CIS Tax Calculator

How CIS Tax Deductions Work

If you work as a subcontractor in the UK construction industry, your contractor is required by HMRC to deduct tax from your payments before you receive them. This is the Construction Industry Scheme (CIS), and it affects hundreds of thousands of tradesmen across the country.

The standard deduction rate is 20% for registered subcontractors. If you haven't registered with HMRC, your contractor must deduct 30%. Subcontractors with an excellent compliance history can apply for gross payment status, which means no deductions are taken at source — but you're still responsible for paying your tax through Self Assessment.

CIS deductions are not an additional tax — they're advance payments towards your annual tax bill. When you file your Self Assessment return, these deductions are offset against what you owe. Many subcontractors actually overpay through CIS and are entitled to a refund. That's why it's important to keep accurate records of all your CIS payment and deduction statements.

How CIS Refunds Work

Because CIS is a flat-rate deduction, it doesn't account for your personal allowance (£12,570 tax-free), your business expenses, or the actual tax bands you fall into. This means most subcontractors overpay through CIS and are owed money back.

For example, a subcontractor earning £3,000/month gross with 20% CIS has £7,200 deducted annually. But after the personal allowance and typical expenses of £4,000, their actual income tax might only be around £4,200 — plus roughly £1,900 in National Insurance. That's a total tax bill of about £6,100, meaning they're owed over £1,000 back from HMRC.

To claim your CIS refund, you need to file a Self Assessment tax return. You'll need your CIS payment and deduction statements (CIS vouchers), records of all business expenses, and details of any other income. The deadline for online returns is 31 January following the end of the tax year.

This calculator helps you instantly see your take-home pay after CIS deductions, plus an annual projection so you can plan ahead. The refund estimator shows how much you could claim back based on the 2025/26 tax year rates — it runs entirely in your browser and we don't store any of your data.