A tax code can change because of a new job, benefits, underpaid or overpaid tax, pension income, second income, Marriage Allowance or updated HMRC records.
HMRC may change a tax code when it receives new information about employment, pension income, taxable benefits, underpaid or overpaid tax, estimated income, allowance transfers or a second job. The changed code can alter estimated monthly take-home pay.
On £35,000 a year with a standard 2026/27 tax code, no pension, no student loan, the figures work out like this. Open a calculator to put your own numbers in.
| Tax/source year | 2026/27 |
|---|---|
| Region | England/Wales/Northern Ireland |
| Tax code basis | 1257L where the page uses PAYE defaults |
| Pension basis | No pension deduction unless this example says otherwise |
| Student loan basis | No student loan unless selected in this example |
| Calculator setting | The answer is more useful when matched to the same tax year, region, tax code, pension and loan settings used in the calculator. |
|---|---|
| Records to compare | Payslips, HMRC records, student-loan notices and pension scheme documents can explain differences. |
| Tax code | HMRC tax-code changes, K codes, BR/D codes, or emergency markers can change PAYE deductions. |
|---|---|
| Cumulative basis | Week 1, Month 1 or non-cumulative payroll can differ from a smooth annual calculator estimate. |
| Pension method | Relief at source, net pay and salary sacrifice can affect taxable pay and take-home pay differently. |
| Payroll timing | Bonus, overtime, arrears, refunds, cut-off dates and corrections can all move a single payslip. |
| Open the salary calculator with editable assumptions | calculator next step |
|---|---|
| Review calculation sources and methodology | source context |
| What is not decided here | PayBreakdown does not decide payroll correctness, lender approval, benefit entitlement or employer compliance, and does not give regulated financial, mortgage, debt, tax, payroll or legal advice. |
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The figures on this page are worked from tax code changed, using the values below. Open the calculator if your example salary differs.
A tax code is based on information HMRC has about the income and allowances for that employment or pension. Changes can move the Personal Allowance, apply a prefix, or put pay on a temporary basis.
A changed code is a possible explanation for a changed payslip, but it does not by itself prove an error.
PayBreakdown cannot see HMRC's live tax-code calculation, benefits records, previous jobs or payroll notices. Use it to estimate the impact of a code change, then check official records where needed.
This is educational guidance on how UK pay and tax work. It does not decide payroll correctness, HMRC treatment, lender approval, benefit entitlement or employment rights, and it is not regulated financial advice. Where a figure matters, check it against your payslip, your HMRC account, or the official guidance linked above.
It may change because of employment or pension updates, benefits, previous tax adjustments, estimated income, allowance transfers, a second job, regional prefixes or emergency/non-cumulative basis.
Yes. If payroll does not have previous income and tax details, the first payslips may use a temporary or emergency basis such as W1, M1, X or NONCUM.
Yes. Taxable benefits, previous underpayments, state benefits, savings interest or other adjustments can reduce the allowance represented by the code.
No. It is one possible reason for a different result. Check the HMRC record, payslip code, gross pay, taxable pay, deductions and payroll timing before treating it as an issue.
Last updated 2026-08-27. Checked by Sean Elsmore. Rates last checked against GOV.UK on 2026-08-07. Estimates are for planning and should be checked against official records where the decision matters.