What Is a Money Market Fund? How It Works, Safety, Pros and Cons

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A money market fund (MMF) is a mutual fund that invests only in high-quality, short-term debt and cash equivalents, such as Treasury bills, repurchase agreements and commercial paper. Its goal is to preserve the money you put in, keep it available on short notice, and pay income that follows short-term interest rates. Most retail money market funds aim to hold a stable share price of $1.00. That price is a target the fund manages toward, not a guarantee. A money market fund is an investment product, so it is not covered by FDIC deposit insurance.

What Is a Money Market Fund?

The “money market” is the wholesale market for borrowing and lending over short periods, from overnight to about a year. Governments, banks and corporations raise short-term cash there. A money market fund buys these short-term IOUs for its shareholders. Each shareholder owns a slice of a diversified pool instead of buying individual instruments, which often require large minimums.

Typical holdings include:

• U.S. Treasury bills and other short-term Treasury securities
• Government agency debt, including securities issued by government-sponsored enterprises
• Repurchase agreements (repos): short-term loans, often overnight, in which the fund buys securities and the seller agrees to buy them back at a set price. The securities serve as collateral.
• Commercial paper: unsecured short-term corporate borrowing
• Certificates of deposit and other bank obligations
• Bankers’ acceptances: short-term obligations guaranteed by a commercial bank
• Municipal notes, held by tax-exempt funds
• Cash

A money market fund, also called a money market mutual fund, is a mutual fund. It is the most conservative category, built around the short end of the debt market rather than stocks or long-term bonds.

How a Money Market Fund Works

Like any open-end mutual fund, a money market fund issues shares when investors buy and redeems them when they sell. A registered investment adviser manages the portfolio within limits set by regulators and the fund’s prospectus. You can buy shares from the fund company directly or through a brokerage or bank.

The stable $1.00 NAV

Net asset value is a fund’s assets minus its liabilities, divided by shares outstanding. A stock fund’s NAV moves every day with the market. A stable-NAV money market fund is designed to stay at $1.00.

Two features make that possible:

1. Very short, high-quality holdings. A Treasury bill maturing in 30 days barely changes in price when interest rates move, because it will soon pay its face value. A portfolio made of such instruments has little price volatility.
2. Pricing conventions. Under Rule 2a-7, government and retail funds may value holdings at amortized cost and round the share price to the nearest penny. Small gains and losses within half a cent do not change the $1.00 price that investors see.

Because the share price stays fixed, return arrives as income rather than price appreciation. Interest earned by the portfolio, minus fund expenses, typically accrues as a dividend each business day and is paid or reinvested monthly. If you reinvest, you receive additional $1.00 shares, so your share count grows instead of the share price.

Floating-NAV funds

Since reforms adopted in 2014 (fully effective in 2016), institutional prime and institutional tax-exempt funds must price shares at market value to four decimal places, such as $1.0000, so their NAV can drift to $0.9998 or $1.0003. The change keeps large institutional investors from redeeming at $1.00 when holdings are worth less, which leaves the loss with remaining shareholders. Government funds and retail funds, which are limited to accounts owned by individuals (natural persons), may still use the stable $1.00 price.

What determines the return

A money market fund’s yield follows the interest rates on the instruments it holds. When the Federal Reserve raises short-term rates, new Treasury bills, repos and commercial paper pay more. Because holdings mature within weeks, the fund reinvests at the new rates quickly, and its yield adjusts with only a short lag. When rates fall, the yield falls just as fast.

Funds usually report a 7-day SEC yield, a standardized annualized figure based on the most recent week’s income after expenses. It shows what the fund currently earns. It does not promise future income, because the yield changes whenever short-term rates change. The expense ratio comes directly out of that income, so between two funds with similar holdings, the lower-cost fund pays more.

Example of a Money Market Fund

These figures are hypothetical and do not describe any real fund or current rates.

Income on a stable-NAV fund. You invest $10,000 in a government money market fund priced at $1.00, so you receive 10,000 shares. Assume the portfolio earns a gross yield of 4.50% and the fund charges a 0.25% expense ratio. Your net yield is about 4.25% annualized.

