Most people treat their pension contribution as a set-and-forget percentage. Tick the box at 5%, let auto-enrolment do its thing, and never look at it again.

But the way you make that contribution can be worth as much as the contribution itself. A salary sacrifice pension can hand you back both income tax and National Insurance on the money you put in, where an ordinary workplace or personal pension only gives you the tax back. The gap is real money, and at some income levels it is significant.

The catch is that "how much should I salary sacrifice" has a genuine answer, and it is not "as much as possible." It depends on where your salary sits against two specific lines in the tax system. Let's walk through it, with 2026/27 numbers and a fully worked example.

What is a salary sacrifice pension?

A salary sacrifice pension is an arrangement where you agree to give up part of your gross salary, and your employer pays that amount straight into your pension instead. Because you never technically receive the money as pay, it is not counted as earnings. So you pay no income tax on it, and, crucially, no National Insurance (NI) on it either.

That last part is what makes salary sacrifice different. Your employer also saves their own NI on the sacrificed amount, and many employers add some or all of that saving into your pot on top.

In short: you swap taxed, NI'd take-home pay for an untaxed pension contribution. Same money, routed more efficiently.

Salary sacrifice vs relief at source: where the extra saving comes from

Compare it to the two common alternatives, and the advantage becomes obvious.

• Relief at source (most personal pensions and SIPPs, plus some workplace schemes): you contribute from your take-home pay, the provider adds 20% basic-rate relief automatically, and higher-rate taxpayers claim the rest back through Self Assessment. You get the income tax back. You do not get National Insurance back, because you already paid it on that salary.
• Net pay arrangement (many workplace pensions): your contribution comes out before income tax is calculated, so you get full-rate income tax relief immediately. Again, no NI relief.
• Salary sacrifice: income tax relief and the National Insurance saving, because the money never counts as pay in the first place.

So the "salary sacrifice National Insurance saving" is simply the NI you would otherwise have paid on that slice of salary. For 2026/27 in England, Wales and Northern Ireland, employee NI runs at 8% on earnings between £12,570 and £50,270, and 2% above £50,270. That is the extra layer salary sacrifice unlocks that relief at source cannot. National Insurance is UK-wide, though note that income tax bands differ in Scotland, which changes the maths there.

How much should you actually sacrifice?

Here is the part most guides skip. Every pound you sacrifice is not worth the same amount, because your combined relief depends on which tax band that pound sits in.

Think of it as an efficiency ladder for 2026/27:

• The higher-rate slice (income above £50,270): 40% income tax plus 2% NI. Combined relief of 42%. This is the most efficient money to sacrifice, because you are clawing back 40p in the pound of income tax.
• The basic-rate slice (£12,570 to £50,270): 20% income tax plus 8% NI. Combined relief of 28%. Still worthwhile, and note the NI saving is actually larger here.
• The £100,000 to £125,140 band: a special case. Here your Personal Allowance is tapered away at £1 for every £2 of income, creating an effective 60% marginal rate. Add 2% NI and salary sacrifice into this band gives you roughly 62% relief. This is the single most efficient pound you can sacrifice, and it pairs directly with the "salary sacrifice and the £100k tax trap" question, restoring up to £12,570 of lost Personal Allowance.

The general principle: sacrificing down through £50,270 is especially efficient because it strips out your 40% income tax first. Beyond that, it is a judgement call about how much take-home you need to live on. This is exactly the kind of multi-variable trade-off Optiml is being built to model on your own numbers, so you can see the whole-retirement impact of a contribution decision rather than guessing at a percentage.

A worked example: Dan, 45, earning £58,000

Dan is a fictional higher-rate taxpayer in England, earning £58,000, deciding how much to salary sacrifice. His salary pokes £7,730 above the £50,270 higher-rate threshold, so that top slice is being taxed at 40% and is only earning him 2% NI territory anyway.

Say he sacrifices exactly that £7,730, bringing his taxable salary down to £50,270. Here is the before and after, on 2026/27 figures.

Read the bottom of the table slowly. Dan gives up £4,483 of take-home pay and puts £7,730 into his pension. If his employer passes on their 15% employer NI saving, it is closer to £8,890. Every £1 in his pension has cost him roughly 58p of spendable income, or about 50p once the employer NI is added.

The reason it is so efficient is the 40% income tax on that slice. Under relief at source, Dan would get the same 40% back eventually, but he would never recover the National Insurance, and he would be waiting on a Self Assessment refund to collect the higher-rate portion. Salary sacrifice delivers the lot at source. These are illustrative figures on stated assumptions, and remember that money in a pension is invested, so its value can fall as well as rise.

What the 2029 National Insurance change means

At the Autumn Budget 2025, the government announced that from April 2029, salary-sacrificed pension contributions above £2,000 a year will become subject to both employer and employee National Insurance. The first £2,000 you sacrifice each year keeps its full NI exemption. Everything above it loses the NI break.

Two things to hold onto here, because the headlines have been dramatic and the reality is calmer:

• It is a future change, not a live one. The rules for 2026/27 are unchanged. The measure takes effect in April 2029.
• Income tax relief is completely untouched. The change only affects the NI layer above £2,000. All of your pension contributions keep their income tax relief, subject to the usual limits.

For Dan, from April 2029 the NI advantage on the portion above £2,000 would narrow. But the biggest prize in his example was never the NI. It was the 40% income tax relief on clearing the higher-rate band, and that survives entirely. Salary sacrifice stays a strong move after 2029; the NI cherry on top is simply capped at £2,000 a year. This is precisely the sort of rule change Optiml UK is designed to model, so you can see how a 2029 shift ripples across the rest of your plan rather than reacting to a headline.

The traps to watch before you sacrifice

Salary sacrifice is powerful, but there are hard limits and one common pitfall.

• The Annual Allowance is £60,000 for 2026/27. That is the total that can go into your pensions each year with tax relief, counting your contributions and your employer's. High earners can see this tapered down towards a £10,000 floor.
• The MPAA trap. If you have already flexibly accessed a defined contribution pension, for example by taking taxable income from drawdown, your Money Purchase Annual Allowance drops to just £10,000 a year. Sacrifice beyond that and you lose the tax relief. If you are 55 or over and dipping into a pension already, check this first.
• You cannot sacrifice below the National Minimum Wage. Employers will not let a salary sacrifice arrangement drop your pay under the legal minimum, so there is a floor on how far down you can go.

None of these are reasons to avoid salary sacrifice. They are reasons to know your number before you commit to it. The right level depends on your whole picture: your other income, your Annual Allowance headroom, whether you are near the £100,000 taper, and how much take-home you actually need this year. When Optiml launches in the UK, modelling that full trade-off across your entire retirement horizon is exactly what it is built to do.

The bottom line

Salary sacrifice is not about giving up as much salary as you can. It is about giving up the right salary, the pounds sitting in the highest tax bands, where the combined income tax and National Insurance relief is greatest.

Get that number right and you convert taxed take-home into pension pounds at a discount the rest of the tax system rarely offers. The 2029 change trims the edges. It does not change the shape of the decision.

It's not about sacrificing more. It's about sacrificing smarter.