Investment Growth Planner (UK)

ISA, SIPP, GIA — you know you should invest. The question is where.

The compound interest maths is the same everywhere. But your real outcome depends on which wrapper you use — Stocks & Shares ISA (tax-free forever), SIPP (pension tax relief now, taxed at withdrawal), or a General Investment Account (capital gains tax applies). This planner models all three with UK tax rules so you can see your actual spendable wealth at retirement.

What's Included

Multi-account modelling: enter current value and monthly contribution for your Stocks & Shares ISA, SIPP (personal pension), workplace pension, and GIA (general investment account)
ISA allowance tracker: monitors your £20K annual ISA limit across all ISA types — flags when you're approaching the cap and suggests overflow to GIA or SIPP
SIPP tax relief calculator: shows the effect of basic rate (20%), higher rate (40%), and additional rate (45%) tax relief on your pension contributions — including the extra relief higher-rate taxpayers need to claim via Self Assessment
Lifetime Allowance awareness: although the LTA charge was abolished in 2024, the sheet tracks your projected pension pot against the £1,073,100 reference point for awareness, and models the new Lump Sum Allowance (£268,275 tax-free)
After-tax withdrawal comparison: ISA withdrawals are completely tax-free. SIPP withdrawals are 25% tax-free lump sum + rest taxed as income. GIA withdrawals face Capital Gains Tax above the £3,000 annual allowance. See the real difference at retirement.
Fee impact comparison: enter your platform fee + fund OCF (e.g. 0.15% platform + 0.07% Vanguard Global All Cap vs 0.45% platform + 0.75% managed fund) — see the pound difference over decades
Drawdown modeller: enter target retirement age and desired monthly income — the sheet shows how long your pot lasts, what the 25% tax-free lump sum is worth, and optimal drawdown order (GIA first to use CGT allowance, then ISA, then SIPP last)
Employer pension match: enter your workplace contribution % and your employer's match — see total annual pension contribution and how it compounds
Scenario comparison: conservative (4%), moderate (6%), aggressive (8%) — plan for the range
Works in Google Sheets (free) and Excel

Who This Is For

UK investors who have a mix of ISA, pension, and GIA accounts and want to see the combined projected outcome
Anyone maxing their ISA allowance who wants to know where the next pound should go — SIPP or GIA
Higher-rate taxpayers who want to see the real impact of pension tax relief on their long-term wealth
People approaching retirement who need to model the optimal drawdown order across account types

How It Works

Enter your age, target retirement age, and each account's current balance
Set monthly contributions per account — the sheet flags ISA/pension limits
Enter your expected return rate and total platform + fund fees
The projection shows real after-tax wealth at retirement, monthly income you can withdraw, and optimal drawdown order
Use the scenario tabs to see how contributing more to SIPP (for tax relief) vs ISA (for flexibility) changes your outcome