Product research and financial education from the PopaDex team.
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FIRE Calculator Methodology: Estimate Your FIRE Number and Date
A FIRE calculator answers two different questions:
- How large might your investment portfolio need to be?
- If your savings and assumptions hold, when might you reach that amount?
The answer is an estimate, not a retirement guarantee. PopaDex uses a deterministic projection: it applies the return, inflation, tax, and contribution assumptions you enter. It does not run Monte Carlo simulations or replay historical market sequences.
Open the free PopaDex FIRE calculator in another tab if you want to follow the formulas with your own figures.
FIRE planning in PopaDex
Keep the estimate connected to your actual net worth
The FIRE tracker separates accessible investments from pensions, records your assumptions, and turns the estimated date into progress you can revisit.
The FIRE number formula
In the calculator’s traditional withdrawal-rate model:
FIRE number = desired annual retirement income ÷ withdrawal rate
If you want a portfolio to provide 50,000 a year and choose a 4% initial withdrawal rate:
50,000 ÷ 0.04 = 1,250,000
This is the same arithmetic as multiplying annual spending by 25. It does not mean that 1,250,000 will fund every retirement indefinitely. It means that 50,000 is 4% of 1,250,000.
The widely cited 4% starting point comes from historical US-market research. William Bengen’s 1994 paper tested inflation-adjusted withdrawals against past market periods. The 1998 study by Philip Cooley, Carl Hubbard, and Daniel Walz tested several stock-and-bond mixes, withdrawal rates, and payout periods. Crucially, the researchers evaluated defined historical periods; they did not prove that one rate works forever or in every market. Read Bengen’s original paper and the Cooley, Hubbard, and Walz study.
For an early retirement that may last longer than 30 years, test more than one withdrawal rate. A lower rate raises the target; a higher rate lowers it. Neither choice removes market, longevity, or spending risk.
| Desired annual retirement income | 3.0% rate | 3.5% rate | 4.0% rate |
|---|---|---|---|
| 30,000 | 1,000,000 | 857,143 | 750,000 |
| 50,000 | 1,666,667 | 1,428,571 | 1,250,000 |
| 80,000 | 2,666,667 | 2,285,714 | 2,000,000 |
Use the table as a sensitivity check, not a recommendation. Your chosen currency does not change the formula.
How PopaDex estimates the date
The calculator starts with these inputs:
- current age;
- accessible investments;
- pension or age-restricted investments;
- annual income and spending;
- desired annual retirement income;
- pension access age and life expectancy;
- the share of contributions going to pensions;
- estimated tax rates for accessible and pension assets;
- expected nominal investment return, inflation, and withdrawal rate.
It derives the savings rate as:
(annual income - annual expenses) ÷ annual income
Annual contributions are then estimated as annual income multiplied by that savings rate. The calculator divides contributions between accessible investments and pensions according to the pension contribution percentage you choose.
Real-return assumption
For the timeline, PopaDex uses a simplified real return:
real return = nominal investment return - inflation rate
With a 7% nominal return and 2% inflation, the projection grows balances at 5% per year in today’s-money terms. This arithmetic shortcut is easy to understand, but it is not the exact inflation-adjusted return formula. It also assumes the same return every year.
Real markets do not behave that way. The SEC notes that investment returns vary, losses are possible, and even long-run averages can hide long periods of poor performance. Fees reduce the amount left to compound as well. See the SEC’s Saving and Investing guide and investor bulletin on fees.
Tax estimate
In the traditional model, the calculator estimates a weighted tax rate from the current split between accessible and pension assets. It then grosses up desired retirement income:
gross retirement income = desired net income ÷ (1 - estimated tax rate)
This is a planning shortcut. It does not model tax brackets, allowances, capital-gains basis, account-specific withdrawal ordering, social insurance, or future tax-law changes. The calculator also assumes a 20% tax benefit on pension contributions and adds that estimated benefit to accessible contributions. Replace the default tax inputs with figures appropriate to your circumstances, and get professional advice before acting on a tax strategy.
Traditional withdrawal-rate model
The traditional option calculates the gross retirement income target, divides it by the chosen withdrawal rate, and projects balances one year at a time.
Before pension access age, only the accessible balance counts toward the projected target. At or after pension access age, the projected accessible and pension balances are combined. Each simulated year applies the smooth real return and adds the estimated contributions.
