An employer match is the only guaranteed return in personal finance. A plan matching 50 cents on the dollar hands you a 50 percent return the moment the money lands, before it has been invested in anything. Nothing else on this site comes close, which is why this is the first calculator anyone with a 401(k) should run.
The obvious mistake is contributing below the threshold your employer matches to. The less obvious one, and the reason this calculator walks the year pay period by pay period rather than annualising it, is front-loading. Contribute aggressively enough and you hit the IRS cap in September, at which point your contributions stop for the rest of the year. On most plans the match stops with them, so October, November and December arrive with nothing from your employer.
That second one costs high earners real money every year and almost nobody notices, because from the outside it looks like doing the right thing faster.
At the absolute minimum, enough to capture the full employer match, because anything below that is declining part of your compensation. Beyond that the honest answer depends on whether you have high interest debt and an emergency buffer, both of which come first. Once those are handled, 15 percent of gross pay including the match is the figure most planning models converge on for a normal retirement age.
A true-up is a provision that looks at your contributions across the whole year and pays any match you missed by hitting the cap early, usually as a lump sum after year end. It exists precisely because front-loading is a known trap. Plenty of plans have one and plenty do not, and the only reliable way to find out is your summary plan description or a direct question to HR. Assuming you have one when you do not is an expensive assumption.
No, and this trips people up constantly. The elective deferral limit applies only to your own money. The match sits under a separate and much larger combined limit covering everything that goes into the account from both sides. So the match does not eat into what you can put in yourself.
The excess has to come back out, and if it is not corrected by 15 April of the following year it gets taxed twice, once in the year contributed and again when eventually distributed. Payroll usually catches this automatically within one employer. It goes wrong when you change jobs mid-year, because your new employer has no idea what you already contributed at the old one. Tracking that is your job, not theirs.
Contribute to the 401(k) up to the full match first, always, because no tax treatment beats free money. After that the Roth IRA is often the better next dollar, since you choose the investments and the fees rather than accepting whatever menu your plan offers. Return to the 401(k) once the Roth is full.
SECURE 2.0 created a larger catch-up contribution for a four year window covering the calendar years you turn 60, 61, 62 and 63. It is meaningfully bigger than the standard catch-up available from 50, and it disappears again at 64, when you revert to the ordinary one. If you are in that window it is the largest contribution allowance you will ever have.
401(k) calculator, 2026 limits
Two ways people give up free money. Contributing below the match threshold, and hitting the IRS cap before December so the last months of match never arrive. This checks both.
The most common formula in the US is 50 percent of the first 6 percent. Your plan document calls this the matching contribution formula.
You put in
$4,500
Employer adds
$2,250
Left on the table
$0
You are capturing the full match, $2,250, and your contributions run to the last pay period of the year. Nothing is being left behind.
| Your contribution limit | $24,500 |
| Catch-up, not yet, from 50 | — |
| Your total cap | $24,500 |
| Everything combined, yours plus your employer's | $72,000 |
$6,750 going in this year. See whether that reaches your retirement number, or what the same money does tax-free in a Roth.
The year is walked one pay period at a time rather than annualised, because that is the only way the front-loading problem shows up. Each period you contribute your percentage of that period's pay, and your employer matches whatever share of it falls inside the match limit for that same period. When your running total reaches the IRS cap, contributions stop, and so does the match.
Limits are the 2026 figures from IRS Notice 2025-67. Elective deferral $24,500 under section 402(g), catch-up $8,000 at 50 and over, $11,250 for ages 60 to 63 under SECURE 2.0, annual additions $72,000 under 415(c), and match is computed on pay up to the 401(a)(17) compensation limit of $360,000.
What this does not model: vesting schedules, after-tax and mega backdoor contributions, non-elective or profit-sharing contributions, or plans that match on an annual rather than per period basis. Your numbers never leave your browser.