A money market fund is a mutual fund that holds short-term debt—mostly government bills, corporate IOUs due soon, and other very safe loans—and pays you interest on your balance.

Unlike a money market account at a bank, a money market fund is not insured by the FDIC. It is run by an investment company (like Vanguard, Fidelity, or Schwab) and holds actual securities rather than deposits. The trade-off is that money market funds often pay slightly higher interest than bank accounts, though the difference shrinks when bank rates are high.

The fund buys bonds and loans that mature in less than a year—usually within 90 days. As those mature and get paid back, the fund uses the cash to buy new ones. You own a share of the whole pool, so you earn a portion of all the interest the fund collects. The interest rate changes daily because the fund's holdings change daily.

Key Takeaways

  • Money market funds hold short-term debt securities and are run by investment companies, not banks, so they carry no FDIC insurance.
  • Interest rates on money market funds fluctuate daily based on what the fund buys, and they typically track the federal funds rate.
  • You can usually withdraw your money within one to three business days, though some funds have restrictions during market stress.
  • Money market funds are safer than stock funds but riskier than bank savings accounts because the principal is not may provide.
  • The fund's expense ratio—the annual cost to run it—directly reduces your return, so comparing fees between funds matters.

How the interest rate is set

A money market fund's interest rate is not fixed by the fund company. Instead, it moves with the market rate for short-term debt. When the Federal Reserve raises its benchmark rate, the interest rates on new short-term loans go up, so the fund buys new securities at higher rates. When rates fall, so does the fund's yield.

This means your rate can change weekly or even daily. The fund publishes a yield—the annualized return based on recent earnings—but that is not a promise. If you hold the fund for a year, your actual return depends on what rates were during that year.

The difference between a money market fund and a money market account

A money market account is a bank deposit product. It is FDIC insured up to $250,000, which means if the bank fails, you are protected. The bank sets the interest rate, and it does not change as often as a fund's rate does. You can usually write checks or use a debit card, though some accounts limit withdrawals.

A money market fund is an investment product. It is not FDIC insured. You own shares of the fund, and the value of those shares can theoretically fall if the securities inside lose value—though this is rare because the fund holds only very short-term, low-risk debt. You cannot write checks directly from most money market funds, though you can usually move money to a linked bank account within a few business days.

If safety and insurance matter most to you, a money market account is the simpler choice. If you want a slightly higher rate and do not mind the lack of FDIC insurance, a money market fund may pay more.

What happens to your money when you invest

When you put money into a money market fund, the fund company buys shares on your behalf. Your account shows the number of shares you own and their current value. As the fund collects interest from its holdings, that interest is either paid out to you (if the fund distributes it) or reinvested to buy more shares (if you choose reinvestment).

The fund's share price stays very close to $1.00 per share. This is by design—money market funds are structured to keep the price stable. However, the price can move slightly, and in rare cases of market stress, a fund might "break the buck" and fall below $1.00. This has happened only a handful of times in history, usually during financial crises.

Fees and how they affect your return

Money market funds charge an expense ratio—an annual percentage fee taken from the fund's assets to pay for management, administration, and other costs. Expense ratios for money market funds typically range from 0.01% to 0.50% per year, depending on the fund company and the specific fund.

This fee comes out of your returns automatically. If a fund yields 5.00% and has a 0.20% expense ratio, you actually earn about 4.80%. Over time, even small differences in fees add up. When comparing money market funds, check the expense ratio on the fund's fact sheet or prospectus—usually available on the fund company's website.

When to use a money market fund instead of a savings account

A money market fund makes sense if you have cash you want to keep safe and accessible, and you want to earn a higher rate than your bank is offering. This works best when interest rates are high enough that the fund's yield beats the bank account's rate by enough to cover the lack of FDIC insurance.

Money market funds are also useful if you are holding cash temporarily while deciding where to invest it, or if you need a place to park money between other investments. Because the fund's value is stable and you can access your money in a few business days, it acts as a bridge between your checking account and longer-term investments.

However, if you need the money within a few days and cannot afford any delay, a bank savings account is safer because you can withdraw instantly. And if the rate difference is small—say, 0.10% or less—the simplicity of a bank account may be worth more than the extra earnings.

Restrictions and when you might not be able to withdraw

Most of the time, you can move money out of a money market fund within one to three business days. However, during periods of market stress or high redemption requests, some funds have the right to impose restrictions. These are rare, but they can happen.

Additionally, some money market funds have minimum investment amounts—often $1,000 to $3,000—and minimum balances you must maintain. Check the fund's prospectus or fact sheet for these rules before you invest. If you need instant access to your money, a bank savings account is more reliable.

Frequently Asked Questions

Can I lose money in a money market fund?

In theory, yes—the fund's share price can fall if the securities inside lose value. In practice, this almost never happens because money market funds hold only very short-term, low-risk debt. The only time a money market fund has "broken the buck" (fallen below $1.00 per share) was during the 2008 financial crisis. For most investors, the risk is extremely low.

Is a money market fund the same as a money market account?

No. A money market account is a bank deposit with FDIC insurance. A money market fund is an investment product run by a fund company with no FDIC insurance. Money market funds often pay higher rates, but accounts offer more protection and easier access to your money.

How often does the interest rate change?

The fund's yield can change daily because it is based on the interest rates of the securities the fund holds. However, the change is usually small from day to day. The bigger shifts happen when the Federal Reserve changes its benchmark rate, which typically happens a few times per year.

What's the difference between a money market fund and a bond fund?

A money market fund holds debt that matures in less than a year, usually within 90 days. A bond fund holds longer-term debt, often with maturities of several years or more. Because longer-term bonds are riskier and more sensitive to interest rate changes, bond funds have more price volatility than money market funds.

Do I have to pay taxes on money market fund earnings?

Yes. The interest you earn is taxable income in the year you earn it, whether or not you withdraw the money. If the fund is in a taxable account (not a retirement account), you will receive a 1099-DIV form at tax time showing your earnings. If it is in an IRA or 401(k), the earnings are tax-deferred or tax-free depending on the account type.