Your 401(k) goes up and down mainly because it holds mutual funds, and those funds are repriced at the end of every business day based on how their underlying stocks and bonds traded. On top of that daily repricing, your balance also moves when new contributions land, when your employer’s match posts, when dividends reinvest, when fees come out, and occasionally when the plan itself adjusts your account. Knowing which force is behind a given change is the difference between normal noise and something worth acting on.
Daily Repricing Is the Main Reason
A 401(k) is a wrapper around investment funds, usually mutual funds that hold dozens or hundreds of individual stocks and bonds. Under SEC Rule 22c-1, every mutual fund calculates its net asset value at least once each business day.1SEC.gov. Amendments to Rules Governing Pricing of Mutual Fund Shares NAV is the total value of everything the fund owns divided by the number of shares outstanding. That per-share figure is the price attached to your holdings.
Throughout the day, the stocks and bonds inside those funds trade on exchanges. Prices rise when demand climbs and fall when sellers outnumber buyers. At market close, those individual moves get baked into the fund’s new NAV. Your plan multiplies that new price by the number of shares you own, and the product is your updated balance. The next business day, it happens again.
Payroll contributions also take time to process, so there can be a short lag between when money leaves your paycheck and when it shows up invested. That lag can make it look like your balance jumped on a random day when a contribution was simply settling.
What You’re Invested In Sets the Size of the Swings
Two coworkers with identical balances can have very different daily moves depending on what they hold. Stocks represent ownership in companies and react sharply to earnings, economic data, and investor confidence. A portfolio weighted toward stock funds swings more in both directions. Bonds are essentially loans that pay interest over time and tend to be steadier, though they’re not immune to bad days.
Federal rules require applicable 401(k) plans to offer at least three diversified options with materially different risk and return profiles.2eCFR. 26 CFR 1.401(a)(35)-1 – Diversification Requirements for Certain Defined Contribution Plans Most plans go well beyond three and include stable value and money market options built to preserve principal. Those barely move day to day. The trade-off is much lower long-term returns.
Many plans also offer target-date funds that gradually shift from mostly stocks to mostly bonds as you approach retirement. A fund built for a worker in their twenties might hold around 90% stocks; by the early seventies, it may sit closer to 30% stocks and 70% bonds. If you’re in one of those, your daily swings will slowly shrink over the decades without you touching anything. It’s also why a younger coworker can lose $2,000 in a rough week while someone closer to retirement barely notices.
Economy-Wide Events That Move Everything at Once
Sometimes your balance drops even though nothing changed about the specific companies you’re invested in. That’s usually a broad market event. The Federal Open Market Committee sets the target for the federal funds rate, which influences borrowing costs across the economy.3Federal Reserve. The Fed Explained – Monetary Policy When the Fed raises rates, bond prices typically fall and stock valuations often get marked down because future earnings are worth less in today’s dollars. A single rate decision can move nearly every fund in your account on the same day.
Inflation reports carry similar weight. High inflation squeezes corporate profit margins and erodes the purchasing power of fixed bond payments. Earnings season adds another layer: public companies file quarterly financial reports with the SEC, and a large miss can drag down every fund that holds the stock.4SEC.gov. Exchange Act Reporting and Registration
Then there’s plain investor sentiment. If enough participants decide the outlook looks shaky, broad selling can push prices down even when individual companies are doing fine. Sentiment-driven drops often reverse once real data contradicts the fear, but they’re unpleasant while they last.
Money Coming In That Pushes the Balance Up
Not every upward move is the market. Three other forces regularly lift your balance and can mask what the market is actually doing.
- Payroll contributions. Every pay period, your pre-tax or Roth contribution buys new fund shares at that day’s price. Annual limits set by the IRS cap how much you can put in.5Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
- Employer matching. If your employer matches, that money arrives on its own schedule, whether every pay period or quarterly, and buys additional shares independent of market performance.
- Dividend reinvestment. Many stocks and bonds inside your funds pay dividends or interest, and in a 401(k) those payments are almost always reinvested automatically. Your share count grows without any action on your part.
These together can lift your balance during a week when the market was actually down. The new money bought shares at lower prices, which doesn’t feel like a win but tends to work in your favor over time. That’s the mechanic behind dollar-cost averaging: fixed regular contributions buy more shares when prices are low and fewer when prices are high, lowering your average cost per share over the years.
