Why Most UK Tax Calculators Get Salary Sacrifice Wrong
David Mohamad · 9 March 2026
Updated 9 March 2026
What Salary Sacrifice Is
Salary sacrifice is a deal with your employer. You give up part of your gross pay and they put it into your pension instead. The key word is “gross”: the money leaves your pay before tax, NI, or student loan are worked out.
This is not the same as a NET pay pension, which lowers your taxable income but not your NI. And it is not the same as relief at source, where pension comes out after tax and your provider claims back basic-rate tax for you.
Why It Changes Everything
Because the sacrifice happens before tax, it lowers the number used for every step after it. Your NI drops. Your student loan drops. Your adjusted net income drops.
Say you earn £135,000 and sacrifice £20,000. Your gross for NI and student loan becomes £115,000 — not £135,000. That one change can cut your NI by hundreds and your student loan by nearly £2,000 a year.
It also matters if you earn above £100,000. You lose £1 of tax-free allowance for every £2 over that mark. A big enough sacrifice can bring you back below the line and restore your full £12,570 allowance.
Where Calculators Go Wrong
We tested seven UK take-home pay calculators against a salary sacrifice scenario. Three had no salary sacrifice input at all. Of the four that did, results still differed by up to £1,883 on the student loan line alone.
The root cause is ordering. If a tool takes off pension after NI and student loan, it shows too much for both. If it only takes a percentage and not a fixed pound amount, rounding errors creep in. HMRC’s own tool has this problem — it only takes a pension percentage, so £20,000 has to be entered as 14.81%, which shifts the numbers.
Use a tool that skips salary sacrifice and enter a normal pension instead? Your take-home figure could be wrong by thousands of pounds a year.
The Numbers
We ran the same inputs through all seven tools: £135,000 gross, £20,000 salary sacrifice pension, Plan 2 student loan, 2025/26 tax year.
| Calculator | Take Home | Tax | NI | Student Loan | Pension |
|---|---|---|---|---|---|
| MSE* | |||||
| HMRC | £64,277 | £36,432 | £4,711 | £9,587 | £19,993 |
| Salary Calc | £66,470 | £36,432 | £4,311 | £7,704 | £20,000 |
| UK Tax Calc | £66,470 | £36,432 | £4,310 | £7,788 | £20,000 |
| UK Salary Calc* | |||||
| ANNA* | |||||
| Take Home | £64,670 | £36,432 | £4,311 | £9,588 | £20,000 |
* Calculator does not support salary sacrifice pension
Tax matched at £36,432 across every tool that supports salary sacrifice. But NI ranged from £4,310 to £4,711, and student loan from £7,704 to £9,588. That is a £1,884 gap on student loan alone — enough to change how much you actually take home each month.
Check Your Own Numbers
The Casomo take-home pay calculator supports salary sacrifice as a fixed pound amount. Enter your gross salary and pension to see how NI, student loan, and take-home pay change.
Frequently Asked Questions
Related
- UK Calculator Comparison — full feature and accuracy review of seven take-home pay tools
- Take-Home Pay Calculator — see your net salary after tax, NI, and pension