Is a pension or an ISA better for tax?

It depends on the rates at each end. With basic rate in and out, the pension returns 6.25% more per pound of net outlay because a quarter leaves tax-free. Higher-rate relief in and basic rate out puts it roughly 41.7% ahead; basic in and higher out can put the ISA ahead.

Where does the 6.25% pension advantage come from?

From the 25% tax-free element. Basic-rate relief grosses £1 up to £1.25; withdrawal keeps 25% tax-free plus 75% taxed at 20%, which is 85p in the pound. 1.25 × 0.85 = 1.0625 — a 6.25% uplift on the same net outlay as an ISA.

Does paying into a pension reduce adjusted net income when an ISA doesn’t?

Yes. A pension contribution reduces adjusted net income pound for pound, which can restore the personal allowance above £100,000 (ITA 2007 s.35) and reduce the High Income Child Benefit Charge between £60,000 and £80,000. An ISA subscription has no effect on ANI.

Can salary sacrifice make the pension side better still?

It adds National Insurance relief that a personal contribution cannot get — 8% or 2% employee NI on the sacrificed pay (SSCBA 1992 s.8) — and the employer may add its 15% secondary NI saving to the contribution. There is no ISA equivalent.

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Pension or ISA: how the tax maths actually compares

On tax alone the question is a round trip: what the relief adds going in against what the tax takes coming out. Worked in pounds at basic and higher rate — with the National Insurance and adjusted-net-income effects the headline round trip misses.

Based on Finance Act 2004 (pension tax relief), ITTOIA 2005 Part 6 Chapter 3 (the ISA exemption), SSCBA 1992 (National Insurance) and ITA 2007 (adjusted net income).

6 min read · Last reviewed 30 Jun 2026

Strip the pension-or-ISA question down to tax and it becomes a round trip: what the Treasury adds on the way in, against what it takes on the way out. Both wrappers grow free of tax while the money is inside, so growth cancels out of the comparison — the whole difference is at the two ends. The pension adds relief at your marginal rate going in (FA 2004 s.192) and taxes 75% of what comes out (PTM063300; ITEPA 2003 s.579A); the ISA adds nothing and takes nothing (ITTOIA 2005 Part 6 Chapter 3).

The basic-rate round trip: 6.25%

Take £8,000 of taxed pay, and run it through each wrapper with tax at basic rate on both ends of the pension leg:

£8,000 of taxed pay · basic rate in and out · 2026/27

Into the ISA: £8,000.00
Out of the ISA (nothing taxed on exit): £8,000.00
Into the pension once relief at source adds £2,000: £10,000.00
Tax-free element on the way out (25%): £2,500.00
The remaining £7,500, taxed at 20%: £6,000.00
Out of the pension: £8,500.00

£8,500 against £8,000 — the pension returns 6.25% more from the same money, and the whole margin comes from the quarter that leaves tax-free. The algebra is compact: relief grosses £1 up to £1.25, and the exit keeps 25% plus 75% at 80%, which is 85p in the pound — 1.25 × 0.85 = 1.0625. If the withdrawal instead lands at higher rate, the exit keeps only 70p in the pound and the same round trip returns £7,000 — less than the ISA. The relief rate in against the marginal rate out decides the direction.

The higher-rate round trip

For a higher-rate taxpayer the front end is worth double. A £10,000 gross contribution attracts £2,000 of relief at source plus £2,000 more through the extension of the basic rate band (FA 2004 s.192(4)):

£10,000 gross · higher-rate relief in · basic rate in retirement · 2026/27

Paid to the pension from taxed pay: £8,000.00
Basic-rate relief added by the scheme (20%): £2,000.00
Higher-rate relief reclaimed via the band extension: £2,000.00
Net cost of the £10,000 contribution: £6,000.00
Net received drawing it at basic rate (£2,500 + £6,000): £8,500.00
The same £6,000 through an ISA: £6,000.00

£8,500 from a £6,000 net outlay is roughly 41.7% more than the ISA’s £6,000 — the combination of 40% relief in, 20% tax out and the 25% element. Even if the withdrawal is taxed at 40% too, the pension leg returns £7,000 against £6,000: about 16.7% ahead, from the tax-free quarter alone. The pension vs ISA tax comparison calculator computes the round trip at your own income now and in retirement.

