A money market fund pools your cash with other investors' money and buys short-term debt that matures in days or months, not years

A money market fund is a type of mutual fund that invests in very short-term loans—usually to governments, banks, and large corporations. When you put money in, your dollars get mixed with thousands of other investors' dollars. The fund manager then uses that pool to buy things like Treasury bills (which mature in a few weeks), commercial paper (short-term corporate IOUs), and certificates of deposit. You own a share of everything the fund holds, and you earn a portion of whatever interest those loans pay.

The key difference from a money market account at a bank: a money market fund is not insured by the FDIC. It is a mutual fund, which means its value can move slightly day to day, though the movement is usually tiny. A money market account is a bank deposit account, which means FDIC insurance protects it up to $250,000. Both are safer than stocks, but they work in completely different ways.

Key Takeaways

  • Money market funds buy short-term debt from governments and corporations, and you own a share of that entire portfolio.
  • The interest rate you earn changes frequently because the fund's holdings mature and get replaced with new loans at current rates.
  • Your money is not locked in—you can usually withdraw it within a few business days—but the fund's value can fluctuate slightly.
  • Money market funds charge a small annual fee (called an expense ratio) that comes out of your returns before you see them.
  • The fund is not FDIC-insured, so in theory the value could drop, though this is extremely rare for funds that stick to the safest securities.

How the fund manager invests your money

When you deposit cash into a money market fund, the manager does not sit on it. Instead, they immediately start buying short-term IOUs. A Treasury bill might mature in 13 weeks. A certificate of deposit might mature in 30 days. Commercial paper from a bank might mature in 90 days. The manager spreads the money across many of these loans so that some are maturing almost every day.

As each loan matures, the borrower pays back the principal plus interest. The manager then takes that cash and buys a new short-term loan at the current interest rate. This constant rolling-over is why your rate changes. If interest rates rise, new loans pay more, so your fund's yield goes up. If rates fall, new loans pay less, so your yield falls. You are not locked into a single rate the way you would be with a certificate of deposit.

The manager also has to follow strict rules about what they can buy. Most money market funds only hold securities rated as very safe—things like U.S. Treasury bills, loans from banks with strong credit ratings, and commercial paper from large, stable companies. This is why money market funds are considered low-risk, even though they are not FDIC-insured.

Why your interest rate changes so often

Money market funds publish a new yield every single day. You might see 5.25% one day and 5.23% the next. This happens because the fund's holdings are constantly turning over. When the manager buys a new loan, it might pay slightly more or slightly less than the loan it replaced. Over time, these small changes add up to a visible shift in your rate.

The broader economy also matters. When the Federal Reserve raises its benchmark interest rate, new short-term loans start paying more, and your fund's yield rises within days or weeks. When the Fed cuts rates, the opposite happens. This is why money market funds are sometimes called a "floating rate" investment—the rate floats with the market, rather than staying fixed.

You will also see your yield affected by the fund's expense ratio, which is the annual fee the fund company charges to manage it. A fund with a 0.10% expense ratio will show a lower yield than an identical fund with a 0.05% ratio, because the fee comes out first. Always check the expense ratio before you choose a fund—it compounds over time.

How you earn money and when you can access it

You earn money in two ways. First, the interest from all those short-term loans gets paid into the fund, and your share of it is added to your account. Most funds let you choose whether to receive that interest as cash (paid monthly or quarterly) or to reinvest it automatically. Second, if you sell your shares for more than you paid, you make a small capital gain, though this is rare because the fund's value stays very stable.

You can usually withdraw your money within one to three business days, depending on the fund company. Some funds let you write checks directly against your balance or link it to a debit card, which makes it feel almost like a checking account. However, the fund is not required to let you withdraw instantly the way a bank account is. During times of extreme market stress, a fund can temporarily restrict withdrawals, though this almost never happens with funds that hold only the safest securities.

