401(k) Calculator

401(k) Calculator is a free, browser-based tool built for one job: project retirement savings with employer match, growth & the 4% rule. It is part of ToolsMonk's calculators collection, so you can finish the work without downloading software, creating an account, or jumping between multiple websites.

People reach for it most often when they need deciding what contribution percentage to set when you start a job, checking you are contributing at least enough to capture the full employer match, and seeing what a one or two percent increase compounds into over a career. With nothing to install, an unexpected one-off job takes no longer to start than your regular routine.

The design goal is simple: practical output on the first try, with a privacy-friendly workflow and the rest of the ToolsMonk library one click away.

The 401(k) Calculator projects a retirement balance from your salary, contribution rate, employer match, current balance and an assumed rate of return, and splits the result into what you contributed, what your employer contributed and what growth added. It also applies the 4 percent rule to give a rough sense of the annual income such a balance might support.

The employer match is the part worth acting on immediately. A common arrangement, 50 percent of the first 6 percent of salary, is an instant 50 percent return on that slice of money before any market growth at all. Nothing else available to an ordinary saver comes close. Contributing below the match cap means declining part of your own compensation, and the calculator makes the size of that gap visible.

The assumed return is the input that dominates the output, and it deserves scepticism rather than a single confident number. Long-run US equity returns have averaged roughly 10 percent nominal and 7 percent real, but that average spans decades containing severe and prolonged drawdowns, and it describes the past rather than predicting the future. Running the projection at several rates, say 5, 7 and 9 percent, is far more informative than picking one, because the spread is the honest answer.

Inflation deserves the same care. A projection in nominal terms produces impressive-looking numbers that overstate what the money will buy, because forty years of inflation is a large multiplier. If you want an answer in today's purchasing power, enter a real return rather than a nominal one and read the result accordingly.

Two things the projection deliberately does not model. Annual IRS contribution limits change most years and a percentage-of-salary contribution can quietly exceed them at higher incomes, so check the current figure if that might apply. And plan fees, which compound against you exactly as returns compound for you, are not included; a difference of half a percent in fees over a career is a meaningful sum.

The 4 percent rule is a useful rule of thumb and no more. It came out of research on a particular market history and a particular asset mix, later work has argued for both higher and lower safe rates, and it says nothing about your own tax position or spending. Treat the withdrawal figure as an order of magnitude that tells you whether you are roughly on track, not as a number to plan a retirement around.