Connecting Your Own App to Xero or QuickBooks via API

What a custom app usually syncs

The goal is simple: anything your app already knows about a sale should reach the accounting system once, correctly, without someone retyping it. In most small business integrations that means a short list of records.

Keep reporting and GST returns inside Xero or QuickBooks. Pulling large amounts of data out for your own reports is where API limits and, increasingly, developer fees bite.

Xero API: limits and developer plans (October 2026)

Xero's API limits page sets these limits per connected organisation: 5 calls in progress at once, 60 calls per minute, and 1,000 calls per day on the Starter tier or 5,000 a day on higher tiers. Exceeding them returns HTTP 429 with a Retry-After header telling you how long to wait.

For a custom app used by one business, Starter is normally enough: you connect one organisation and a few hundred invoices a day fit comfortably within 1,000 calls if you batch them. Xero suggests up to about 50 invoices per request. Xero's limits page also says each organisation can connect a maximum of five uncertified apps, so count what else is already connected.

QuickBooks Online API: limits and the App Partner Program

Intuit's API limits page lists 500 requests per minute per company (realm ID), 10 requests per second per company and app, and a recommended maximum of 30 payloads per batch request. Queries return at most 1,000 records, so larger reads need pagination. When throttled you get HTTP 429 and should wait 60 seconds.

Since 2025, Intuit's App Partner Program splits calls into Core and CorePlus. Core calls, mostly data-in operations such as creating or updating invoices, customers and payments, are unmetered and free. CorePlus calls, mostly reads, queries and reports, are metered. New apps start in the free Builder tier, which Intuit's limits page says includes 500,000 CorePlus calls per workspace per month before calls are throttled.

How the connection works: OAuth, IDs and GST codes

Both Xero and QuickBooks use OAuth 2.0: someone with admin rights in the accounting file approves your app once, and your app stores tokens that let it call the API on the company's behalf. Tokens expire and must be refreshed, so the app needs to store them securely and handle refresh failures.

1. 1

   Register a developer app

   Create it in the Xero developer portal or Intuit Developer, under an account your company controls.

2. 2

   Map your data

   Agree which account codes, GST tax codes, tracking categories or classes and items each sale should use. Your accountant should sign this off.

3. 3

   Build against a test company

   Use Xero's demo company or a QuickBooks sandbox. Never test against live books.

4. 4

   Store the accounting system's IDs

   Save the Xero or QuickBooks ID for every contact, invoice and payment you create, so updates hit the same record and retries do not create duplicates.

5. 5

   Connect the live file and run a pilot

   Sync a week of real transactions, then check them against the bank and with your accountant before switching off manual entry.

Getting GST codes right matters more than anything technical. A wrong tax code on every invoice means a wrong GST return. If you are still choosing a system, our Xero vs QuickBooks comparison covers which accountants prefer and how each handles Singapore GST.

Error handling that keeps the books right

Integrations fail quietly. A token expires on a Saturday, an invoice uses a deleted account code, or the API is throttled during a busy hour. Without proper handling, records go missing and nobody notices until month end.

Also decide what happens when someone edits a synced invoice in Xero or QuickBooks directly. The usual rule is that the app owns the record until it is paid, and accounting changes after that are made by the accountant only.

Who should own the integration

This is the question most small businesses forget. If a developer builds the integration under their personal developer account, the app, its credentials and any paid developer plan belong to them, not you.

Ask for this in writing before work starts. It costs nothing extra to set up correctly at the beginning and is painful to untangle later. Integration work is also a common line item in larger web projects; our website cost guide shows where it fits in a typical budget.

Do it yourself or get help?

If your app is a standard platform like Shopify, Stripe or a booking tool, check for an existing connector first; it is often cheaper than custom code. Custom integrations make sense when your own system holds the sales data and no connector exists.

Our Xero / QuickBooks Integration (S$390) connects bank feeds and existing payment or sales apps to your accounting. For your own app talking to Xero or QuickBooks through the API, a Custom Business App (from S$4,900) is scoped and fixed-priced in writing, with the developer app and code registered in your company's name. IT Sorted is a Singapore-based team of developers with over 10 years of experience. You pay upfront, send your brief through the site chat or contact form, and track progress on your order page.