FIRE Finance

ISA vs SIPP Calculator UK

Model the real after-tax difference between investing in an ISA and a SIPP for your FIRE journey.

Your Inputs

Enter the amount you have available to invest (after tax). We'll calculate what each wrapper produces with the same out-of-pocket cost.

Same out-of-pocket amount for both wrappers

6–7% real return is typical

ISA vs SIPP: The UK FIRE Dilemma

For UK FIRE pursuers, the ISA vs SIPP question is one of the most consequential decisions in portfolio construction. Both are tax-advantaged wrappers, but they work very differently — and the right answer depends on your current tax rate, your planned retirement age, and your expected income in retirement.

The good news is that for most people in accumulation, the answer is not either/or. It is both — in the right order, in the right proportions.

How Each Wrapper Works

Stocks & Shares ISA

You contribute post-tax money (up to £20,000/year in 2025/26). All dividends, capital gains, and growth are completely free of tax. You can withdraw at any time, at any age, with no tax whatsoever. This flexibility is the ISA's superpower for FIRE: it is the only genuinely unrestricted tax-free account in the UK.

SIPP (Self-Invested Personal Pension)

You receive tax relief at your marginal rate when you contribute. A basic rate taxpayer paying £800 gets £1,000 in their SIPP. A higher rate taxpayer paying £600 (reclaiming 20% via self-assessment) also gets £1,000 in. Growth is tax-free inside the SIPP. At retirement, 25% of the fund can be taken tax-free (capped at £268,275 lifetime), and the rest is taxed as ordinary income.

The SIPP minimum access age is currently 55, rising to 57 in April 2028. Anyone planning to retire before 57 must have sufficient ISA savings to bridge the gap.

The Optimal Strategy for UK FIRE

The most common recommendation in the UK FIRE community is:

  1. Maximise employer pension match (free money — always do this first)
  2. Fill your Stocks and Shares ISA up to the £20,000 annual limit
  3. Contribute to your SIPP, especially if you are a higher-rate taxpayer
  4. Use a GIA for any remaining investable capital

Higher-rate taxpayers often benefit from prioritising the SIPP above ISA (after the employer match) because the 40% relief makes the effective cost so low. However, if you plan to retire before 57, you always need enough ISA/GIA capital to bridge the gap to SIPP access.

Frequently Asked Questions

Should I use an ISA or a SIPP for FIRE in the UK?

Ideally both. The ISA gives flexibility (accessible at any age) while the SIPP gives tax relief (20–45% depending on your rate). For early retirees, the ISA is essential to bridge the gap before SIPP access at 57.

What is the annual ISA allowance?

£20,000 per person in 2025/26. This can be split across multiple ISA types. The Stocks and Shares ISA is the primary choice for long-term investment growth towards FIRE.

How much tax relief do I get on SIPP contributions?

Basic rate: 20% — you pay £800 and £1,000 goes in. Higher rate: 40% effective — you pay £600 after reclaiming 20% extra via self-assessment. Additional rate: 45% effective — you pay £550. The relief is one of the best guaranteed returns available in UK personal finance.

Can I access my SIPP before retirement age?

No — the minimum pension access age is 55 (rising to 57 in 2028). Anyone planning to retire before 57 must have sufficient non-pension savings (ISA, GIA) to fund those years without SIPP access.

This calculator is for illustrative purposes only and does not constitute financial advice. Tax rules, rates, and pension access ages can change. The 25% tax-free lump sum is capped at the lifetime allowance figure (currently £268,275 following the 2023 Budget changes). For advice specific to your circumstances, consult a qualified financial adviser.

Related Calculators

Your Financial Freedom Awaits

Every Journey Begins with a Single Step

Imagine waking up each day knowing you're one step closer to financial freedom.

No more anxiety about money. No more working just to pay bills. Just the peace of mind that comes from being in complete control of your financial future.

Join the community taking control of their financial future