Why Salary Calculators Give Different Results

If you have used more than one salary calculator in the UK, chances are you have been perplexed by the fact that the displayed results are different. On most occasions, it is not a software bug but a result of configuration that governs how deductions are applied by different systems.

Pension contribution procedure

The biggest source of disagreement is the pension contribution procedure. There are three prevailing methodologies and each will give a different take-home pay on the same figures:

Relief at source – The contribution is taken out from your net pay. Basic rate tax relief at 20% is reclaimed, whereas you continue to pay tax on your entire salary amount. The pension is “relief at source” by the pension provider.
Net pay scheme – The contribution is deducted from your gross pay before income tax is calculated. You receive the tax rebates on your full gross wage, taking that payment straight into your pension, while the government still deducts NI on the original gross figure.
Salary sacrifice – An arrangement in which the employee gives up part of their salary. Taxes are deducted after the reduction and may provide a higher take-home pay out of the three methods, including an employee NI saving.

Case study: £35,000 with a 5% pension

Assuming a £35,000 salary with £1,750 in pension contributions, the annual take-home pay would differ based on the three methods:

Relief at source: tax and NI are calculated on £35,000. The £1,750 pension is taken out of the net wage.
Net pay arrangement: tax is calculated on £33,250 (net of pension). NI is calculated on £35,000.
Salary sacrifice: tax is calculated on £33,250, similarly for NI.

If two calculators (where calculation includes pension) are showing different values, chances are they are using different methods. Ask your employer, check your payslip, or speak to HR to confirm which method applies.

Student loan repayment plan

In the UK, the student loan system includes five plans, each with a unique income repayment threshold. Plan 1 starts its repayment only after an income of £26,065 is earned, whereas Plans 5 and 2 have threshold limits of £25,000 and £28,470 respectively. The difference between the highest and lowest thresholds is over £11,000 per year.

Some calculators automatically assume Plan 2, and this is not always made clear in their options. If you have a Plan 1 or Plan 4 loan and the calculator defaults to Plan 2, the amount stated for take-home pay will be incorrect. The Student Loans Company (SLC) will be able to confirm which plan you are assigned.

Scotland vs rest of UK

Scotland has six income tax bands compared to three in the rest of the UK. The lowest Scottish rate is 19% (starter) and the highest is 48%. For those who work within England, Wales, or Northern Ireland, the rates are 20%, 40%, and 45% respectively.

A Scottish taxpayer pays approximately £7,852 income tax on a £50,000 salary, compared to £7,486 paid by workers in England on the same gross salary. That amounts to around £366 annually.

If a calculator does not ask which country you live in, it is probably using England/Wales rates by default. Scottish taxpayers should look for calculators that support Scottish tax codes (those beginning with “S”).

Rounding and timing conventions

HMRC rounds tax and NI to specific decimal places on each payroll run. Small rounding differences can accumulate over 12 months. A calculator that computes everything annually and then divides by 12 may produce a slightly different monthly figure than one that calculates each month individually.

When student loan thresholds are divided into monthly figures, rounding is done towards zero (divide the annual threshold by 12 and take any decimal down to the nearest pound). Not all calculators do this rounding, leading to a small variation in the total annual repayment amount.

How this calculator works

Our calculator uses the 2025/26 tax rates set out by HMRC and supports both English and Scottish tax codes along with all five student loan plans. Pension contributions are calculated on gross qualifying earnings and are not mandatory. You can decide whether to include them.

We use monthly calculations, dividing the threshold by 12 and always using the smaller integer per HMRC. The result is then multiplied by 12 for an annual total.

Salary sacrifice, benefits in kind, marriage allowance, and gift aid do not feature in our calculator. A comprehensive set of assumptions is available on the Salary Calculator Assumptions & Methodology page.