How this calculator works
This tool solves one specific problem: you're on track to earn between £100,000 and £125,140 this tax year, and you want to know exactly how much extra pension contribution, made via salary sacrifice, gets you back out of that zone before the tax year ends. It's built for mid-year use, not a from-scratch annual plan, which is why it starts by asking what month you're in and what you've already earned and sacrificed so far.
The £100,000 to £125,140 band is expensive because of the Personal Allowance taper. HMRC withdraws £1 of your tax-free allowance for every £2 you earn above £100,000, so income in that band is effectively taxed twice, once at your normal marginal rate, and again through the shrinking allowance. In England, Wales and Northern Ireland that combination produces an effective rate of 60%. In Scotland it's higher, 67.5%, because the marginal rate operating through the taper zone is the 45% Advanced band rather than the 40% higher rate used elsewhere in the UK.
The calculator projects your full-year gross income from what you've earned so far plus your expected pay for the remaining months, works out how far over £100,000 that puts you, and calculates the extra sacrifice needed to bring your taxable income back down to exactly £100,000. It then spreads that figure evenly across your remaining pay periods, so you know what to actually change in your payroll each month.
The saving shown combines two effects: the actual Income Tax difference from having the Personal Allowance restored, calculated properly band by band, plus the National Insurance you avoid by sacrificing instead of taking the money as pay. Because you're well above the main NI threshold in this income range, that NI saving is usually 2%, not the 8% rate that applies to income lower down.
Understanding your inputs
Tax Month and your year-to-date figures tell the calculator exactly where you stand right now, rather than assuming a clean, even year.
Forward Monthly Gross is your expected pay for the remaining months, which can differ from your year-to-date average if you've had a pay rise, a bonus, or irregular pay during the year so far.
Pension Scheme Type matters because the National Insurance saving only applies to salary sacrifice specifically. If you're currently on relief at source, the calculator will flag that switching, if your employer offers it, would save you additional NI on top of the tax saving already shown.
Annual Allowance Used So Far, Employer Contribution % and Carry Forward feed into a separate check: whether the extra sacrifice this tool recommends would push your total pension contributions for the year over the £60,000 Annual Allowance. If it would, you'll see a warning rather than a silent miscalculation.
Worked example
Six months into the tax year, you've earned £52,000 so far and expect £8,700 a month for the remaining six months, with no bonus and nothing sacrificed yet. That projects to £104,200 for the full year, £4,200 over the £100,000 taper start.
To bring taxable income back down to exactly £100,000, you'd need to sacrifice £4,200 across the remaining six months, £700 a month. Income Tax drops from £29,532 to £27,432, a saving of £2,100, calculated properly rather than estimated. National Insurance, at the 2% rate that applies well above the main threshold, saves a further £84. That's a total saving of £2,184 against a £4,200 sacrifice, meaning the real cost to you is £2,016, roughly 48p in every £1 sacrificed, for money that goes straight into your pension rather than your take-home pay.
2026/27 figures used in this planner
| Figure | 2026/27 value |
|---|---|
| Personal Allowance taper zone | £100,000–£125,140 |
| Effective marginal rate in taper zone (England, Wales, NI) | 60% |
| Effective marginal rate in taper zone (Scotland) | 67.5% |
| National Insurance above £50,270 | 2% |
| Pension Annual Allowance | £60,000 (plus any unused carry forward) |
Ask your employer: whether they offer salary sacrifice specifically, not just a standard workplace pension, and how late in the tax year you can still change your contribution rate. Some payroll systems have a cut-off before the final pay period.
Common Mistakes
The most common one is treating the 60% (or 67.5% in Scotland) effective rate as the amount you'll save, rather than as an explanation of why the taper zone is expensive. The actual saving depends on your specific numbers and is calculated properly here, but it's worth understanding that the headline percentage is a rate, not a cash figure.
Another is forgetting that only salary sacrifice delivers the National Insurance saving. If your pension is relief at source or net pay, you still get the Income Tax benefit of restoring your Personal Allowance, but not the NI saving this calculator includes, unless you specifically switch schemes.
People also frequently overlook the Annual Allowance interaction. Sacrificing enough to escape the taper zone is excellent tax planning on its own, but if it pushes your total pension contributions, including your employer's, over £60,000 for the year, the excess can trigger a separate tax charge that offsets some of what you just saved.
Finally, leaving this until the last pay period is a real risk. The later in the year you act, the more of your remaining pay each additional sacrifice consumes, and some payroll systems can't process a change in time for the final payslip of the tax year.
Sources and verification
Figures verified against HMRC on 03 September 2026. Applies to the 2026/27 tax year.
Personal Allowance taper mechanics and thresholds: HMRC. Income Tax bands, England/Wales/Northern Ireland and Scotland: HMRC and the Scottish Government technical factsheet, published 13 January 2026. National Insurance thresholds and rates: HMRC. Pension Annual Allowance and carry forward rules: HMRC.