NumberToRetire.com  ·  8 min read

How to Calculate Your FIRE Number With Multiple Income Sources

The standard FIRE number formula — 25 times your annual expenses — assumes your portfolio is your only income source in retirement. The moment you add rental income, a pension, a side business, or part-time work, the formula breaks down. Here is how to calculate it correctly when your retirement picture is more complicated than a single number.

Why 25x Is a Starting Point, Not the Answer

The 25x rule comes from the 4% safe withdrawal rate — the idea that a diversified portfolio can sustain annual withdrawals of 4% of its starting value indefinitely. Spend $80,000 per year, need $2,000,000 saved. Simple and useful as a rough benchmark.

The problem is what it assumes: that every dollar of your retirement spending comes from portfolio withdrawals. For a growing number of people — those with rental properties, pensions, side income that continues into retirement, or plans for part-time work — this assumption overstates how much portfolio you actually need.

The correct version of the formula is not 25 times your total expenses. It is 25 times your portfolio-dependent expenses — the spending that your investments actually need to cover after all other income sources are accounted for.

The Adjusted FIRE Formula

Start with your annual spending target. Then subtract any income that will continue in retirement and does not come from your portfolio. The remainder is what your portfolio needs to fund. Multiply that by 25.

That $600,000 difference is not a rounding error. It represents years of additional work if you ignore it, or years you could have retired earlier if you model it correctly.

Each Income Source Has Its Own Logic

The adjustment is straightforward in concept but requires careful treatment for each income type. Not all non-portfolio income is created equal — each source has a different start date, inflation behavior, and degree of reliability.

Rental income. Net rental income (after expenses) that continues after retirement reduces your required withdrawal dollar for dollar. The key modeling question is whether it adjusts with inflation. If your rent tracks inflation over time, your real spending gap stays constant. If it is flat, the portfolio needs to cover an increasing share of expenses as inflation erodes the rental's real value over a 20- or 30-year retirement. See how rental income changes your FIRE number for a full breakdown.

Pension income. A pension with a COLA (cost-of-living adjustment) behaves like inflation-adjusted rental income — it reliably covers a fixed portion of your expenses in real terms. A pension without a COLA erodes in real value each year. The start date also matters: if your pension begins at 65 and you plan to FIRE at 50, you have a 15-year gap to bridge entirely from portfolio withdrawals before the pension kicks in. See how much extra to save when you have a pension for a deeper look at this gap problem.

Side income or part-time work. Income from freelance work, consulting, or part-time employment after retirement is real but it is the least reliable of the three. It depends on your health, market demand, and continued willingness to work. It is reasonable to include it in your model, but with a defined end age and without inflation adjustment — model it conservatively. For a full breakdown of how semi-retirement changes your FIRE number, see how part-time income changes your retirement number.

Social Security. Social Security functions like a COLA pension — it is inflation-adjusted and starts at a specific age. Most people planning to FIRE early will not draw it until 62 at the earliest, and often 67 or 70. The gap between FIRE age and Social Security start age is the most commonly underestimated planning challenge in early retirement. If most of your savings are in retirement accounts, also see how to bridge the gap before 59½.

The Start Age Problem

The adjusted FIRE formula works cleanly when all your income sources start at the same time you retire. In practice, almost no one has that alignment. Income sources stack in over time, each with its own start age.

Consider someone planning to FIRE at 48 with the following income sources:

This person's portfolio-dependent spending is not a single number — it changes four times over the course of retirement. From 48 to 58 they have rental plus consulting income. From 58 to 60 they lose the consulting. From 60 to 67 the pension adds a layer. At 67 Social Security fills in further.

A single adjusted FIRE formula cannot capture this. The correct approach is a year-by-year projection that models each income source activating at its start age and applies inflation adjustments where appropriate. The portfolio drawdown rate changes at each transition point.

Why This Changes When You Can Actually FIRE

The practical implication of modeling this correctly is that your FIRE date is often earlier than the 25x formula suggests — sometimes significantly.

If you have income sources that kick in during retirement, you do not need to fully fund all of your expenses from the portfolio on day one. You need enough portfolio to cover the gap in each phase until the next income source starts. This is a fundamentally different — and lower — savings target than 25x total expenses.

The Mistakes People Make With Multiple Income Sources

Counting income that is not guaranteed. Side income, freelance work, and part-time consulting feel reliable until they are not. Build your base FIRE number without them, then model them as a buffer that accelerates your timeline if they continue.

Ignoring the gap years. Pension and Social Security income that starts years after FIRE is not available from day one. The portfolio has to carry full weight during the gap. Model this explicitly rather than averaging the income over the full retirement period.

Using gross income instead of net. Rental income after expenses, pension income after any reduction for a survivor benefit, Social Security after tax — the numbers that matter are what you actually receive, not the headline figures.

Not accounting for inflation asymmetry. Income sources that are not inflation-adjusted become less valuable every year. A $1,000/month flat pension at age 60 is worth meaningfully less in real terms by age 80. If most of your non-portfolio income is not inflation-adjusted, your portfolio needs to compensate for the growing gap over time.

Running a single static calculation. The 25x formula gives you a snapshot. A year-by-year projection shows you the actual drawdown curve — where the portfolio is most stressed, how each income source changes the picture, and whether you actually have enough at each phase of retirement.

How to Model It

The right tool for this is a calculator that treats each income source as a separate input with its own start age, inflation behavior, and retirement dependency — not a single "other income" field that collapses everything into one number.

For each income source you plan to have in retirement, you need four inputs: the net annual amount, the age it starts, whether it adjusts with inflation, and whether it continues if you stop working before it begins. With those inputs, a year-by-year projection shows you exactly when your portfolio is under the most pressure and what your actual FIRE number is — not the 25x approximation, but the real number that accounts for the full shape of your retirement income.