Under the Construction Industry Scheme a contractor takes tax off a subcontractor's payment before it is paid and sends it to HMRC. The deduction is not the subcontractor's tax bill; it is money on account of it, taken from labour at a flat rate with no regard to the personal allowance or to any expenses. That mismatch is why CIS subcontractors so often finish the year having overpaid, and why the calculation matters twice: once when the payment is made, and again when the return works out what was actually owed.
What the deduction is taken from
The deduction applies to the labour element of a payment. Materials the subcontractor genuinely bought for the job, plant hire from a third party, and VAT where charged are excluded, so an invoice that separates labour from materials produces a smaller deduction than one that does not. The rate depends on the subcontractor's status with HMRC: verified subcontractors have a standard rate applied, unverified ones a higher rate, and those with gross payment status have nothing deducted at all. Getting the invoice split right is the whole of the arithmetic at this stage.
Why a refund is the usual outcome
The deduction ignores the personal allowance, ignores the rate bands and ignores every expense the subcontractor incurs. A subcontractor with a normal year of tools, travel, insurance and other allowable costs will therefore have had more taken than the return eventually calculates as due. The return reconciles the two: the tax actually owed is worked out from profit in the ordinary way, the CIS already deducted is set against it, and the difference is repaid. That is the whole mechanism behind what people search for as a CIS rebate.
Getting the deduction onto the return
The amount deducted goes on the self-employment pages, and it is credited only if HMRC can match it. That is why the monthly statements contractors are required to give you matter: they are the evidence of what was taken and by whom. Keep them, reconcile them against your own invoices as the year goes, and chase a missing one before January rather than during it. A deduction you cannot evidence is a refund you may not get.
What the refund is not
It is not a bonus and it is not automatic. It arrives because too much was taken, and it is only paid once a return has been filed and processed, which is another reason to file early rather than in the last week of January. It is also reduced by anything else you owe, including payments on account for the following year, so the figure that reaches your bank can be smaller than the overpayment even when the calculation is right.
Questions people ask about cis calculator
Is the CIS deduction my tax bill?
No. It is money taken on account of it, at a flat rate on labour, ignoring your allowance and your expenses. The return works out what was actually due and repays the difference.
Are materials included in the deduction?
No, provided they are genuinely materials you bought for the job and the invoice shows them separately. Plant hired from a third party is also excluded.
When will I get the refund?
After the return for that tax year is filed and processed, not before. Filing early rather than in January is the only lever you have on the timing.
What if a contractor never gave me a statement?
Ask for it - they are required to provide one. Reconcile your invoices against the payments received and chase gaps well before the filing deadline, because an unevidenced deduction is hard to claim.