ISA vs Pension Calculator

For the same gross income contributed each year, which UK long-term savings vehicle leaves you with more spendable money at retirement? This calculator compares the after-tax outcomes of an ISA (or LISA) against a pension, including employer matching.

The fundamental math

ISAs and pensions both grow tax-free. The only differences are when you pay income tax and who adds top-up money:

• ISA: Pay income tax now, contribute the after-tax remainder, withdraw tax-free.
• LISA: Same as ISA, but the government adds 25% on contributions up to £4,000/year. Withdrawals locked until age 60 or first home (no penalty); other withdrawals lose the bonus plus 6.25% — effectively a 25% confiscation, so don't put money in if you might need it.
• Pension: Contribute gross (no income tax now), 25% tax-free at withdrawal, the remaining 75% taxed at your marginal rate at retirement.

The single most important variable

Forget the headline rules. The dominant factor is the comparison between your marginal tax rate now and your marginal tax rate at retirement.

• Higher now than later: Pension wins, often by a lot. You defer tax at 40% / 45% now and pay 20% (or even 0%) in retirement when income is lower. This is the classic case for high earners.
• Same in both periods: Pension still wins narrowly because of the 25% tax-free lump sum effect. Roughly 6-7% better than ISA for the same contribution.
• Lower now than later: ISA wins. Pay tax at today's lower rate, save the higher future rate.
• LISA (under 40, saving for first home or retirement): Hard to beat. 25% bonus is roughly equivalent to 20% income tax relief inside a pension — but withdrawals are tax-free, and you can access the money at 60 with no further tax.

Why the employer match is non-negotiable

If your employer offers a workplace pension match, capture it first, before any other long-term saving. Every £1 of your contribution unlocks (say) £1 of free money from the employer plus income tax relief at your marginal rate. The instant return is 100%+ — nothing else in personal finance comes close.

Standard sequencing:

1. Contribute to the workplace pension up to the full employer match.
2. Pay off any high-interest debt (above ~8% APR).
3. Build the emergency fund (3-6 months of expenses).
4. If you're under 40, consider opening a LISA and contributing up to £4,000/year for the bonus.
5. Beyond that, the ISA-vs-pension choice depends on your marginal-rate forecast.

Worked example

A higher-rate taxpayer (40% marginal) contributes £8,000 gross per year for 25 years, with 6% annual return and a 5% employer pension match. Assume retirement marginal rate of 20%.

• ISA: Net contribution after 40% tax = £4,800/year. Final pot after 25 years at 6% ≈ £263,000. Withdraw tax-free.
• Pension: £8,000/year gross + £400/year employer (5% of £8,000) = £8,400/year. Final pot ≈ £460,000. 25% lump sum = £115,000 tax-free. 75% (£345,000) taxed at 20% = £69,000 tax owed. Net: £391,000.
• Pension wins by £128,000 in this scenario, or about 49% more spendable money.

When ISA wins

ISAs beat pensions in several specific scenarios:

• Basic-rate now, higher-rate at retirement. Rare but possible — e.g., expecting to inherit, sell a business, or move into a higher tax country.
• Need access before age 55/57. ISAs can be drawn at any age; pensions are locked. If there's any chance you'll need the money before 55, ISA flexibility is worth the tax hit.
• Hitting the pension annual allowance or LSA. The pension annual allowance (£60,000 in 2025/26) tapers down to £10,000 for very high earners. The Lump Sum Allowance caps tax-free withdrawals at £268,275. Beyond these limits, ISAs become the next-best option.
• Estate-planning considerations. ISAs lose tax-free status at death (eventually); pensions can pass income-tax-free to nominated beneficiaries (this is changing in 2027 — check current rules).
• Already maxing pension and the LSA looks tight. Once the lifetime allowance / LSA equivalent is in sight, additional pension contributions get diminishing returns.

When pension wins

• Higher-rate now, basic-rate at retirement. The standard case — defer 40-45% tax, pay 20%.
• UK PA taper trap (£100k-£125,140). Effective marginal rate is 62% in this band. Pension contributions extract you from the taper and produce eye-watering effective returns (often 80%+ on the contribution).
• Salary sacrifice with NI savings. A salary sacrifice pension contribution saves both income tax and the 8%/2% National Insurance. ISAs save neither.
• Strong employer match. Even with otherwise mediocre pension parameters, a meaningful match (4%+) tips the balance decisively.
• Long horizon. The longer the lockup, the more the tax-deferred compounding rewards the pension structure.

What this calculator does NOT model

• National Insurance savings on salary sacrifice pensions. Typically worth an extra 8% / 2% on your contribution.
• Pension annual allowance taper. For incomes above £260k adjusted, the £60k allowance tapers to £10k.
• Lump Sum Allowance. 25% tax-free is capped at £268,275 from 2024/25; above that, withdrawals are fully taxable.
• State pension. Affects your retirement marginal rate.
• LISA early withdrawal penalty. 25% confiscation if withdrawn before age 60 for non-house purposes — meaningfully worse than a refund of the bonus.
• ISA flexibility. Being able to access funds at any age has value not captured in a numerical comparison.
• Tax rules change. Pension rules have changed at least once a decade for the last 30 years. Long projections are speculative.
• Inheritance tax treatment. Major rule changes for pensions are scheduled for April 2027.

Disclaimer

Educational only. The ISA-vs-pension decision is heavily personal and tax-law-dependent. Talk to a regulated UK financial adviser (preferably one with chartered status) for advice tailored to your circumstances.