Type "how long will my money last in retirement calculator" into Google and you'll get dozens of tools that all ask for the same handful of numbers — but not all of them do the same math once you hit "calculate." Here's what's actually happening behind a good one, and why it matters for the number you get back.
What a retirement calculator needs from you
Almost every retirement calculator, including this one, asks for some version of the same four inputs: your current savings, how much you plan to spend, what return you expect your investments to earn, and an inflation assumption. What separates a rough tool from an accurate one is what it does with those numbers next.
The shortcut most calculators take
The fast way to estimate how long money lasts is a single algebraic formula that assumes a constant withdrawal and a constant return, solved for the number of years until the balance hits zero. It's quick, but it quietly assumes your spending never changes — which isn't how retirement actually works once inflation is in the picture.
The more accurate way: month-by-month simulation
A more accurate retirement calculator simulates your finances one month at a time, for up to several decades:
- Your balance earns its share of the annual return for that month
- Your withdrawal for that month comes out
- Once a year, your withdrawal amount increases to keep pace with inflation
- The loop repeats until the balance hits zero, or a set number of years passes
This is closer to how an actual bank or brokerage statement behaves than a single formula, and it's the approach this calculator uses. It's also the only reliable way to answer "how long will my money last in retirement" once you factor in that your cost of living keeps rising.
Why the assumptions matter more than the tool
Two people using the exact same retirement calculator can walk away with very different confidence levels, because the tool is only as good as the numbers going in. A 7% return assumption is optimistic for a conservative portfolio; 3% inflation is a reasonable long-run average but can run hotter for stretches of several years. It's worth running your numbers more than once — once with your expected case, and again with a more conservative return and higher inflation — to see how much the answer moves.
Questions worth asking any retirement calculator you use
- Does it recalculate month by month, or use a single simplified formula?
- Does your withdrawal amount increase with inflation, or stay flat the whole projection?
- Does it show you a year-by-year breakdown, or just a single final answer?
A retirement calculator can't predict the market. What it can do is make the math behind your assumptions honest.