Fees That Quietly Pull the Balance Down
Fees only move your balance in one direction, and most of them don’t appear as line-item deductions. The biggest ongoing cost is the fund expense ratio, a percentage of assets that the fund manager pulls directly out of the fund’s NAV every day. You never see a separate charge; your returns are just slightly lower than the raw performance of the underlying holdings. A fund with a 0.26% expense ratio, roughly the average for 401(k) equity funds in 2024, takes about $2.60 per year for every $1,000 invested. That sounds trivial, but the Department of Labor has shown that a 1% difference in annual fees can reduce your final balance by tens of thousands of dollars over a 35-year career.6U.S. Department of Labor. A Look at 401(k) Plan Fees
Your plan provider may also charge administrative fees for recordkeeping and compliance, sometimes deducted quarterly or monthly straight from your balance. Those do appear as visible deductions. You may also see transaction-based fees for specific actions like taking a plan loan or processing a qualified domestic relations order in a divorce.7Internal Revenue Service. Retirement Topics – Fees If you see a small unexplained dip on a day when the market was flat, fees are usually the answer.
Drops That Aren’t Market Losses
Some balance decreases have nothing to do with fund prices. These are the ones most likely to catch you off guard.
Unvested Employer Money Coming Off
Your own contributions are always fully yours, but employer matching contributions often come with a vesting schedule. Defined contribution plans generally use one of two structures: cliff vesting, where you own nothing until you hit three years of service and then own everything, or graded vesting, where ownership grows from 20% at two years to 100% at six.8Office of the Law Revision Counsel. 26 USC 411 – Minimum Vesting Standards Most dashboards display your total balance, including the unvested portion. If you leave before fully vesting, that unvested piece gets forfeited and removed. That can look like a dramatic drop to anyone who didn’t realize part of the number was conditional. Look for a separate “vested balance” figure; that’s what you’d actually walk away with today.
Corrective Distributions and Excess Contributions
Occasionally the plan administrator pulls money out of your account. This happens when the plan fails annual nondiscrimination testing. The IRS requires 401(k) plans to run ADP and ACP tests, which compare how much higher-paid employees contribute relative to everyone else. When the gap is too wide, the plan refunds excess contributions to those highly compensated employees to stay in compliance.9Internal Revenue Service. 401(k) Plan Fix-it Guide – The Plan Failed the 401(k) ADP and ACP Nondiscrimination Tests That refund is taxable in the year you receive it.
A related issue is going over the annual contribution limit, which can happen if you switched jobs mid-year and contributed to two different plans. If the excess isn’t corrected by the tax filing deadline, you could face double taxation on it. If a withdrawal shows up you didn’t authorize, call your plan administrator. A corrective distribution is usually the reason.
Loans and Early Withdrawals
Taking a loan from your 401(k) creates a balance shift that looks nothing like market movement. The IRS allows you to borrow up to 50% of your vested balance or $50,000, whichever is less.10Internal Revenue Service. Retirement Topics – Plan Loans Shares are sold, cash is lent to you, and your balance drops by the loan amount. As you repay, the money buys new shares, and the interest you pay goes back into your own account. While the loan is outstanding, those dollars aren’t invested.
Defaulting is where it gets expensive. If you leave your job or miss repayments, the outstanding balance is treated as a taxable distribution. Under 59½, that means ordinary income tax plus a 10% early withdrawal penalty. A direct early withdrawal works the same way: 20% is withheld for federal taxes off the top, and the 10% penalty applies unless you qualify for an exception.11Internal Revenue Service. 401(k) Resource Guide – Plan Participants – General Distribution Rules Between taxes and penalties, cashing out $10,000 early could leave you with as little as $7,000.
Required Minimum Distributions
Once you reach age 73, the IRS requires you to start withdrawing from your traditional 401(k) each year through required minimum distributions.12Internal Revenue Service. Retirement Plan and IRA Required Minimum Distributions FAQs The amount is based on your balance and a life expectancy factor. Those mandatory withdrawals reduce your balance regardless of what the market is doing and are taxed as ordinary income. If you’re still working at 73, your current employer’s 401(k) may be exempt, but any 401(k) from a previous employer is not.
What to Do When It Drops
The instinct when a balance falls is to move everything to a stable value fund or cash. That impulse is usually costly. Selling stock funds after they’ve dropped locks in the loss, and then you face a second decision about when to buy back in, which most people get wrong. Markets have historically recovered from every downturn, though the timeline varies. If retirement is a decade or more away, the account has time to recover on its own.
Regular contributions actually work harder during downturns. When fund prices are lower, the same dollar buys more shares, and those extra shares participate fully in the eventual recovery. The dips that feel terrible in the moment are quietly lowering your average cost basis.
Where action makes sense is on your allocation, not your emotions. If a drop reveals you’re less comfortable with risk than you thought, adjusting your target allocation going forward is reasonable. Dumping stocks after a 20% decline and hoping to jump back in when things feel better is not. By the time they feel better, the recovery is already priced in. The people who build the most retirement wealth are almost always the ones who set a sensible allocation and left it alone.