Your 401(k) balance goes up and down because the mutual funds sitting inside it are repriced every business day based on how their underlying stocks and bonds traded that day. On top of that daily repricing, your balance shifts whenever new contributions land, your employer match posts, dividends get reinvested, fees come out, or vesting rules change what’s actually yours. Knowing which force is driving a given change is what separates normal movement from something worth acting on.
How Daily Repricing Works
A 401(k) is a wrapper around investment funds, most often mutual funds, and those funds hold dozens or hundreds of individual stocks and bonds. Under SEC Rule 22c-1, every mutual fund has to calculate its net asset value at least once each business day.1SEC.gov. Amendments to Rules Governing Pricing of Mutual Fund Shares NAV is just the total value of everything the fund owns divided by the number of shares outstanding. That per-share figure is the price you see on your statement.
During the trading day, the stocks and bonds inside those funds are bought and sold on exchanges. When buyers outnumber sellers on a stock, its price climbs; when sellers dominate, it drops. By market close, those changes are baked into the fund’s updated NAV. Your provider then multiplies the new per-share price by the number of shares you own, and that product is your new balance.
Trades don’t settle instantly, though. Since May 2024, the standard settlement cycle for most securities is T+1, meaning the transaction finalizes one business day after the trade.2FINRA.org. Understanding Settlement Cycles: What Does T+1 Mean for You Payroll contributions also take time to process, so there’s often a short lag between money leaving your paycheck and showing up as invested shares. If your balance seems to jump on a random day with no market news, a settling contribution is often the reason.
What You’re Invested In Shapes the Size of the Swings
Two coworkers with identical balances can see very different daily moves. The difference is what each person owns. Stocks represent partial ownership in companies, and their prices react sharply to earnings, economic data, and investor confidence. A portfolio heavily weighted toward stock funds will move more in both directions. Bonds are essentially loans that pay interest over time and tend to be steadier, though not immune to bad days.
Federal rules require applicable 401(k) plans to offer at least three diversified investment options with materially different risk and return profiles.3eCFR. 26 CFR 1.401(a)(35)-1 – Diversification Requirements for Certain Defined Contribution Plans Most plans offer well beyond that, including stable value and money market options designed to preserve principal. Those barely move day to day. The trade-off is lower long-term returns.
Many plans also include target-date funds, which automatically shift from mostly stocks to mostly bonds as retirement approaches. A target-date fund for someone in their twenties might hold 90% stocks. By the time that same person reaches their early seventies, the fund may hold closer to 30% stocks and 70% bonds. If you’re in one of these, your daily swings will shrink gradually over the decades without you touching anything.
Economic Events That Move the Whole Market
Sometimes your balance drops even though nothing specifically changed about the companies you own. That’s usually a market-wide event. The Federal Open Market Committee sets the target for the federal funds rate, which influences borrowing costs across the economy.4Federal 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 plan 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, since public companies must file quarterly financial reports with the SEC.5SEC.gov. Exchange Act Reporting and Registration When a large company misses expectations, every fund holding that stock takes a hit.
Investor sentiment is a looser factor but a real one. If enough market participants decide the economy looks shaky, broad selling can push prices down even when the underlying businesses are performing fine. Sentiment-driven drops often reverse when concrete data contradicts the fear, but they’re unnerving while they’re happening.
Upward Movements That Aren’t the Market
Not every rise comes from markets going up. Three other forces regularly push your balance higher.
- Payroll contributions. Every pay period, your pre-tax or Roth contribution buys new fund shares at the current price. For 2026, you can contribute up to $24,500, with higher catch-up limits once you turn 50.6Internal 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 hits your account on its own schedule, sometimes every pay period, sometimes quarterly. The match buys additional shares and bumps your balance regardless of what the market did that day.
- Dividend reinvestment. Many stocks and bonds inside your funds pay dividends or interest, and in a 401(k) those payments are almost always reinvested to buy more shares. Your share count rises on its own, which adds to your balance over time even when prices stay flat.
