The model

How ExitAge calculates your exit date.

A plain-language explanation of the model behind the number.

Starting point

The question we start with

Most retirement calculators ask: how big will your portfolio be? ExitAge asks a different question — at your current trajectory, how many years will your money last, and does that get you to where you want to go?

The answer is your retirement runway. Your exit date is the age at which your runway is long enough.

How it works

A month-by-month simulation of your financial life.

ExitAge runs a deterministic, month-by-month simulation from today through your chosen life expectancy. Deterministic means it produces a precise outcome based on the inputs you provide, rather than running thousands of randomised probability scenarios. Your numbers in, your numbers out. The simulation has two connected phases.

Phase one
Pre-retirement growth
From today until your retirement age, ExitAge projects how your assets grow and your debts reduce. Each asset compounds monthly at its own return rate — because a savings account, an investment portfolio, and a rental property do not all behave the same way. Monthly contributions are added until you stop working. Debts accrue interest and are reduced by their scheduled repayments. The result is your net wealth at the moment you retire.
Phase two
Retirement drawdown
From retirement age through your life expectancy, ExitAge simulates how you spend from that wealth. Your retirement spending is drawn down month by month, adjusted for inflation each year. If you have income sources in retirement — a pension, rental income, part-time work — those offset your withdrawals. Assets continue to earn a return during this phase. The model tracks whether your wealth sustains your lifestyle through your full life expectancy, or the age at which it runs out.
Design decisions

What makes it precise

Individual asset return rates
Every asset carries its own pre-retirement and post-retirement return rate. This matters because your asset allocation changes as you approach retirement, and treating everything as a single blended rate introduces meaningful error over a 20–30 year horizon.
Monthly compounding
All calculations use monthly compounding — annual rates are converted to their monthly equivalent. Over decades, this produces materially more accurate results than annual compounding, which can overstate or understate portfolio values depending on the timing of contributions and withdrawals.
Inflation applied to spending
Retirement spending is inflated forward from today's dollars. A lifestyle that costs £4,000 per month now will not cost the same in fifteen years. The model inflates each spending item through your entire retirement, so the figures reflect real purchasing power, not nominal cost.
Liabilities are modelled, not ignored
If you carry a mortgage or other debt into retirement, the model accounts for ongoing repayments and interest accrual. These are real cash demands on your assets that most calculators simply omit — and omitting them produces an optimistic result that reality will correct.
Partner timing
If you plan with a partner, each person's assets, contributions, and retirement age are tracked individually. Contributions stop at each person's retirement age, and the drawdown phase reflects when both have retired — not an averaged assumption.
Retirement spending stages
Spending in retirement is not static. The model supports staged spending — your outgoings in your sixties are likely different from your eighties. You can model these shifts explicitly rather than assuming a flat spend for thirty years.
Scope and limits

What the model does not do

ExitAge is not a Monte Carlo simulator
It does not run thousands of randomised market scenarios to produce a probability distribution of outcomes. It produces a single projection based on your inputs. This is deliberate: the aim is clarity, not false statistical precision. If you want to stress-test your plan, the Market Shock feature lets you model a specific downturn scenario directly.
ExitAge does not model tax
Returns, withdrawals, and contributions are modelled on a pre-tax basis. Tax treatment of retirement assets varies significantly by jurisdiction, account type, and individual circumstances — this is precisely where independent financial advice adds the most value.
ExitAge does not provide financial advice
The projections are estimates based on the inputs and assumed return rates you provide. They are not guaranteed outcomes. ExitAge is a planning tool — it tells you what your current trajectory implies, and lets you explore what changes it would take to move your exit date. Always seek independent financial advice before making investment or retirement decisions.
The result

The output

The engine produces a single runway result — and from either outcome, you can adjust your inputs to see exactly which levers move your exit date.

Sustainable
Your assets last the distance
Your wealth sustains your retirement lifestyle through your chosen life expectancy, with a positive balance remaining. Your current plan works.
Depleting
Assets are exhausted before life expectancy
The model shows the age at which your wealth runs out under your current assumptions. Adjust your inputs to find the changes that close the gap.

The model is fast enough that exploring your plan is the experience. Every adjustment — retirement age, spending, contributions, return rates — recalculates instantly so you can see which levers have the most impact on your exit date.

ExitAge is built by a chartered accountant with 20+ years in financial services, who ran these calculations for himself before building them into software. The methodology reflects what a financially literate person would model on a spreadsheet — except it runs in seconds, and it never stops updating as your life changes.

Warren Brown — Founder, ExitAge · FCCA
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