IT contractors bill differently from almost everyone else: high day rates, monthly invoicing, agencies or end clients on 30-day terms, and tax rules (IR35 and VAT) that decide how much of the day rate you actually keep. This guide covers what invoicing software needs to do for an IT contractor in 2026, and how to set it up so nothing slips.
- Umbrella? You do not invoice anyone. Your umbrella does.
- Limited company: invoice the agency or client, track POs, chase at day 1 late.
- B2B services to overseas clients are generally outside the scope of UK VAT.
- Inside IR35 you are taxed like an employee; outside, you invoice gross.
- Keep timesheets and PO references: disputes are about hours, not prices.
Why IT contractors need different invoicing software
An IT contractor’s billing life is shaped by how they engage:
| Engagement | Who invoices | What the software must handle |
|---|---|---|
| Umbrella company | Nobody: the umbrella employs you and pays you net of tax | Nothing beyond keeping your own records |
| Limited company, direct to client | Your company invoices the client | Client records, PO numbers, VAT, 30-day terms |
| Limited company via agency | Your company invoices the agency | Agency details, timesheet references, rates per contract |
| PAYE via an agency | Nobody: the agency pays you | Timesheet confirmation only |
If you run a limited company, invoicing is a monthly, repeatable process: one or two invoices, exactly the same rate, exactly the same terms. The software’s job is to make that repetition boring and to chase the money when terms lapse.
The features that matter for contractors
- Recurring invoices. A fixed monthly invoice for an ongoing contract is the textbook use for recurring billing. Set it once, review it at renewal.
- Client and agency records. The invoice goes to the paying entity, which may be an agency, while the work name belongs to the end client. Both need to be on the invoice correctly.
- Timesheet and PO references. Corporate accounts-payable teams reject invoices that do not match their purchase order or timesheet reference. Store the reference per client and carry it onto every invoice.
- VAT handling. Standard 20% once registered, correct treatment for overseas B2B clients, and clean records for the flat rate scheme if you use it.
- Reminders. A 30-day term silently becomes 40 days unless something chases. Sendinvo sends payment reminders automatically, which matters more than any other single feature for contractors.
Sendinvo is deliberately the invoicing-and-getting-paid half: no timesheets, no diary. For IT contractors who already track time in their own tool, that split works well: the timesheet produces the hours, the invoicing software turns them into a paid invoice.
Set up invoicing in one sitting
- Create the paying entity as the client: agency or end client, with their exact legal name and address from the contract.
- Store the contract references: PO number, contract name, agreed day rate, payment terms.
- Set the invoice frequency: monthly in arrears is standard; recurring templates handle it.
- State terms on the invoice: 30 days is normal; anything you agree is enforceable.
- Turn on reminders so day-1-late chases happen without you.
VAT and IR35 in one paragraph each
VAT. Once your taxable turnover passes the £90,000 registration threshold you register and add 20% to UK invoices. Most B2B services to business clients outside the UK are outside the scope of UK VAT: see how to invoice overseas clients for the wording. If you use the flat rate scheme, note that some supplies fall outside it, and check HMRC’s current percentage for your trade rather than trusting an old blog.
IR35 / off-payroll working. Since April 2021, medium and large private-sector clients determine your employment status for tax; for small clients, your own company does. The outcome changes who taxes the engagement: inside IR35, payments are taxed broadly like employment income; outside, your company invoices normally. Invoicing software does not decide IR35, but it must record what was agreed: the status determination, the rate, and who the contract is with.
The monthly invoicing routine
A contractor on one contract needs about ten minutes a month:
- Confirm the timesheet or deliverable against the PO.
- Send the recurring invoice on the same date every month, with the PO reference on it.
- Log the payment when it lands; let reminders handle the gap if it does not.
The failure mode is not complexity: it is drift: a rate changed at renewal but not on the template, an agency renamed after a merger, a PO number that rotated. Review client records at each contract renewal.
What late payment looks like here
Agency chains create late payment: the end client pays the agency, the agency pays you. Your remedy is the same as any UK business: statutory late payment interest at 8% above base rate plus a fixed compensation sum on business-to-business debts, from the day after the due date. State it in the reminder; it collects most debts without escalation.
Compare the options briefly
| Option | Best for | Watch for |
|---|---|---|
| Sendinvo | Recurring monthly invoicing, reminders, clean records | Invoicing only: no timesheets or diary |
| Xero | Contractors wanting a full company ledger with accountant access | Monthly bookkeeping discipline |
| QuickBooks | Contractors who also want bookkeeping and receipt capture | Heavier than invoicing alone |
| FreeAgent | UK limited companies wanting built-in tax timelines | UK-market focus |
| Zoho Invoice | Occasional invoicing on a budget | Feature depth varies |
For the full comparison across trades, see the 2026 shortlist.
FAQ
Do IT contractors need invoicing software if they use an umbrella company?
No, the umbrella employs you and issues all invoices. You keep your own timesheet records and payslips. Invoicing software only becomes relevant when you contract through your own limited company.
What should an IT contractor’s invoice include?
Your company details, the paying entity’s details, the contract or PO reference, the period covered, the agreed rate, VAT at 20% if registered or the reason none is charged, and your payment terms.
Do I charge VAT to a client outside the UK?
Generally not for B2B services. The supply is outside the scope of UK VAT. State the reason on the invoice and keep evidence of the client’s business status.
Does IR35 change how I invoice?
Inside IR35 the client or fee-payer taxes the engagement largely like employment, so your company’s role shrinks. Outside IR35, your company invoices normally. Record the status determination with the contract either way.
What payment terms should an IT contractor use?
30 days is standard for agencies and corporate clients. State the terms on every invoice and quote: without stated terms, UK business-to-business law defaults to 30 days anyway.
Can I charge interest when an agency pays late?
Yes. The Late Payment of Commercial Debts (Interest) Act 1998 gives 8% above the Bank of England base rate plus a fixed compensation sum on business-to-business debts, from the day after the due date.
One last thing
The contractor invoices that go unpaid are almost never wrong about money. They are wrong about paperwork: a stale PO number, an agency legal name that changed, a timesheet the client never countersigned. Ten minutes reviewing client records at contract renewal prevents every one of them.