Retirement calculator: will your savings actually last?

How this calculator works

This tool answers one question in three parts: what will you have, what will you need, and will the money last. It does that with a straightforward month-by-month simulation rather than a single compound-interest formula, because month-by-month is where inflation, raises, and withdrawals actually play out. Every figure updates the instant you change an input, so you can feel how each lever moves the outcome.

During your working years — the accumulation phase — the calculator steps forward one month at a time. Each month your existing balance earns your assumed pre-retirement return, then your contribution is added on top. Contributions themselves grow: if you save a percentage of pay, that percentage rides your rising salary; if you save a fixed dollar amount, it is indexed to inflation so it keeps its real value instead of quietly shrinking. Your salary grows each year by the raise rate you set. The result is the single “projected savings at retirement” figure.

At retirement the model flips into the drawdown phase. It works out the monthly income you actually need — your target budget, inflated to your retirement date, minus any other income such as Social Security or a pension. That net need then rises with inflation every month you are retired, while the remaining balance earns your (usually more conservative) post-retirement return. If the balance ever hits zero, the calculator records the age your money runs out.

The amount you need is not guessed from the drawdown loop — it is computed directly as the lump sum at retirement that would fund every inflating withdrawal down to exactly zero. Mathematically that is the present value of a growing annuity:

need = netMonthlyNeed × (1 + i) × (1 − ((1 + i) / (1 + r))^N) / (r − i)

where i is the monthly inflation rate, r is the monthly post-retirement return, and N is the number of months in retirement. When the post-retirement return and inflation are nearly equal, that formula is replaced by its stable limit, need = netMonthlyNeed × N. Subtract the amount you need from what you are projected to have and you get your surplus or shortfall — and if it is a shortfall, the calculator solves for the extra monthly contribution that would close it.

Every input, explained

Good inputs make good projections. Here is what each field means, where to find your real number, what a reasonable value looks like, and how much it actually moves the result.

Current age

Your age today, the starting line for the projection. Its main effect is time: every extra year of compounding before retirement matters more than almost anything else, which is why starting to save in your twenties beats saving more in your forties.

Annual pre-tax income

Your gross yearly pay before taxes and deductions. It anchors two things: how a percentage-based contribution grows, and — if you choose the percentage option — how big your retirement budget is. Use your steady base pay; if your income is lumpy, use a conservative average.

Current retirement savings

The total already earmarked for retirement: your 401(k), 403(b), IRAs, and any other retirement-specific accounts, added together. This is the number people most often get wrong by leaving accounts out. Do not include your emergency fund, checking balance, or home equity — those are not funding your retirement withdrawals in this model.

Monthly contribution

What you add to retirement each month — and this is the field most people underestimate, because you should include your employer match. If you contribute 6% and your employer adds 3%, that is 9% going in. Our 401(k) growth calculator breaks the match out separately and warns you if you’re leaving any of it unclaimed. Toggle between a percentage of pay and a fixed dollar amount; the calculator shows the live equivalent of whichever you are not typing. Contributions are one of the two most powerful levers here — small, sustained increases compound dramatically over decades.

Desired monthly budget in retirement

How much you want to spend each month once retired, in today’s dollars. The familiar “70% of pre-retirement income” replacement rate is a reasonable starting point — you typically stop saving for retirement and may have a paid-off home — but it is a heuristic, not a law. If you plan to travel heavily or expect high healthcare costs, budget more; if you will have a modest, paid-off lifestyle, you may need less. This is the other high-impact lever: the budget drives the entire “amount needed” figure.

Other monthly retirement income

Income you will receive in retirement outside your own savings: Social Security, a pension, rental income, or an annuity. Enter it in today’s dollars; the model inflates it to your retirement date and keeps adjusting it upward each year (a cost-of-living assumption). Every dollar here is a dollar your personal savings does not have to provide, so it can meaningfully shrink the amount you need.

Retirement age

When you plan to stop working and begin drawing down. It is unusually sensitive: raising it gives you more years to save and fewer to fund, and vice versa. Even one or two years can swing a shortfall into a surplus.

Life expectancy

The age your plan should fund you through. Planning to the mid-90s is prudent — roughly half of 65-year-olds will live past their mid-80s, and running out at 88 is a real risk if you plan only to 85. Longer horizons raise the amount you need.

Pre- and post-retirement returns

Your assumed average annual investment return before and during retirement. The post-retirement rate is usually lower, reflecting a more conservative allocation once you are living off the portfolio. Returns are the most uncertain input of all — nudge them down a couple of points to see whether your plan still holds. A plan that only works at an optimistic return is a fragile plan.

Annual inflation

How fast prices rise each year. It is applied to your target budget, your withdrawals, and your other income, so it shapes the entire “needed” side of the equation. The long-run average is around 3%. Its effect compounds quietly but powerfully — which is exactly why the today’s-dollars toggle exists.

Annual income increase

The average yearly growth of your salary, from raises and promotions. It lifts future income and, if you save a percentage, your future contributions. A rate modestly above inflation (say 2–3%) reflects a typical career; flatten it if your pay has plateaued.

Assumptions and limitations

Every calculator makes assumptions; the honest ones say so out loud. Here is what this model does and, just as importantly, what it does not.

• It is a straight-line, deterministic projection, not a Monte Carlo simulation. It assumes your returns arrive smoothly every month at the rate you set. Real markets do not — they lurch, and the order in which good and bad years arrive matters.
• It ignores sequence-of-returns risk. A market crash early in retirement is far more damaging than the same crash later, because you are selling assets while they are down. A smooth average hides that danger entirely.
• It ignores taxes and fees. Withdrawals from traditional accounts are taxable, Roth withdrawals generally are not, and fund fees skim returns every year. Real spendable income will differ from these pre-tax, pre-fee figures.
• It does not model market volatility, healthcare shocks, or long-term care. These are among the largest risks to a real retirement and none of them appear in a straight-line model.
• The assumed return is an input, not a promise. Entering 6% does not make 6% happen. Treat the output as a framework for thinking, and stress-test it with more conservative numbers.

Used with that context, a deterministic calculator is genuinely useful: it builds intuition, shows the direction and rough magnitude of each decision, and tells you whether you are in the right ballpark. It is a compass, not a GPS.