Reverse FIRE Calculator
Work backwards from a target retirement age to find the exact monthly savings required. All calculations happen locally — nothing leaves your browser.
What is a reverse FIRE calculation? A reverse FIRE calculation fixes the retirement date and solves for the savings rate, rather than fixing the savings rate and solving for the date. It works out your FIRE number as annual expenses divided by your safe withdrawal rate, grows your existing savings forward at a real return, and then solves the future-value-of-an-annuity formula for the constant annual contribution that closes the remaining gap exactly on your target birthday.
How to Use the Reverse FIRE Calculator
- Set your current age and target FIRE age — The difference between them is the whole contribution window. The target must be later than your current age, and shortening the window raises the required saving far faster than you would guess — a decade earlier can more than double the monthly figure.
- Enter what you have already saved — Everything invested for retirement, across every account. This balance compounds on its own for the whole window, so it does more work the earlier you start and shrinks the amount you have to add.
- Enter your annual expenses in retirement — This is the input that sets the target, not your current salary. Use what you expect to spend in a year once you stop working, in today's money — the calculator works in real terms throughout.
- Set the return, inflation and withdrawal rate — Return and inflation are combined into a real return, so a 7% return with 3% inflation is treated as about 3.88%. The withdrawal rate decides the multiple: 4% means 25 times expenses, 3.5% means about 28.6 times.
- Read the required monthly figure against your actual income — The headline is what you must invest every month from now until your target age. If it is more than you can reasonably save, the target age is the input that has to move — not the return assumption.
- Check the sensitivity table before you commit — It re-runs the calculation two percentage points either side of your return assumption. The spread between those two rows is the honest uncertainty in the answer, and it is usually wider than people expect.
How the Required Savings Figure Is Calculated
Most FIRE calculators fix what you save and tell you when you finish. This one fixes the finish and tells you what you have to save, which is the more useful direction when the date is the thing you actually care about. It runs in four steps.
1. The FIRE number. Your target portfolio is annual expenses divided by the withdrawal rate:
FIRE number = annual expenses ÷ (withdrawal rate ÷ 100)At $45,000 of expenses and a 4% withdrawal rate that is $1,125,000 — the familiar 25× figure, arrived at by division rather than by rule of thumb.
2. The real return. Nominal return and inflation are combined properly rather than subtracted:
real return = (1 + r) ÷ (1 + i) − 1A 7% return with 3% inflation gives 3.8835%, not 4%. The difference looks trivial and compounds into tens of thousands over twenty-five years. Working in real terms is what lets every other figure on the page stay in today's money.
3. Growing what you already have. Existing savings are compounded across the window on their own: future value of current savings = savings × (1 + real return)^years If that alone clears the FIRE number, the tool reports $0 a month and says you are already on track — you have reached Coast FIRE and could stop contributing entirely.
4. Solving for the contribution. Whatever gap remains is closed by a level annual contribution, found by rearranging the future value of an ordinary annuity:
annual contribution = (FIRE number − grown savings) × real return ÷ ((1 + real return)^years − 1)The monthly headline is that annual figure divided by twelve. That is a deliberately slightly conservative presentation: because contributions are modelled once a year rather than every month, investing the same total in twelve instalments would compound a little more and get you there marginally sooner.
A worked example
Age 30, targeting 55, with $40,000 saved and $45,000 of annual expenses, at 7% return and 3% inflation with a 4% withdrawal rate. The FIRE number is $1,125,000. The real return is 3.88%, so over 25 years the existing $40,000 grows to about $103,700 on its own — a useful reminder that a modest starting balance is not the deciding factor over a long window. The remaining $1,021,300 requires about $24,900 a year, or roughly $2,076 a month, invested consistently for twenty-five years.
The chart plots the projected balance against a dashed line at your FIRE number. Read the two headline figures rather than the exact point where the lines meet: the projection line is drawn on the nominal return while the FIRE number is expressed in today's money, so the crossing point sits earlier than your target age.
Why the Target Age Dominates Everything Else
Holding every other input steady and moving only the target age shows how unforgiving the arithmetic is. Using the worked example above — age 30, $40,000 saved, $45,000 expenses, 7% return, 3% inflation, 4% withdrawal:
| Target age | Years to save | Required per month | Required per year |
|---|---|---|---|
| 45 | 15 | $4,425 | $53,105 |
| 50 | 20 | $2,944 | $35,325 |
| 55 | 25 | $2,076 | $24,911 |
| 60 | 30 | $1,514 | $18,172 |
| 65 | 35 | $1,127 | $13,526 |
Retiring at 45 instead of 55 costs more than twice as much per month — $4,425 against $2,076 — even though the window is only 40% shorter. Compounding is the reason: the contributions made in the final decade barely have time to grow, so each one has to do almost all its own work. Pushing the target from 55 to 60 cuts the monthly requirement by about 27%, which is often the difference between a plan you can actually run and one you abandon in year three.
This is also why the honest response to an unaffordable result is to move the date rather than the return assumption. Raising expected return from 7% to 9% makes the required monthly figure drop, but it does not make the money appear — it just relocates the risk into an assumption you cannot control. Lowering expenses is the one change that helps twice, because it cuts the FIRE number and frees up money to contribute at the same time.
