How Money Market Mutual Funds Work and What They're For
What a money market mutual fund is
A money market mutual fund is a mutual fund that holds short-term debt — mostly government bills, corporate IOUs due within a few months, and cash. It works like any other mutual fund: you buy shares, a manager invests the money, and you own a piece of the holdings. The difference is what the fund holds and why.
Money market funds exist to be stable and liquid. They aim to keep the share price at exactly $1.00 and to let you withdraw your money quickly, usually within a day or two. In exchange, they pay a lower return than stock funds or bond funds. You use them when you need a place to park cash that earns a little interest but won't drop in value.
The fund manager buys short-term IOUs — Treasury bills (government debt due in weeks or months), commercial paper (corporate debt due soon), and certificates of deposit from banks. Because these mature so quickly, the fund's value stays steady even when interest rates move around. That stability is the whole point.
Key Takeaways
- Money market funds hold short-term debt and cash, not stocks, so they aim to keep share price at $1.00 and rarely lose value.
- They pay interest based on current rates, which means the yield changes monthly or quarterly as the fund's holdings mature and are replaced.
- You can usually withdraw money within one business day, making them more liquid than bond funds or stock funds.
- Money market funds are not the same as money market accounts at banks, which are deposit products insured by the FDIC.
How the yield works
Money market funds don't pay a fixed rate. Instead, the yield — the interest you earn — changes based on what the fund manager can buy in the market right now. When interest rates are high, new Treasury bills and commercial paper pay more, so the fund's yield goes up. When rates fall, the yield falls too.
The fund publishes a distribution yield or SEC yield, usually shown as an annual percentage. This is what you would earn if the current rate stayed the same for a year. It changes daily or weekly as the fund's holdings turn over. You won't see a steady paycheck; instead, interest accrues and is either paid out monthly or reinvested into more shares, depending on which share class you own.
Because the fund holds only short-term debt, it has very little interest rate risk. If rates rise tomorrow, the fund doesn't lose money — it just buys new holdings at the higher rate when the old ones mature in days or weeks. This is why the share price stays at $1.00 (or very close to it) even when the broader bond market moves.
Money market funds versus money market accounts
The names are similar, but they are different products. A money market account is a bank deposit product. Your bank holds the money, pays you interest, and insures the account up to $250,000 through the FDIC. You can write checks or use a debit card. The rate is set by the bank and changes when the bank decides to change it.
A money market mutual fund is an investment. You own shares in a fund that buys Treasury bills and commercial paper. There is no FDIC insurance — if the fund loses money, you lose money. You can't write checks directly from most money market funds, though some allow transfers. The yield is set by the market and changes as the fund's holdings mature.
Money market accounts are safer and more convenient for everyday banking. Money market funds are for investors who want to hold cash in their investment account and earn a market rate without taking on stock or bond risk. If you're choosing between the two, ask yourself whether you need FDIC insurance and check-writing, or whether you're parking cash within a brokerage account.
Why investors use money market funds
Investors hold money market funds for three main reasons: as a temporary holding place while deciding where to invest, as a safe place to keep an emergency fund within a brokerage account, or as a way to earn interest on cash without taking risk.
If you have $50,000 you just inherited and you're not sure whether to buy stocks or bonds, you might put it in a money market fund for a few weeks while you think. The money stays liquid and earns interest instead of sitting in a checking account earning nothing. Once you decide, you move it to your chosen investment.
Some investors also use money market funds to hold the cash portion of a diversified portfolio. If your plan says you should keep 10% in cash, a money market fund lets you earn the current rate on that cash instead of leaving it idle. The rate changes with market conditions, so you're not locked into a low return.
Costs and tax treatment
Money market funds charge an expense ratio — an annual fee taken from the fund's assets. This ratio is usually very low, often between 0.05% and 0.30% per year, because the fund doesn't require much active management. The manager is mostly just rolling over maturing debt into new debt. Compare the expense ratio when choosing between funds; a difference of 0.10% per year adds up over time.
Interest earned in a money market fund is taxed as ordinary income at your federal tax rate, plus any state and local taxes. If you hold the fund in a taxable brokerage account, you'll owe tax on the interest each year. If you hold it in a retirement account like an IRA or 401(k), the interest grows tax-deferred.
Some money market funds hold municipal bonds — debt issued by cities and states — and the interest is exempt from federal tax (and sometimes state tax too). These are called tax-exempt money market funds and pay a lower yield because of the tax benefit. They make sense only if you're in a high tax bracket and hold the fund in a taxable account.
The difference between government and prime money market funds
Money market funds come in a few flavors based on what they hold. A government money market fund holds Treasury bills and other U.S. government debt, plus cash. These are the safest because the U.S. government backs them. The yield is usually the lowest because government debt is the safest.
A prime money market fund (also called a general-purpose fund) holds Treasury bills, commercial paper from corporations, and bank CDs. The yield is higher than a government fund because corporate debt pays more than government debt. The risk is still very low — commercial paper is short-term and issued by stable companies — but it's not zero.
A tax-exempt money market fund holds short-term municipal debt. The interest is free from federal income tax. These funds pay a lower yield than prime funds, but the after-tax return may be higher if you're in a high tax bracket. They only make sense in a taxable account; in a retirement account, the tax exemption is worthless.
How to buy a money market fund
You buy money market funds through a brokerage account the same way you buy any mutual fund. Open an account at a broker like Fidelity, Vanguard, Charles Schwab, or your bank's investment arm. Search for money market funds by name or filter by fund type. Read the fund's prospectus to see what it holds, what the expense ratio is, and what the current yield is.
Most brokers let you buy money market funds with no transaction fee. Some brokers have their own money market funds and may waive the expense ratio if you buy them through that broker. Compare a few options before you decide; a 0.10% difference in expense ratio is small but real over years.
Once you own shares, you can check the yield and current value any time through your brokerage account. You can sell the shares and move the money to another investment whenever you want. Most sales settle within one business day, so you can access the cash quickly if you need it.
Frequently Asked Questions
Can a money market fund lose money?
Very rarely. Money market funds are designed to keep the share price at $1.00. In normal conditions, they don't lose value. However, they are not may provide by the government and are not FDIC-insured. In extreme market stress, a fund could "break the buck" (drop below $1.00), though this has happened only a handful of times in history and usually only to funds holding risky commercial paper.
What's the difference between a money market fund and a savings account?
A savings account is a bank deposit insured by the FDIC up to $250,000. A money market fund is an investment with no insurance. Savings accounts usually pay less interest than money market funds, especially when rates are high. Money market funds are better for earning yield; savings accounts are better for safety and FDIC protection.
Should I use a money market fund or a short-term bond fund?
Money market funds are for cash you might need soon or that you want to keep very stable. Short-term bond funds hold bonds due in one to three years and can go up or down in value as interest rates change. If you need the money within a year or want zero volatility, use a money market fund. If you're willing to accept some price movement for higher yield, a short-term bond fund may pay more.
Do I have to pay taxes on money market fund interest every year?
Yes, if you hold the fund in a taxable brokerage account. The interest is taxed as ordinary income each year, even if you don't withdraw it. If you hold the fund in an IRA, 401(k), or other retirement account, the interest grows tax-deferred and you don't owe tax until you withdraw money from the account.
Why would I choose a money market fund over just keeping cash in my checking account?
A checking account usually pays little or no interest. A money market fund pays the current market rate, which can be 4% to 5% or higher depending on conditions. If you have cash you won't need for a few weeks or months, a money market fund lets you earn interest instead of leaving the money idle. The tradeoff is that you can't write checks directly from most money market funds.