Type your savings, monthly contribution, and years to retirement into any retirement calculator and it hands back a single, precise-looking number. It's tempting to treat that number as a prediction. It isn't -- it's the output of a formula built on assumptions you chose, and a couple of those assumptions do almost all the work in determining whether the final figure is realistic or wildly off.
The three inputs, and which one matters most
A retirement projection generally needs your current balance, your contribution rate, and your time horizon. Of the three, time horizon has the most leverage, because compounding is exponential, not linear. Money contributed in your 20s has decades to compound and ends up contributing far more to the final balance than the same dollar amount contributed in your 40s, even though it's the identical contribution. This is why "start small but start now" consistently beats "wait until I can contribute more."
The assumption doing all the hidden work
The rate of return you assume is the single biggest lever in the whole calculation, and it's also the input people think about the least. Bump an assumed annual return from 6% to 8% over a 30-year horizon and the projected ending balance doesn't nudge slightly -- it changes dramatically, because that difference compounds every single year. There's no way to know your actual future return in advance, so the honest way to use a calculator is to run it at more than one rate -- a conservative one and an optimistic one -- and look at the range rather than anchoring on a single output.
Employer match is part of the model, not a bonus on top
If your employer matches retirement contributions, that match needs to be an input in the projection, not something you mentally add afterward. A calculator that only counts your own contributions will understate your actual trajectory, sometimes significantly, since an employer match is effectively an immediate, guaranteed return before any market performance even factors in.
Nominal dollars vs. what they'll actually buy
A projection typically outputs a nominal dollar figure -- what the account balance will read in the future, not what that balance will be worth in today's purchasing power. A number that looks comfortable decades from now can be a lot less comfortable once you mentally adjust it for inflation between now and then. Some calculators let you apply an inflation-adjusted (real) rate of return instead of a nominal one; if yours doesn't, it's worth doing that adjustment yourself before treating the output as a lifestyle target.
Use it as a range, not a verdict
The most useful way to run a retirement calculator isn't once -- it's periodically, with updated numbers, and at a couple of different assumed rates each time. Treat the output as a directional check ("am I roughly on track, given reasonable assumptions?") rather than a fixed target to hit exactly. Because small changes in the rate-of-return assumption move the result so much, a single run at a single rate tells you far less than the same calculation re-run under a low and a high scenario.
The Retirement Calculator projects your balance from your current savings, contributions, and expected return -- run it at more than one assumed rate to see how much the outcome actually depends on that one number.