Choosing a Withdrawal Rate
The withdrawal rate is a single number that moves the target by hundreds of thousands, so it deserves more thought than the default. The widely quoted 4% figure comes from William Bengen's 1994 research in the Journal of Financial Planning and was reinforced by the 1998 Trinity Study, both of which tested historical US portfolios over a 30-year retirement. An early retiree planning for forty or fifty years is outside the horizon those studies examined, which is why lower rates are common in the FIRE community.
| Withdrawal rate | Multiple of expenses | FIRE number on $45,000 | Typically chosen by |
|---|---|---|---|
| 3.0% | 33.3× | $1,500,000 | Very long horizons, or people with no willingness to earn again |
| 3.5% | 28.6× | $1,285,714 | A common early-retirement compromise for a 40-plus year horizon |
| 4.0% | 25.0× | $1,125,000 | The classic figure, researched against a 30-year retirement |
| 4.5% | 22.2× | $1,000,000 | Shorter horizons, or plans with flexible spending |
| 5.0% | 20.0× | $900,000 | Late retirement, or portfolios backed by other income |
Moving from 4% to 3.5% adds $160,714 to the target on these numbers — a 14% larger portfolio for a half-point of caution. Whether that is worth it depends less on the arithmetic than on how much flexibility you have: someone willing to cut spending in a bad market year, or to earn again, is buying far less protection with a low withdrawal rate than someone with fixed obligations and no intention of returning to work.
Two things this model cannot see are worth holding in mind. It assumes a single steady real return every year, whereas real markets deliver their averages in a jagged sequence, and a poor first decade of returns hurts a retiree far more than a poor last decade — the sequence-of-returns problem. It also ignores tax entirely, both on the way in and on the way out, so a portfolio held largely in taxable accounts needs to be larger than one held in tax-advantaged accounts to fund the same spending.
This is an educational projection, not financial advice, and it is not a prediction of investment returns. It assumes a constant real return and models no tax, fees, market volatility or changes in circumstance. Speak to a qualified financial professional before making decisions about retirement, contributions or asset allocation.
Frequently Asked Questions
Reverse FIRE flips the usual question. A standard FIRE calculator takes your savings rate and tells you when you can retire; a reverse calculation takes the retirement age you want and tells you what you must save to get there. Mathematically it solves the future-value-of-an-annuity formula for the payment instead of for the number of periods. It is the more useful direction when the date is fixed by something in your life.
Annual expenses divided by the withdrawal rate expressed as a decimal. At $45,000 of expenses and a 4% rate that is $1,125,000, which is the same as multiplying by 25. Note that it is driven by your expected spending in retirement, not by your income — two people on identical salaries with different spending have very different FIRE numbers.
Because the target is expressed in today's money. Combining return and inflation into a real return — (1 + r) ÷ (1 + i) − 1 — keeps every figure on the page in the same units, so the FIRE number you see is the spending power you are aiming for rather than a nominal balance whose value you would then have to discount. A 7% return with 3% inflation is a 3.88% real return, not 4%.
It is exact for the assumptions you enter and only as good as those assumptions. It models a constant real return every year, level contributions, no tax, no fees and no change in circumstances. Real returns arrive in an uneven sequence, which the model cannot capture. Use the sensitivity table to see the range rather than treating the headline as a precise requirement.
No. Everything runs in JavaScript in your browser with no server call. Two local behaviours are worth knowing: your inputs are written into the page URL so a scenario can be bookmarked, and shared values such as age, savings, expenses and rate assumptions are saved in your browser's local storage so the other FIRE calculators on this site pre-fill consistently. Clear the URL before sharing a link.
It means your existing savings, compounded at the real return for the years remaining, will reach the FIRE number without any further contributions. That is the Coast FIRE position: you still need income to cover living costs until the target date, but you no longer need to invest for retirement. Use the Coast FIRE Calculator to see when you crossed that line.
The 4% figure was researched against a 30-year retirement. If you plan to stop at 45, you are funding 45 or 50 years, which is outside the horizon those studies tested, and many early retirees use 3.25% to 3.5% for that reason. The trade is concrete: dropping from 4% to 3.5% raises the target from $1,125,000 to $1,285,714 on $45,000 of expenses. Flexibility about spending is worth more than a fractionally lower rate.
Because contributions are modelled as one annual payment at the end of each year, and the monthly headline is that figure divided by twelve for readability. Investing the same total in twelve monthly instalments would compound slightly more and reach the target marginally sooner, so treating the monthly number as a target rather than a minimum leaves you a small margin rather than a shortfall.
Use Cases
Testing Whether a Target Age Is Realistic
You want out at 50. Enter it, read the required monthly figure, and compare it against what is actually left after rent and living costs. The answer is usually either obviously yes or obviously no, and both are useful.
Pricing the Cost of Retiring Five Years Earlier
Run the same inputs at two target ages and subtract. Seeing that five years costs an extra $900 a month turns a vague preference into a decision you can weigh against what that money would otherwise do.
Setting a Contribution Rate for a New Job
When a salary changes, the required monthly figure gives you a payroll deduction percentage to set on day one, rather than picking a round number and discovering years later that it was never enough.
Seeing What Lower Expenses Are Worth
Drop retirement expenses from $60,000 to $50,000 and the FIRE number falls by $250,000 at a 4% rate. Cutting spending is the only lever that shrinks the target and frees up contributions at the same time.
Agreeing a Plan With a Partner
Two people with different instincts about risk can each run the sensitivity table and see the same spread. Arguing about a range of outcomes is more productive than arguing about a single confident number.
An Annual Plan Review
Re-run it every January with your updated balance and a target age that is now one year closer. If the required monthly figure has fallen, you are ahead; if it has risen, you know before the gap becomes hard to close.