Published: 07 March 2026
Why your payslip differs across calculators
Many people assume a pension contribution is just a pension
contribution. In payroll, the method matters. The same gross pay and
same pension target can still produce different income tax, NI, and
student loan deductions.
The biggest reason for mismatches: pension method
UK payroll commonly uses one of three pension approaches:
-
Salary sacrifice: reduces contractual pay before
tax, NI, and student loan calculations.
-
Net pay arrangement: reduces taxable pay for income
tax, but NI and student loan bases can stay higher.
-
Relief at source: pension is deducted from net pay
and basic-rate relief is added in the pension scheme.
If a calculator does not ask for pension method, it can easily miss
your actual payslip outcome.
NI and student loan are usually pay-period based
PAYE income tax often feels annual, but NI and student loan deductions
are typically assessed per payroll period. That means frequency
matters (weekly, fortnightly, four-weekly, or monthly), and so does
how pay is adjusted by pension setup.
How to debug a mismatch quickly
- Check pension method on your actual payroll documentation.
- Match your pay frequency exactly.
- Use the same tax code as your payslip.
-
Confirm student loan plan (and whether postgraduate loan also
applies).
-
Compare taxable pay, NI-able pay, and loanable pay as separate
lines.
We built a dedicated tool for this comparison:
Take-Home Pay Calculator (Payslip Truth).
Official HMRC references