How is the FIRE number calculated?

It divides your annual retirement spending by your safe withdrawal rate. For example, £40,000 a year at a 4% withdrawal rate gives a target of £1,000,000.

The 4% rule suggests you can withdraw 4% of your portfolio in the first year of retirement, adjusted for inflation thereafter, with a low chance of running out over 30 years. You can change the rate to be more or less conservative.

Why does it say I never reach the target?

If your monthly contribution plus the growth on your current balance does not exceed what the target requires, the timeline goes to infinity. Raise your monthly contribution or your expected return, or lower your target spending, and a finite timeline will appear.

Does the FIRE number account for inflation?

The target itself is in today's money. Because the safe withdrawal rate assumes you raise withdrawals with inflation each year, the portfolio is designed to keep pace — but the calculator does not separately inflate your spending figure, so review it periodically.

What expected return should I use?

A diversified portfolio is often modelled at 5–7% a year after fees but before inflation. Using a lower, more conservative return lengthens the timeline but gives a safer plan.

Is my income or savings data stored?

No — all calculations run entirely in your browser and nothing you enter is uploaded or saved on a server.

FIRE Retirement Calculator

FIRE retirement calculator

FIRE stands for Financial Independence, Retire Early — reaching a portfolio big enough that investment returns cover your living costs indefinitely. This calculator does two things: it computes your FIRE number (the target portfolio) from your spending and withdrawal rate, then estimates how many years of saving it takes to get there. It is for anyone planning early retirement or financial independence who wants a concrete target and timeline.

How it works

The target uses the safe-withdrawal-rate rule:

FIRE number = annual spending ÷ (safe withdrawal rate ÷ 100)

To find the timeline, it grows your current savings and monthly contributions at the monthly rate i = annual return ÷ 12 ÷ 100 and solves for the number of months where the balance first reaches the target. The closed-form solution is:

months = ln((target × i + PMT) ÷ (current × i + PMT)) ÷ ln(1 + i)

where PMT is your monthly contribution. If you are already at or above the target it returns zero; if contributions can never close the gap it flags the target as unreachable.

Example

Spending 40,000 a year with a 4% withdrawal rate gives a FIRE number of 40,000 ÷ 0.04 = 1,000,000. Starting with 50,000, adding 1,500 a month at a 6% return reaches that million in roughly 21 years 11 months.

Annual spending	Withdrawal rate	FIRE number
30,000	4%	750,000
40,000	4%	1,000,000
40,000	3.5%	1,142,857
60,000	4%	1,500,000

The 4% rule comes from research suggesting a portfolio can sustain 4% annual withdrawals (rising with inflation) for 30+ years. Lower the rate to 3–3.5% for a more conservative, longer-lasting plan.

FIRE variants — which number to target

The classic FIRE number assumes indefinite retirement from any paid work. Several popular variants adjust it:

Variant	Description	Effect on target
Lean FIRE	Very frugal lifestyle, minimal spending	Lower target — shorter timeline
Fat FIRE	Comfortable or luxury lifestyle, higher spending	Much larger target
Barista FIRE	Semi-retire; part-time or flexible work covers basic costs	Partial target — smaller portfolio needed
Coast FIRE	Save enough early so contributions stop; growth alone reaches the target by traditional retirement age	Target is the “coast number” at current age

If you plan part-time income in early retirement (Barista FIRE), you can reduce the annual spending figure in the tool by the income you expect to earn, which shrinks your required portfolio and shortens the timeline.

The math behind the timeline

The calculator uses a closed-form compound-growth formula rather than a year-by-year simulation:

months = ln((target × i + PMT) / (current × i + PMT)) / ln(1 + i)

where i is the monthly interest rate (annual return / 12) and PMT is your monthly contribution. This gives an exact answer for a fixed rate. Real markets are volatile — in some years the portfolio will grow faster, in others it will shrink. A Monte Carlo simulation (running hundreds of market scenarios) would give a probability distribution rather than a single number, but the formula gives a useful central estimate.

Sensitivity: which inputs matter most

Spending is the most powerful lever. Because the FIRE number is annual spending divided by withdrawal rate, reducing spending cuts the target and simultaneously reduces the monthly saving needed. A £5,000/year reduction in planned retirement spending cuts the FIRE target by £125,000 at a 4% withdrawal rate.

Return assumptions matter over long timelines. At a 5% return, £50,000 of current savings takes roughly 35 years to become £1,000,000 without contributions. At 7%, that shrinks to about 27 years. Over a 20-30 year accumulation phase, even a 1% difference in expected return shifts the timeline by several years.

Withdrawal rate affects both the target and sustainability. A 4% withdrawal rate gives a £1M target for £40K spending. Dropping to 3.5% raises the target to about £1.14M but gives a more conservative, longer-lasting plan — important if you retire at 35 and need the portfolio to last 60+ years.

Everything is computed in your browser, so none of your financial figures ever leave your device.