Together, these can lift your balance in a week when the market actually fell. The new money bought shares at lower prices, which doesn’t feel like a win but works in your favor over time.
Fees That Quietly Pull Your Balance Down
Fees push in only one direction, and most of them never appear as a line item on your statement. The biggest ongoing cost is the fund expense ratio, a percentage of assets that the fund manager deducts directly from the fund’s NAV every day. You don’t see a separate charge; your returns are simply a touch lower than the raw performance of the underlying holdings. A fund with a 0.26% expense ratio takes about $2.60 per year for every $1,000 invested. The Department of Labor has shown that a 1% difference in annual fees can reduce your final balance by tens of thousands of dollars across a 35-year career.7U.S. Department of Labor. A Look at 401(k) Plan Fees
Your plan may also charge administrative fees for recordkeeping and compliance, sometimes deducted quarterly or monthly directly from your balance. Transaction-based fees can apply for specific actions like taking a plan loan or processing a qualified domestic relations order during a divorce.8Internal Revenue Service. Retirement Topics – Fees If you see a small, unexplained dip on a day the market was flat, fees are often the explanation.
Vesting Can Make the Displayed Balance Misleading
Your own contributions are always 100% yours. Employer matching contributions often come with strings attached called a vesting schedule. Under federal law, defined contribution plans must 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 your ownership grows from 20% at two years to 100% at six years.9Office of the Law Revision Counsel. 26 USC 411 – Minimum Vesting Standards
Here’s where confusion sets in: most 401(k) dashboards display your total balance, including unvested employer contributions. That figure feels like yours, but it isn’t all legally yours yet. If you leave your job before fully vesting, the unvested portion gets forfeited and removed from your account. That can look like a dramatic drop. Check whether your plan shows a separate “vested balance” figure, since that number represents what you’d actually walk away with today.
Withdrawals You Didn’t Make
Occasionally your balance drops because the plan administrator takes money out, not you. 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. If the gap is too wide, the plan must refund excess contributions to the highly compensated employees to stay in compliance.10Internal 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 similar issue arises if you accidentally exceed 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 that amount. If you get an unexpected check or notice a withdrawal you didn’t authorize, call your plan administrator. Corrective distributions are usually what you’re looking at.
Loans, Early Withdrawals, and Required Distributions
Taking a loan from your 401(k) looks different from market movement. The IRS allows you to borrow up to 50% of your vested balance or $50,000, whichever is less.11Internal Revenue Service. Retirement Topics – Plan Loans When you take the loan, shares are sold and the cash is lent to you. Your balance drops by the loan amount. As you repay, the money goes back in and buys new shares, and the interest you pay goes into your own account. While the loan is outstanding, though, those dollars aren’t invested and aren’t earning market returns.
Defaulting is where it gets expensive. If you leave your job or fail to repay on time, the outstanding balance gets treated as a taxable distribution. If you’re 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 right off the top, and the 10% penalty applies unless you qualify for a specific exception.12Internal Revenue Service. 401(k) Resource Guide – Plan Participants – General Distribution Rules
At the other end of your career, once you reach age 73, the IRS requires you to start pulling money out of a traditional 401(k) each year through required minimum distributions.13Internal Revenue Service. Retirement Plan and IRA Required Minimum Distributions FAQs The amount is calculated from your balance and a life expectancy factor. These 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 plan may be exempt, but any 401(k) from a previous employer is not.
Why Short-Term Drops Rarely Matter Long-Term
The natural response to a falling balance is to move everything into something stable. That impulse usually costs you money. 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 from months to years. If retirement is a decade or more away, your account has time to recover without you doing anything.
The regular contributions coming out of your paycheck are actually working harder during a downturn. When fund prices are lower, the same dollar amount buys more shares, and those extra shares participate fully in any recovery. The dips that feel awful at the time are quietly lowering your average cost per share.
Where it does make sense to act is on your allocation, not your emotions. If a market drop reveals that you’re more uncomfortable 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 usually isn’t. By the time it feels better, the recovery is already priced in.