How to connect Xero to Karbon: what accounting firms are building

Yes, Xero connects to Karbon natively through Karbon’s official integration, which syncs client contacts and provides billing functionality between the practice management and accounting systems. Formulaic builds custom work-to-invoice automations on top that track WIP against budgets in real time, auto-generate invoices when work items complete, and surface practice profitability metrics that neither system provides alone. This matters most to accounting practices using Karbon for workflow management and Xero for billing.

Short answer: Yes, Xero connects to Karbon natively. Client contacts and billing data sync between systems. Formulaic builds custom automations linking Karbon work items to Xero invoicing and WIP tracking.

What data moves between Xero and Karbon

Contact data syncs bidirectionally. Karbon client records map to Xero contacts, including: company name, primary contact name, email, phone, and address. When a new client is added in either system, it appears in the other.

Billing data flows primarily from Karbon to Xero. Karbon work items (tax returns, annual accounts, bookkeeping engagements) can generate Xero invoices with the work item description, budgeted amount, and client reference. Invoice status flows back from Xero to Karbon, showing whether the invoice is draft, sent, paid, or overdue.

What doesn’t sync natively: time entries from Karbon to Xero line items, WIP calculations, budget vs actual comparisons, team member allocation costs, or profitability per work item. Karbon tracks the work. Xero tracks the money. But the connection between work completed and money earned requires manual reconciliation.

For practices operating on fixed-fee engagements, the data gap is manageable. For practices doing any time-based billing or wanting to track profitability per engagement, the native sync falls short.

The native integration

Karbon’s native Xero integration handles the essentials for practices that invoice on a simple fixed-fee basis. You complete a work item in Karbon, create a Xero invoice from within Karbon, and the client gets billed. The invoice status syncs back. For a practice with straightforward billing, this covers the core workflow.

The integration also provides useful context within Karbon. When you open a client’s profile, you can see outstanding invoices, payment history, and billing status without switching to Xero. This saves time for team members managing client relationships.

Where the native integration leaves gaps: there’s no automated invoicing. Completing a work item doesn’t automatically generate an invoice. Someone has to manually create the invoice from the work item. For a practice completing 200+ work items per month, this creates a billing lag. Work gets done in week one, the invoice goes out in week three.

WIP tracking is also missing from the native integration. Karbon tracks time against work items, and Xero tracks invoiced amounts, but there’s no automated way to compare the two. Understanding whether a client engagement is profitable requires pulling data from both systems and reconciling manually in a spreadsheet.

What Formulaic builds

Formulaic builds a billing automation layer that connects Karbon’s workflow completion to Xero’s invoicing, with real-time WIP and profitability tracking.

The system monitors Karbon work items via the API. When a work item moves to a “Complete” or “Ready to bill” status, the system automatically generates a corresponding Xero invoice. For fixed-fee work, the invoice matches the engagement amount. For time-based work, the system pulls Karbon time entries, applies the relevant charge-out rates, and generates an itemised Xero invoice.

The WIP tracking component runs continuously, comparing time logged in Karbon against budgets and invoiced amounts in Xero. This produces a live dashboard showing: work in progress by team member and client, budget burn rate per engagement, unbilled time ageing, realisation rate (time recorded vs. time billed), and collection rate (time billed vs. payment received).

The profitability layer adds cost data. By mapping team member charge-out rates from Xero (or a separate rate card) against time logged in Karbon, the system calculates gross margin per engagement, per client, and per service line. Partners can see which clients and service lines generate the highest margins and which are eroding profitability.

One mid-tier practice we built this for had 120 active clients across tax, accounts, and advisory service lines. They were invoicing an average of 12 days after work completion and had no visibility into engagement profitability. The custom build reduced billing lag to 2 days (automatic invoice generation on work completion), increased monthly billing by 8% (by catching work items that were completed but never invoiced), and provided the first practice-wide profitability dashboard the partners had ever seen.

For HMRC-registered practices, the system applies correct VAT treatment based on the service type configured in Karbon, ensuring that tax returns, accounts preparation, and advisory work are invoiced with the appropriate VAT rates.

DIY vs Zapier vs custom build

	DIY / Manual	Zapier / Make	Formulaic Custom Build
Setup time	N/A	Not available	2-4 weeks
Cost	Free (your time)	N/A	Custom-scoped
Data accuracy	Manual reconciliation	N/A	Real-time WIP tracking
Customisation	Spreadsheet-based	N/A	Full: profitability, auto-invoicing
Compliance	Manual VAT checking	N/A	Automated VAT treatment
Maintenance	You	N/A	60-day warranty

Timeline and investment

Every build is scoped and priced based on complexity. Simpler data syncs take less time than multi-system orchestration with compliance requirements. Start with an audit to get a clear proposal.

Take the AI Opportunity Scorecard to see which integrations would deliver the highest ROI for your firm.