Salary sacrifice: the pension-side extra

The round trips above use a personal contribution, which saves income tax only. Where an employer offers salary sacrifice, the pension leg also saves National Insurance — 8% on pay between £12,570 and £50,270 and 2% above (SSCBA 1992 s.8) — and the employer’s own 15% secondary NI saving (SSCBA 1992 s.9) can be added to the contribution too. £1,000 sacrificed by a basic-rate employee costs £720 of take-home and can land £1,150 in the pension. No ISA route has an equivalent. Salary sacrifice for pensions, explained works the figures.

SSCBA 1992 s.8 and s.9 · NICs (Secondary Class 1 Contributions) Act 2025.

The side effects the round trip misses

A pension contribution reduces adjusted net income pound for pound; an ISA subscription does not. Between £100,000 and £125,140 of ANI the personal allowance is withdrawn at £1 per £2 (ITA 2007 s.35) — the 60% band — and between £60,000 and £80,000 the High Income Child Benefit Charge claws back child benefit at 1% per £200 (ITEPA 2003 ss.681B–681H). A contribution that pulls ANI back through either threshold carries relief well beyond the headline rate.

The effect reverses in retirement. The taxable 75% of a pension withdrawal is income, so it counts towards the same thresholds in the year it is drawn; an ISA withdrawal never enters adjusted net income at all. The two wrappers differ not just in how much tax is paid but in which years the money is visible to the income tax system — a fact of timing the single-year round trip cannot show.

ITA 2007 s.35 and s.58 · ITEPA 2003 ss.681B–681H.

Common questions

Is a pension or an ISA better for tax?: It depends on the rates at each end. With basic rate in and out, the pension returns 6.25% more per pound of net outlay because a quarter leaves tax-free. Higher-rate relief in and basic rate out puts it roughly 41.7% ahead; basic in and higher out can put the ISA ahead.
Where does the 6.25% pension advantage come from?: From the 25% tax-free element. Basic-rate relief grosses £1 up to £1.25; withdrawal keeps 25% tax-free plus 75% taxed at 20%, which is 85p in the pound. 1.25 × 0.85 = 1.0625 — a 6.25% uplift on the same net outlay as an ISA.
Does paying into a pension reduce adjusted net income when an ISA doesn’t?: Yes. A pension contribution reduces adjusted net income pound for pound, which can restore the personal allowance above £100,000 (ITA 2007 s.35) and reduce the High Income Child Benefit Charge between £60,000 and £80,000. An ISA subscription has no effect on ANI.
Can salary sacrifice make the pension side better still?: It adds National Insurance relief that a personal contribution cannot get — 8% or 2% employee NI on the sacrificed pay (SSCBA 1992 s.8) — and the employer may add its 15% secondary NI saving to the contribution. There is no ISA equivalent.

Sources & grounding

Pension relief in / tax out: FA 2004 s.192 (relief at source) and s.192(4) (basic-rate-limit extension for higher/additional relief); 25% tax-free / 75% marginal on withdrawal (PTM063300; ITEPA 2003 s.579A). ISA exemption: ITTOIA 2005 Part 6 Ch 3; TCGA 1992 s.151.
Worked round-trip figures (£8,000 → £10,000 → £8,500 = 6.25%; £6,000 net cost → £8,500 ≈ 41.7%): rule-based arithmetic at the legislated 20%/40% rates (basicRateTax 2000 / higherRateTax 4000, configs/2026-27.json) and the statutory 25%/75% split — the same computation the wrapper comparison calculator runs through the engine.
Salary-sacrifice NI extra: SSCBA 1992 s.8 (employee Class 1: 8% primary threshold £12,570 → UEL £50,270, 2% above = nationalInsurance block, configs/2026-27.json) and s.9 (employer secondary Class 1: 15% above £5,000; NICs (Secondary Class 1 Contributions) Act 2025).
Adjusted-net-income side effects: personal-allowance taper £1 per £2 over £100,000 (ITA 2007 s.35; paAbatementThreshold 10000000, configs/2026-27.json); ANI definition ITA 2007 s.58; HICBC £60,000–£80,000 at 1% per £200 (ITEPA 2003 ss.681B–681H; hicbcLowerLimit 6000000 / hicbcChargeStep 20000, configs/2026-27.json).

For planning and illustration purposes only. Verify all inputs against source documents. This explainer does not constitute financial or tax advice.