How to Create Custom Functions in Google Sheets with Google Apps Script

As a seasoned Google Sheets user and Apps Script developer with years of experience automating workflows, I've helped countless teams supercharge their spreadsheets. If you've ever needed calculations beyond built-in formulas, custom functions via Google Apps Script are the solution.

Google Sheets offers robust built-in functions for sums, averages, lookups, and more. But for complex logic like dynamic tax calculations varying by location, nested IFs become unwieldy. Here's how to build reusable custom functions efficiently.

Understanding Built-in Functions

Start with what's available: SUM or AVERAGE handle basics effortlessly.

For dynamic needs, like adding location-based taxes to purchases, long IF chains like '=IF(A2="PA",B2*0.06,IF(A2="CA",B2*0.0625,B2*0))' quickly spiral out of control. Custom functions keep things clean and scalable.

Creating a Custom Function

Google Apps Script is beginner-friendly. This tax calculator example demonstrates the process.

1. Open the Script Editor

From your Sheet, go to Extensions > Apps Script (updated menu).

2. Write Your Function

Name it descriptively, like calculateTax. Parameters go in parentheses.

Paste this into the editor:

function calculateTax(amount, location) {
  var rate = 0;
  switch (location) {
    case 'PA':
      rate = 0.06;
      break;
    case 'CA':
      rate = 0.0625;
      break;
    default:
      rate = 0;
  }
  return amount * rate;
}

Note: Add more cases as needed for additional locations.

3. Save Your Project

Click File > Save, name your project, and hit OK.

Using Your Custom Function

Treat it like any built-in: Enter =calculateTax(B2, A2) where B2 is the amount and A2 the location.

Drag to autofill rows, just like SUM.

Add more functions to the same script anytime.

Result:

Reusing Custom Functions

Save effort across sheets:

1. Store in a blank template Sheet and copy as needed.

2. Copy-paste code between Script editors.
3. Publish to Google Sheets Template Gallery (public or domain-limited with Workspace).

Documenting Your Functions

Use JSDoc for professional help text:

Custom functions unlock Google Sheets' full potential, often surpassing Excel for collaboration. For more, check Google's custom functions guide.

Have you built custom functions? Share in the comments!