This makes the result useful for comparing scenarios. For example, you can see the direction and approximate scale of changing spending, contributions, or inflation. It is not a probability-of-success score.
Bridge-to-pension drawdown model
The alternative drawdown model is designed for someone with both accessible investments and money locked until a pension access age.
For each possible retirement year, it checks two conditions:
- Are the accessible investments large enough to fund annual withdrawals until pension access age?
- Is the pension balance large enough to fund the period from pension access age to the life-expectancy input?
The bridge amount uses the present value of an annuity due, which assumes a withdrawal at the start of each year:
bridge amount = annual gross withdrawal × [(1 - (1 + r)^-n) ÷ r] × (1 + r)
Here, r is the simplified real return and n is the number of years until pension access. If the real return is zero, the model uses annual gross withdrawal multiplied by the number of bridge years.
The pension test uses the same annuity-due approach from pension access age through the life-expectancy input. The calculator stops at the first whole year in which both balances pass their tests. Because it checks at annual intervals, the displayed accessible balance can be higher than the theoretical minimum.
This model intentionally allows planned depletion. It is more sensitive to the access-age, life-expectancy, return, tax, and spending inputs than the traditional target. It does not model statutory pension income or other guaranteed income unless you adjust the spending need outside the calculator.
What the calculator does not model
The current version does not include:
- random or historical sequences of annual returns;
- Monte Carlo probability testing;
- investment fees as a separate input;
- detailed country-specific tax rules;
- Social Security, state pension, annuity, rental, or part-time income streams;
- healthcare shocks or long-term care;
- changes in spending over different retirement phases;
- exchange-rate changes between currencies;
- rebalancing, asset allocation, or asset-class-specific returns;
- contribution limits or early-withdrawal penalties.
You can partially account for some omissions by using a lower net return, higher spending, or a lower withdrawal rate. That still does not turn the output into a forecast of what markets will do.
How to use the result without fooling yourself
1. Separate money by access date
Do not treat a pension you cannot access for 20 years as if it were available tomorrow. Put taxable brokerage accounts, ISAs, and other accessible investments in the early-access field. Put 401(k)s, IRAs, SIPPs, and similar restricted accounts in the pension field, subject to the rules that apply to you.
2. Use spending, not salary, for the target
The portfolio needs to fund what you expect to spend after tax. Start with at least 12 months of actual household spending, then add costs that may change in retirement, such as healthcare, housing, travel, dependants, and tax.
3. Deduct fees from the return assumption
If your expected return is before investment and advice fees, reduce it before entering the figure. Small annual fees compound into meaningful differences over long periods.
4. Run a range, not one optimistic case
Keep one base case, then test a lower return, higher inflation, higher spending, and a lower withdrawal rate. If the plan only works with the rosiest inputs, the date is fragile.
5. Review after life changes
Update the inputs after material changes to income, spending, family, location, pension rules, tax, or portfolio value. A FIRE estimate should move when the underlying facts move.
FIRE calculator versus FIRE tracker
A calculator is best for a one-off scenario. A tracker is useful when you want to compare that scenario with current net worth over time.
The PopaDex FIRE tracker app keeps the target, accessible investments, pensions, currencies, and progress in one workflow. It is the commercial product page for people choosing an ongoing FIRE tracking tool. This methodology page exists to document how the estimate is produced and where it can fail.
Frequently asked questions
Is 4% a guaranteed safe withdrawal rate?
No. It is a historical rule of thumb tied to specific portfolios, markets, withdrawal patterns, and time horizons. Use it as one scenario, not a promise.
Does PopaDex run a Monte Carlo simulation?
No. It applies a constant real-return assumption to a deterministic year-by-year projection. That makes scenarios easy to compare, but it does not show the probability of success across volatile market paths.
Why are pensions separate from accessible investments?
Access restrictions matter if you plan to stop working before pension age. The bridge model tests whether accessible investments can cover the gap while the pension balance continues to grow.
Should I include my home in current savings?
Only if your plan clearly turns home equity into spendable retirement assets, for example through a planned sale or downsizing. A home you intend to keep does not directly fund portfolio withdrawals.
Is the result financial advice?
No. It is general planning information based on the inputs you choose. Retirement, tax, and investment decisions can have lasting consequences; consult qualified professionals where appropriate.
Calculate your FIRE number and date, then use the FIRE tracker app if you want to keep the estimate connected to your financial picture.