How to Calculate CIS Deductions Correctly

A CIS deduction can look simple on a payslip, but the figure is only right if the contractor has used the correct tax status and deducted from the correct part of the invoice. Knowing how to calculate CIS deductions helps you check what you have been paid, spot mistakes early and keep proper records for your tax return.

For most construction subcontractors, the starting point is not the total amount on the invoice. CIS is normally deducted from the labour element, not from materials or VAT. That distinction matters. If materials have been included incorrectly, you could have too much tax withheld and less cash in hand when you need it.

What is a CIS deduction?

The Construction Industry Scheme (CIS) requires contractors to deduct money from payments made to many subcontractors working in construction. The contractor sends the deduction to HMRC and gives the subcontractor a monthly deduction statement.

For a sole trader or partnership subcontractor, CIS deductions are payments on account towards the tax and National Insurance due through Self Assessment. They are not an extra tax charge. Your final bill still depends on your full income, allowable business expenses and personal circumstances. If too much has been deducted, you may be due a CIS tax refund after your return is filed.

The deduction rate depends on how HMRC has verified you:

• 20% if you are registered for CIS

• 30% if you are not registered or cannot be verified

• 0% if HMRC has granted you gross payment status

Gross payment status means the contractor pays you without deductions. It does not mean the income is tax-free. You still need to report it and pay any tax due through your return.

How to calculate CIS deductions from an invoice

The calculation itself is straightforward once the invoice has been separated properly. Start with the amount that is subject to CIS, apply the rate confirmed by HMRC, then subtract that deduction from the total payment due.

A worked 20% CIS example

Imagine you send a contractor an invoice for £3,000 made up of £2,000 labour, £500 materials and £500 VAT.

The contractor should usually calculate CIS as follows:

£2,000 labour x 20% = £400 CIS deduction

The total invoice is £3,000, so your payment should be:

£3,000 - £400 = £2,600

The £500 materials and £500 VAT are not included in the CIS calculation. The contractor pays £400 to HMRC and should show it on your deduction statement.

If you were on the 30% rate, the deduction would be £600 instead. If you had gross payment status, there would be no CIS deduction and you would receive the full £3,000.

Why invoices need clear labour and materials figures

A vague invoice that says only “building work £3,000” causes problems for both parties. The contractor may deduct CIS from the full amount because there is no clear evidence of materials, or they may ask you to reissue the invoice. Either way, it can delay payment.

List labour, materials and VAT separately. Keep receipts for materials you have supplied yourself. A contractor is entitled to ask for evidence where the materials figure seems unusually high, particularly if the work is labour-heavy.

Not every cost is automatically treated as materials. The rules can be less clear around plant hire, fuel, travel, accommodation, consumables and subcontracted labour. The correct treatment depends on what has been supplied, who paid for it and how it relates to the contract. If you are unsure, ask before invoicing rather than trying to untangle an over-deduction months later.

Check the CIS rate before work begins

The contractor is responsible for verifying a subcontractor with HMRC before making payment. Verification confirms whether deductions should be taken at 20%, 30% or not at all under gross payment status.

Registering as a subcontractor is usually worthwhile if CIS applies to your work. A 20% deduction is still significant, but it is much better for cash flow than 30%. If you move address, change from sole trader to limited company, or alter your business details, make sure the contractor has the right information. A small mismatch in names, UTRs or National Insurance details can lead to verification issues.

It is also worth remembering that CIS status does not decide whether someone is genuinely self-employed. Employment status is a separate question. A worker who is treated like an employee in practice may need to be paid through PAYE, even if both parties have been using CIS. This is an area where getting advice early can prevent a much larger problem later.

What should appear on a CIS deduction statement?

Your contractor should provide a deduction statement for every tax month in which they make a deduction. Keep these statements safely, alongside invoices and bank records. They are the evidence needed to claim credit for tax already paid on your behalf.

Check that each statement shows your name, the contractor’s details, the gross payment, the materials amount where relevant, and the CIS tax deducted. Compare it with your invoice and the amount that reached your bank account.

A common issue is receiving a payment that is lower than expected but no statement. Do not simply assume the shortfall is CIS. Ask the contractor for the statement promptly. Without the paperwork, it is harder to reconcile the deduction and ensure it is reported correctly to HMRC.

CIS deductions and your Self Assessment return

As a sole trader, you normally include your construction income in the self-employment section of your Self Assessment tax return. You then claim the CIS deductions shown on your statements. Your allowable expenses are deducted from your business income in the normal way, so your final tax position is based on profit, not just turnover.

This is why a refund is possible. A subcontractor may have had 20% deducted from labour payments throughout the year, but after expenses such as tools, insurance, vehicle costs, protective clothing, accountancy fees and legitimate use-of-home costs are claimed, their actual tax bill may be lower.

The opposite can also happen. CIS deductions may not cover everything due if your profits are strong, you have other income, or payments on account apply. Treat the money paid by contractors as an advance towards your bill, not as proof that all tax obligations have been settled.

Limited company subcontractors work differently. CIS deductions suffered by the company are generally set against the company’s PAYE and National Insurance liabilities through payroll reporting, rather than claimed on a director’s personal Self Assessment return. The records need to be handled carefully, especially where the company has no employees or deductions exceed its PAYE bill.

Mistakes that lead to over-deductions

Most CIS errors come down to poor paperwork rather than complicated maths. Contractors sometimes deduct from VAT, fail to remove materials, use the 30% rate when 20% applies, or place a deduction against the wrong subcontractor record. Subcontractors can also create confusion by invoicing through a company but providing sole trader details, or by not retaining their monthly statements.

Deal with discrepancies quickly. Provide the invoice, statement, payment date and amount received, then ask the contractor to check their CIS return. A correction is far easier while the payment is recent than at the end of the tax year.

Good CIS record-keeping is not glamorous, but it protects your cash flow and makes your tax return far less stressful. If your statements, invoices and material receipts are in order, you can see where your money has gone and make sure every deduction gets the credit it deserves.