What Is a 401(k)?

A 401(k) is a retirement savings plan offered through your employer, named after the section of the tax code that created it. You agree to have a slice of each paycheck contributed automatically, before the money ever hits your checking account, and that money is invested for the long term. The defining feature is the tax treatment: contributions and growth are sheltered from tax in ways a regular brokerage account is not, in exchange for rules that nudge the money toward actually being used in retirement.

Two things make a 401(k) different from saving on your own. First, it is workplace-based, so you can only contribute through an employer that offers a plan, and the investment menu is chosen by that plan. Second, many employers match part of what you put in, adding money to your balance that you would not get in an account you opened yourself. If you do not have access to a 401(k) at work, an IRA is the individual equivalent with its own rules and limits.

Most plans offer two versions, and the difference is when you pay tax. A traditional 401(k) takes contributions pre-tax: the money comes out of your paycheck before income tax is calculated, lowering your taxable income for the year. The balance grows tax-deferred, and you pay ordinary income tax on whatever you withdraw in retirement. It rewards people who expect to be in a lower tax bracket later than they are now.

A Roth 401(k) flips the timing. Contributions are made with after-tax dollars, so there is no deduction today, but qualified withdrawals in retirement, including all the investment growth, come out completely tax-free. It tends to suit people who expect a higher tax rate in retirement, or who simply value locking in a known tax bill now. Both share a single combined annual contribution limit, so you split that limit between them rather than getting a separate allowance for each. Walnut is not a tax advisor, and this is not tax advice; how the two compare for you depends on your own tax situation. For the related individual account, see Roth IRA vs 401(k).

The employer match is often the most valuable feature of a 401(k). Your company adds money to your account based on what you contribute, commonly a formula like 50 cents per dollar up to 6% of your pay, or dollar-for-dollar up to a few percent. Because it is extra compensation you only get by participating, contributing at least enough to capture the full match is a widely cited starting point. Note that an employer match has traditionally gone into the pre-tax side of the account even when your own contributions are Roth, though newer rules let some plans offer Roth matching.

Vesting is the catch. The money you contribute is always 100% yours, but employer contributions may come with a vesting schedule that determines how much you keep if you leave. Some plans vest the match immediately; others use graded vesting (you earn a rising percentage over several years) or cliff vesting (you get nothing until a set date, then everything). If you change jobs before you are fully vested, you can forfeit the unvested portion of the match, so it is worth knowing your plan's schedule.

The IRS caps how much you can contribute each year, and the numbers usually rise with inflation. For 2026 the employee contribution limit is $24,500. If you are age 50 or older, you can add a catch-up contribution of $8,000, and a higher catch-up of $11,250 applies for ages 60 to 63 under current rules. There is also a larger overall limit that counts your contributions plus the employer match together, which is higher than the employee figure.

These amounts are specific to 2026 and change most years, so verify current limits with the IRS before you plan around them. A few finer points also shift over time: under recent rules, higher earners may be required to make catch-up contributions on a Roth basis rather than pre-tax. The practical takeaway is steady: the contribution limit is generous, the match is on top of it, and capturing both is the core of how a 401(k) builds wealth.

A 401(k) is a wrapper, not an investment by itself. Once money lands in the account, you choose how it is invested from a menu your plan provides, and that choice drives your long-run results far more than the account type does. The menu is typically a curated list of mutual funds: broad stock index funds, bond funds, international funds, and often target-date funds that hold a diversified mix and gradually get more conservative as the target retirement year approaches.

Target-date funds are popular precisely because they are a single, hands-off choice, while building your own mix from index funds gives you more control over the stock-and-bond split. Either way, the point is that picking your investments is your job inside the plan, and many balances sit in default options that may not match the owner's goals. Our guide to the best ETFs for a 401(k) and how to build a diversified portfolio cover the building blocks. Walnut is informational and is not a financial advisor; this is descriptive, not a recommendation.

The whole appeal of a 401(k) is the tax shelter. In a traditional 401(k), contributions reduce this year's taxable income and the account compounds without yearly taxes on dividends or gains; you settle up at ordinary income rates when you withdraw in retirement. In a Roth 401(k), you pay tax up front and qualified withdrawals later are tax-free. In both, that tax-advantaged compounding over decades is what makes the account powerful.

The flip side is that the money is meant for retirement, and the rules enforce it. Withdrawals before age 59 and a half generally trigger a 10% early-withdrawal penalty on top of any ordinary income tax due, which can consume a large share of the amount. There are exceptions, such as the rule of 55 (penalty-free withdrawals from a current employer's plan if you leave in or after the year you turn 55) and certain hardship and medical situations. The exceptions are specific and change, so verify the current rules with the IRS. Walnut is not a tax advisor; this is not tax advice.

When you leave a job, your 401(k) does not have to stay behind. You generally have a few options: leave it in the old employer's plan, roll it into your new employer's 401(k), or roll it into an Individual Retirement Account (IRA). A direct rollover moves the money between accounts without it passing through your hands, which avoids triggering taxes or the early-withdrawal penalty.

Rolling an old 401(k) into an IRA is common because an IRA usually opens up a much wider universe of investments than a plan's fixed menu, and it consolidates accounts left at past jobs into one place. Pre-tax balances typically roll to a traditional IRA and Roth balances to a Roth IRA to keep the tax treatment intact. A rollover IRA is exactly the kind of account Walnut can help you look at once it is at a connected broker, since you control the investments inside it directly.

The 2026 figures above are specific to that year and change regularly; verify current limits with the IRS, and remember that the employee limit is shared across both account types combined, not doubled. Which version fits depends on your own tax picture, which Walnut, being informational and not a tax advisor, cannot decide for you.

A 401(k) is one of the hardest accounts to actually see. The investments sit behind a plan menu, the balance is easy to ignore for years, and most people never check whether their funds overlap, how concentrated they are, or how they are doing against a benchmark. When you roll an old 401(k) into an IRA at a broker, those holdings become visible and adjustable, and the useful questions get concrete: how diversified am I, what am I really paying in fees, and where am I doubling up on the same companies.

That is where Walnut fits. It connects your existing brokerage through SnapTrade and lets you ask, in plain language through Claude, ChatGPT, or a built-in assistant, how the investments inside an account like a rollover IRA are positioned and how each one is doing against the S&P 500. It is read-only until you choose to trade, and you approve any change. Walnut is informational and is not a financial or tax advisor; this is not tax advice, and it does not host 401(k) plans.

A 401(k) is an employer-sponsored retirement plan that lets you invest part of your paycheck with major tax advantages, often boosted by an employer match you should know the vesting rules for. The traditional version is pre-tax now and taxed later; the Roth version is after-tax now and tax-free later. For 2026 the employee contribution limit is $24,500 with catch-up amounts above it, but those numbers move, so verify current limits with the IRS. Inside the account, you pick the investments, which matters more than the wrapper itself.

Capture the match, choose investments that fit your time horizon, avoid early withdrawals and their 10% penalty, and roll old accounts forward when you change jobs rather than losing track of them. Once a rollover IRA or other account is at a connected broker, you can dig into any ETF or stock it holds. Rules, limits, and tax treatment change; treat the specifics here as a starting point and confirm with the IRS or a qualified professional before deciding.