Why the second number matters more than the first
Most calculators stop at the take-home figure. The more useful number is the marginal rate — what the taxman takes from the next pound you earn — because that is what decides whether a pay rise, a bonus, or an extra shift is worth it.
For most people it is 28%: twenty pence of income tax and eight pence of National Insurance in every pound. Cross £50,270 and it becomes 42%, because income tax steps up to 40% while NI drops to 2%. Add a Plan 2 student loan and it is 51%.
The 60% band nobody legislated
Between £100,000 and £125,140 the personal allowance is withdrawn at £1 for every £2 earned. You are taxed at 40% on the pound itself, and at 40% again on the 50p of allowance it destroys. With National Insurance that is an effective 62% on every pound in that band — a higher rate than anyone pays at £1,000,000.
This is why a £5,000 rise from £100,000 to £105,000 hands you about £1,900. Putting the same £5,000 into a pension instead costs you roughly £1,900 of take-home and puts £5,000 into your pot.
Salary sacrifice versus a normal pension
Both get you full income tax relief. Only salary sacrifice avoids National Insurance, because the pay never legally becomes yours — your employer contributes it directly.
- Net pay / auto-enrolment: the contribution comes out before income tax, but after NI.
- Salary sacrifice: comes out before both, so a basic-rate taxpayer keeps an extra 8% of whatever they contribute, and it lowers student loan repayments too.
On a £50,000 salary contributing 10%, that difference is £400 a year for the identical £5,000 going into the pension.