in the parentheses are all of the individual cells you're adding. This can be a list, like (A1, B2, C10) or a range, like (A1:B10). The range option is how you'll add up columns and rows.
How to Enter a SUM Function in Google Sheets

Enter the information you want to add up into your spreadsheet. Click or tap the cell where you want to place your formula.

Tap Enter text or formula to pull up the keyboard.

Type "=sum(" to start the formula.

Next, you'll choose which numbers you want to add together. One way to do this is to tap the cells you want. They'll start appearing inside the parentheses in your formula.

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

Close the parentheses ")" to end the function, and then tap the checkmark to run the function.

The function will run, and the sum of all of the numbers you selected will appear in the cell you chose.

If you change any of the values in the cells you selected, the sum will update automatically.
Creating 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 your data, and then select the cell in which you want the sum to appear, as before.

Instead of typing your entry into the text box, click or tap the Function button to the left of it. The function button looks like the letters "fx."
On the desktop version of Google Sheets, the Function is in the farright corner of the formatting bar and looks like the Greek letter sigma (∑).

A list of function categories will appear. 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, and tap SUM.

You'll return to the spreadsheet. Enter the range of numbers you want to add together as usual.