CIS Tax Refund Calculator

Why most CIS subcontractors are owed a refund

CIS deducts 20% or 30% from your gross labour income as an advance against tax — but your actual Income Tax and Class 4 National Insurance bill is charged on profit, after allowable expenses and your Personal Allowance. Because the deduction rate doesn't know about your expenses, most subcontractors have suffered more tax during the year than they actually owe, which comes back as a refund once you file your Self Assessment return.

What this calculator estimates

This tool takes your CIS gross income, materials, allowable expenses, any other income, and either your actual CIS deductions suffered or an estimate based on your registration status, then works out your Income Tax and Class 4 NI liability and nets it against the tax already deducted. The result is always labelled an estimate — it won't match your SA302 until every income source, relief and expense is accounted for.

Gross vs net — the box 9 trap

Every Payment and Deduction Statement (PDS) shows a gross amount and a net amount after the CIS deduction — but your bank statement only ever shows the net figure landing in your account. Total up what hit your bank rather than the gross PDS figure, and you'll under-report your CIS income (SA103S box 9) and under-claim the deductions you're owed credit for (box 38) — the single most common reason subcontractors leave part of their refund unclaimed.

The Payment and Deduction Statement and box 38

A contractor must give you a PDS within 14 days of the end of each tax month you were paid — your evidence of both the gross payment and the deduction taken. Total CIS deductions suffered across every contractor you worked for during the year go in box 38 of your SA103S; HMRC nets that against your calculated liability — if box 38 exceeds what you owe, the difference is refunded. Keep every PDS, or a missing statement makes it harder to prove the total if HMRC queries your return.

The £50,000 Making Tax Digital threshold

From April 2026, self-employed subcontractors with qualifying income over £50,000 — turnover before expenses, including CIS-deducted gross income — must start using Making Tax Digital for Income Tax instead of a single annual SA100; the threshold drops to £30,000 from April 2027 and £20,000 from April 2028. It's based on gross turnover, not the net amount a contractor actually pays you after deduction, so a busy subcontractor can cross it before the money in their account feels 'high income'.

A worked example

A registered subcontractor has £35,000 of CIS gross income, £2,000 of materials and £6,000 of allowable expenses, with £5,600 of CIS suffered across the year's statements (20% of the £28,000 labour element). Their profit is £29,000, giving an Income Tax and Class 4 NI liability of roughly £4,375. Against £5,600 already deducted, that's an estimated refund of about £1,225 — the gap between tax withheld on gross labour income and tax actually owed on profit.

Common mistakes to avoid

The most frequent error is entering the net bank amount instead of the gross PDS figure, which understates both your income and the deduction you're owed credit for. The second is missing allowable expenses — tools, travel, protective equipment, materials not already netted off — that would lower your bill and increase your refund. The third is treating this figure as final: it only matches your SA302 once your full return has been filed.

Getting the actual refund

Filing your Self Assessment return is what turns this estimate into a real refund — HMRC doesn't repay CIS deductions automatically. ac-co's Self Assessment service totals every PDS and expense from your bank feed and documents, then calculates and files the return, so the number here should land close to what HMRC actually pays out.