401k Calculator: What Your Paycheck Today Could Become by Retirement
A 401k calculator turns a few payroll numbers into a retirement picture you can actually judge: how much your account could hold when you stop working, how much of that total came from your own paychecks, how much your employer added, and how much came from growth doing its quiet work over decades. It answers the question most savers really ask, which is not "am I saving something" but "is what I am saving enough, and what happens if I change it."
The reason it surprises people is time. Money put in at 30 has 35 years to compound before 65. Money put in at 50 has 15. The same monthly amount can end up worth several times more simply because it started earlier, and no later burst of saving fully makes up for a late start. That is the argument for starting now, even with a small percentage, then raising it later.
How a 401k grows, in plain English
A 401k is a workplace retirement account. You choose a percentage of your pay, your employer takes it from each paycheck before income tax in a traditional plan, and the money goes into investments you pick from the plan menu. It grows without yearly tax on the gains. You pay income tax when you withdraw in retirement. Some plans also offer a Roth 401k, where you pay tax now and withdraw tax free later, but this calculator models the traditional shape: pre-tax contributions, tax-deferred growth, taxable withdrawals.
Three forces build the balance. Your contributions are the fuel. The employer match is free money on top, and it is the only part of investing that arrives as an instant return. Growth is the engine: each year the whole balance earns the assumed return, so later years add far more dollars than early ones even though the percentage stays the same.
Every input, explained
Current age. Your age now. It sets how many years the money has to grow.
Retirement age. The age you plan to stop contributing. The gap between the two ages is the whole runway.
Current 401(k) balance. What is already in the account. Even a modest starting balance matters, because it compounds for the full period.
Annual salary. Your gross pay for the year. Contributions and the match are both figured as percentages of this number.
Raise (%/yr). How much your salary is assumed to rise each year. Raises lift your dollar contributions even when your percentage stays fixed.
You contribute (% of salary). The share of pay you defer. This is the lever you control most directly.
Employer matches (%). The percentage of your contribution your employer adds. A 50% match means 50 cents from them for every dollar you put in, up to the limit.
Of the first (% salary). The match limit. If it reads 6%, the employer only matches contributions on the first 6% of your salary, no matter how much more you save.
Expected return (%/yr). The average yearly growth assumed for the investments. It's an assumption, not a promise, and real returns bounce around it.
A worked example: 30 to 65 on an $80,000 salary
Take a saver who starts at 30 with $25,000 already saved, earns $80,000, gets 3% raises, contributes 10% of pay, and works for an employer matching 50% of the first 6% of salary. Assume a 7% average return and retirement at 65, which gives 35 years of growth. The calculator works year by year: each year it grows the balance by 7%, then adds that year's contribution and match, then raises the salary by 3% for the next year.
- Your contributions. You start at $8,000 a year (10% of $80,000) and the amount climbs with each raise. Over 35 years you put in $483,696.65.
- The employer match. Your employer pays 50% on the first 6% of salary, which is 3% of pay each year. Over 35 years that adds $145,109.00.
- Growth. The balance at 65 is $2,311,221.48. Subtract your $483,696.65, the $145,109.00 match, and the $25,000 you started with, and growth supplied $1,657,415.83, about 72% of the final balance.
- What it pays. As a rough guide, withdrawing 4% a year from $2,311,221.48 gives about $7,704.07 a month before tax. That's a planning yardstick, not a withdrawal plan.
Milestones along the way
The balance does not grow in a straight line. It crawls at first, then climbs steeply, because each year's return applies to a bigger pile. Watching the milestones helps you stay patient in the slow early years, when the account looks like it is barely moving.
| Age | Balance | Salary that year |
|---|---|---|
| 35 | $98,315.98 | $90,040.70 |
| 40 | $211,219.88 | $104,381.85 |
| 50 | $633,271.15 | $140,280.48 |
| 65 | $2,311,221.48 | $218,552.42 |
The jump between 50 and 65 is bigger than everything saved in the first 20 years combined. That is compounding showing its hand late, and it is why pausing contributions in your fifties costs far more than pausing them in your thirties, even though the paused amounts look similar.
