How This Calculator Works
This calculator finds your earliest viable retirement age by simulating your finances year by year from your current age. For each potential retirement age it asks: if you stopped working now, would your SIPP and ISA last until your plan-to age?
All figures are in real (inflation-adjusted) terms — the numbers you see are in today’s purchasing power.
What It Models
Accumulation phase — from today until you retire:
- Your SIPP and ISA grow at your chosen real growth rate
- Monthly contributions are added each year
Drawdown phase — from retirement until your plan-to age:
- SIPP withdrawals use UFPLS rules: each withdrawal is 25% tax-free, 75% taxable
- Drawdown is tax-optimised: SIPP is drawn first to fill your Personal Allowance (£12,570), then ISA covers the remainder — minimising tax paid
- State pension (if included) starts at 67, reducing the amount needed from your pots
- DB pension (if included) starts at your chosen age
The year-by-year breakdown table shows exactly where your income comes from each year, what tax is paid, and what your pots are worth.
SIPP Access Age
Your SIPP cannot be accessed until age 57 (rising from 55 in 2028). If the calculator finds that retirement is viable before 57, your ISA must be large enough to cover the gap years entirely. If your ISA runs out before SIPP access, that retirement age is not viable.
The State Pension
The UK state pension (£11,502/yr, 2024/25) starts at age 67. It counts as taxable income, which means it partly uses your Personal Allowance — reducing the space to draw SIPP income tax-free after that point. The calculator handles this automatically.
Read more about the state pension and FIRE →
ISA vs SIPP
- ISA — accessible any time, all withdrawals tax-free
- SIPP — accessible from 57, each UFPLS withdrawal is 25% tax-free / 75% taxable as income
The optimal strategy for most people is to draw SIPP up to the Personal Allowance each year (paying no tax on the 75% taxable portion), then use ISA for any additional spending. This calculator applies that strategy automatically.