The difference between money market funds and money market accounts

Both are safe places to park cash, but they are not the same thing. A money market account is a bank deposit account. It is FDIC-insured up to $250,000, which means if the bank fails, your money is protected. The interest rate is usually fixed for a set period, though banks can change it whenever they want. You can withdraw money whenever you want, and the account works like a hybrid between a savings account and a checking account.

A money market fund is a mutual fund. It is not FDIC-insured, which means if something goes very wrong, you could theoretically lose money. However, the risk is extremely low if the fund sticks to safe securities. Your rate changes constantly as the fund's holdings turn over. You can withdraw money, but it might take a few business days. The fund charges an annual expense ratio, whereas a bank account might charge monthly fees instead.

Right now, both typically pay similar rates because they both invest in short-term, safe debt. The choice between them often comes down to whether you want FDIC insurance (choose the bank account) or slightly lower fees and more flexibility (choose the fund).

What can go wrong and how rare it is

The biggest risk is that the fund's value drops. This happens when the securities it holds lose value, which can occur if interest rates rise sharply or if a borrower's credit rating falls. However, because money market funds hold only short-term debt, the impact is usually tiny. A one-percentage-point rise in interest rates might cause a fund's value to drop 0.1% or less.

A more serious risk is that a borrower defaults—meaning they do not pay back the loan. If a large bank or corporation fails to repay, the fund loses money and all shareholders take a small hit. This is extremely rare. The 2008 financial crisis saw one money market fund "break the buck" (drop below $1 per share), but that was an exceptional event. Since then, regulations have tightened, and most funds hold only the safest securities.

Another consideration: if you need your money in a true emergency, a money market fund might not be as instantly accessible as cash in a checking account. Most funds process withdrawals within one to three business days. If you need money today, you might have to wait.

How to choose a money market fund

Start by comparing expense ratios. A fund charging 0.05% per year will leave you with more money than one charging 0.25%, all else being equal. Over 10 years, that difference compounds. Check the fund's holdings to see what it actually buys—some funds focus on Treasury bills, others on bank CDs, others on a mix. Treasury-only funds are the safest but sometimes pay slightly less. Mixed funds often pay a bit more but carry slightly more risk.

Look at the fund's current yield and ask whether it is competitive with other money market funds and with money market accounts at banks. Yields change daily, so compare on the same day. Also check the fund's minimum deposit—some require $1,000 or $2,500 to start, while others have no minimum.

Finally, consider where you will hold the fund. If you have a brokerage account (at Fidelity, Vanguard, Charles Schwab, or another firm), you can usually buy money market funds there with no extra fees. If you want to hold it at a bank, ask whether they offer money market funds or only money market accounts. Some banks do not offer funds at all.

Frequently Asked Questions

Can I lose money in a money market fund?

Theoretically yes, but it is extremely rare. If a borrower defaults or interest rates rise sharply, the fund's value can drop slightly. However, because the fund holds only short-term debt, the swings are usually tiny—often less than 0.1%. The 2008 financial crisis saw one fund break the buck, but regulations have since tightened.

How often does the interest rate change?

The fund publishes a new yield every business day. However, the change is usually small—often just a few basis points (hundredths of a percent). The rate can shift noticeably when the Federal Reserve changes its benchmark rate, which typically happens a few times per year.

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

No. A money market account is a bank deposit account with FDIC insurance. A money market fund is a mutual fund with no FDIC insurance. Both are safe and pay similar rates, but they work differently and carry different risks.

What is the expense ratio and why does it matter?

The expense ratio is the annual fee the fund company charges to manage the fund. It is expressed as a percentage of your balance and comes out before you see your returns. A fund charging 0.10% per year will pay you less than one charging 0.05%, even if they hold identical securities.

How quickly can I withdraw my money?

Most money market funds process withdrawals within one to three business days. Some funds offer check-writing or debit card access, which makes withdrawals faster. However, funds are not required to process withdrawals instantly the way bank accounts are.