The match comes first, always
Run the same example with no employer match and the balance at 65 falls to $1,839,458.34. The match alone is worth $471,763.14 over 35 years, because each matched dollar also compounds. Turning down the match to save a little cash now is the most expensive choice in this whole guide, and it's also the easiest to fix: contribute at least up to the match limit before anything else.
Beyond the match, each extra point of contribution still pays, but without the instant bonus. The table below holds every other input fixed and varies only your contribution rate.
| You contribute | You put in | Balance at 65 |
|---|---|---|
| 6% (match limit) | $290,217.99 | $1,682,203.96 |
| 10% | $483,696.65 | $2,311,221.48 |
| 15% | $725,544.98 | $3,097,493.39 |
Common mistakes
- Contributing below the match limit. Every dollar below the limit leaves matched money on the table, and that money would have compounded for decades.
- Treating the projected balance as spendable cash. A traditional 401k balance is pre-tax. Withdrawals are taxed as income, so the spendable amount is smaller than the headline number.
- Using a heroic return assumption. A 10% or 12% return makes any plan look fine on paper. A 6% or 7% assumption is a sterner test, and a plan that passes at 6% is a plan you can trust more.
- Cashing out when changing jobs. Cashing out triggers tax plus, before age 59 and a half, usually a 10% penalty, and it wipes out decades of compounding on that sum. Rolling it into a new plan or an IRA keeps the clock running.
- Ignoring the salary growth in the projection. Flat-salary projections understate your contributions, because raises lift the dollar amount each year even at a fixed percentage.
Practical tips
Start with the full match, then build toward 15% of pay including the employer contribution. Check your investment mix once a year, switch on automatic escalation if your plan offers it, and keep an eye on fees: a fund charging 1% a year takes a large slice of growth over 35 years compared with one charging 0.1%.
Frequently asked questions
How much of my salary should I put in my 401k?
At minimum, contribute enough to collect the full employer match. From there, a common target is 15% of pay including the match. Our example at 10% plus a 3% match reaches 13%, close to that target, and lifting to 15% lifts the age 65 balance to $3,097,493.39.
What does a 50% match on the first 6% actually pay me?
It pays 3% of your salary each year, as long as you contribute at least 6%. On $80,000 that is $2,400 in the first year, rising with your salary. Contributing only 3% would halve the match, so the match limit, not your total rate, sets the floor.
Why is most of my projected balance growth, not contributions?
Because returns apply to the whole balance every year, including past returns. Over 35 years at 7%, growth supplied $1,657,415.83 of our $2,311,221.48 example, while contributions plus match supplied the rest. Shorter runways flip that split toward contributions.
Is the projected 401k balance what I can spend?
No. A traditional 401k is pre-tax, so withdrawals in retirement are taxed as ordinary income. The $2,311,221.48 in our example is the account balance, and the after-tax spending power depends on your retirement tax rate and any state tax.
What happens to my 401k if I change jobs?
The money stays yours, including vested employer matches. You can leave it, roll it into your new employer's plan, or roll it into an IRA. Cashing it out is the costly option, since tax and usually a 10% early penalty apply before age 59 and a half.
Related calculators: Inflation Calculator, IRA Calculator, and Pension Calculator.
A 401k rewards two boring habits above all else: starting early and never leaving the match behind. Get those two right, raise the rate when pay rises, and the arithmetic in this guide does most of the rest on its own.
Disclaimer: this guide explains 401k math for education only and is not financial, tax, or investment advice. Contribution limits, match rules, vesting, and tax treatment vary by plan and change over time, so confirm your own plan's rules and current IRS limits before making contribution decisions.
Further reading: the IRS explains how 401k plans work, including contribution limits and distribution rules, on its 401(k) plans page, which is worth checking before you change your deferral rate.