Methodology

How it works

This simulator gives you a realistic, high-level view of how your retirement plan could have performed across many different historical periods. It is designed to be clear and approachable, not overly technical.

What is financial independence?

Financial independence means your invested assets can sustainably cover your spending, so work becomes optional. It is often estimated using a withdrawal rate such as 3.5% or 4%, but real sustainability depends on market history, inflation, taxes, and portfolio allocation.

What is the FI number?

The FI number is the estimated portfolio size required to fund your annual spending without employment income. It is commonly calculated as spending divided by a baseline withdrawal rate (for example 4%), and shown alongside resilience results.

1. We use real historical data

We use long-run historical market data for UK, US, and global assets. Each year in the dataset represents how those assets actually performed in the past. This helps us model realistic ups and downs.

2. We blend your portfolio mix

Your allocation (stocks, bonds, cash) is used to calculate a single “blended” return for each year. If you choose more stocks, your results will tend to be higher and more volatile. More bonds usually means smoother results.

3. We run many historical scenarios

We don’t rely on just one market period. Instead, we test your plan across many different historical windows to see how it might have held up under a range of conditions.

4. We adjust spending for inflation

Spending is adjusted for UK inflation so the buying power stays consistent over time. This keeps the results grounded in real-world costs.

5. We account for annual fees

You can include a simple annual fee to reflect fund and platform costs. This reduces portfolio returns by the fee rate each year to keep projections realistic.

6. We apply UK tax rules at a high level

The simulator includes simplified UK tax rules and common account types. This is not personal tax advice, but it helps produce more realistic estimates for UK residents.

Limitations

This tool is a historical backtest, not a forecast. It uses simplified UK tax rules and does not model every pension strategy or edge case. It is best used for planning and comparison rather than personalised advice.

The goal is to offer a clear, honest picture of what could happen—not a promise. This tool is best used for planning, comparison, and stress-testing your ideas.

Data sources

We use multiple academic and market datasets depending on the asset class. The sources below are summarized in plain language:

UK equities: JST UK dataset.
US equities: Shiller S&P 500 total return.
Global equities: JST global proxy (1871–2000) + MSCI World GBP (2001–present).
UK bonds: JST UK government bonds.
Global bonds: JST global bonds (1871–2000) + IGLO/AGGG global bond index (2001–present).
Inflation: UK CPI (JST).