Both are tax-efficient wrappers, but they hand you the tax break at opposite ends. Get the timing right and it's worth real money. Here is how they differ, when each one wins, and a simple worked example.
A Stocks and Shares ISA is funded from money you've already paid tax on. It grows tax-free and, crucially, everything comes out completely tax-free, at any age, for any reason.
A SIPP (a self-invested personal pension) works the other way round. You get tax relief going in, so your contribution is topped up before it's invested. It grows tax-free, and when you access it (from age 55, rising to 57 in 2028) you can take 25% tax-free, with the rest taxed as income.
| ISA | SIPP / pension | |
|---|---|---|
| Tax relief going in | No | Yes |
| Growth taxed | No | No |
| Tax coming out | None | 25% free, rest as income |
| Access age | Any age | 55 (57 from 2028) |
Two things stack in the pension's favour: the relief that tops up your contribution, and the 25% tax-free lump sum at the end. Take £100 out of your pocket and imagine the pot doubles over time.
| £100 out of pocket | ISA | SIPP |
|---|---|---|
| Basic-rate now & in retirement | £200 | £213 |
| Higher-rate now, basic in retirement | £200 | £283 |
Even when you're taxed at the same rate in and out, the SIPP edges ahead, purely because a quarter of it comes out tax-free. And if you get 40% relief now but only pay 20% in retirement, the pension wins comfortably.
The rule of thumb: the SIPP wins whenever your relief in, plus the 25% tax-free lump sum, beats the tax you pay on the way out. That's true for most people, and dramatically so for higher-rate taxpayers who'll draw at the basic rate.
Put in the same monthly amount, your return and your tax rate now and in retirement, and see which leaves you more after tax.
Open the ISA vs SIPP calculator →The ISA's edge isn't tax, it's flexibility:
When you access a defined-contribution pension you can normally take 25% of the pot tax-free, up to a Lump Sum Allowance of £268,275. The other 75% is taxed as income as you withdraw it, at whatever rate applies to you that year. That tax-free quarter is a big part of the pension's advantage.
For most people it isn't ISA or SIPP. A common approach is to use the pension for the tax relief (especially if you're a higher-rate taxpayer or your employer matches contributions) and an ISA for flexibility and money you might want before pension age. See exactly what the taxman adds with the Pension Tax Relief calculator, and how a pot grows over time with Compound Growth.
On tax alone a SIPP usually wins, thanks to relief in and the 25% tax-free lump sum. An ISA wins on flexibility, because you can access it at any age.
For a higher-rate taxpayer now who'll be a basic-rate taxpayer in retirement, markedly so. For the same rate in and out, the pension still edges it because of the tax-free lump sum.
You can normally take 25% of a defined-contribution pension tax-free (up to £268,275). The rest is taxed as income on withdrawal.
When you need access before pension age, might pay more tax in retirement than you saved, or are saving for a first home via a Lifetime ISA.
Illustrative and educational only, not financial advice. The worked example assumes the same return for both and that pension relief is reinvested; real outcomes depend on contribution limits (£20,000 ISA, £60,000 pension), your tax rates, how you draw the pension, and market returns. Pension money is locked until 55 (57 from 2028). Your capital is at risk. Always check gov.uk or speak to a qualified adviser.