• Daily income at that rate is roughly $10,000 × 4.25% ÷ 365 ≈ $1.16.
• Over a 30-day month, accrued dividends are about $34.93.
• If dividends are reinvested, you end the month with about 10,034.93 shares, still priced at $1.00.

This simple calculation ignores compounding within the month and assumes the yield holds steady for 30 days. In practice the daily rate shifts as holdings mature and are reinvested.

How a fund breaks the buck. Assume a prime fund holds $1 billion in assets against 1 billion shares, so its NAV is $1.00. It owns $20 million of commercial paper from a single issuer, which is 2% of assets. The issuer files for bankruptcy, and the paper is marked down to 25 cents on the dollar, a $15 million loss.

• New assets: $1,000 million − $15 million = $985 million
• NAV per share: $985 million ÷ 1,000 million shares = $0.985

Rounded to the nearest cent, that is $0.99, below the $1.00 target. If shareholders rush to redeem, the fund may have to sell holdings into a stressed market, which can push the value lower still. This is a run, and it is the scenario most post-2008 rules are designed to prevent.

Types of Money Market Funds

Money market funds are grouped by what they hold and by who may invest.

Government money market funds

A government fund must keep at least 99.5% of its total assets in cash, government securities, or repurchase agreements fully collateralized by cash or government securities. Treasury funds are a narrower subset that holds U.S. Treasury bills, notes and bonds, or repos backed by them. Some hold Treasuries alone.

Government funds are not required to impose liquidity fees, although a fund can adopt them if it discloses that in advance. Income from direct Treasury obligations may be exempt from state income tax. Repo income generally does not get that treatment, and state rules vary.

Prime money market funds

Prime funds can hold non-government debt such as commercial paper, bank CDs and corporate floating-rate notes, along with government securities. This extra credit exposure has historically let them pay somewhat more than government funds. It also means a corporate or bank default can hit them directly. The Reserve Primary Fund was a prime fund.

Tax-exempt (municipal) money market funds

These funds hold short-term debt issued by states, cities and other municipal entities. Their income is generally free from federal income tax. A single-state fund that holds only one state’s municipal debt may also be exempt from that state’s income tax for residents. Some municipal income can be subject to the alternative minimum tax. Tax-exempt funds tend to show lower stated yields than taxable funds, so the comparison that matters is your after-tax yield.

Retail vs. institutional funds

Retail funds are restricted to individual investors and often have low minimums. Institutional funds serve corporations, pension plans and other organizations, often with minimums of $1 million or more. Institutional prime and tax-exempt funds use a floating NAV and, since the 2023 reforms, face mandatory liquidity fees during heavy redemptions.

How Money Market Funds Are Regulated

U.S. money market funds are registered investment companies under the Investment Company Act of 1940, and Rule 2a-7 under that act sets their portfolio limits:

• Credit quality: holdings must present minimal credit risk, as determined by the fund’s adviser.
• Maturity of individual securities: generally no more than 397 days, about 13 months.
• Weighted average maturity (WAM): 60 days or less. WAM is the average time to maturity (or to the next interest rate reset) of all holdings, weighted by each one’s share of the portfolio. A short WAM keeps the fund’s value insensitive to rate moves.
• Weighted average life (WAL): 120 days or less. WAL measures final maturity without counting rate resets, which limits how much long-dated floating-rate debt a fund can own.
• Diversification: taxable funds generally may not put more than 5% of assets in any one issuer. Government securities and repos backed by them are exempt from this cap.
• Liquidity: at least 25% of total assets must be in daily liquid assets (cash or holdings convertible to cash within one business day) and at least 50% in weekly liquid assets.
• Stress testing and disclosure: funds must stress test their portfolios and publish holdings on a regular schedule.

Breaking the Buck: A Short History

1970s: the first funds. The first U.S. money market fund, the Reserve Fund, launched in 1971, when the Federal Reserve’s Regulation Q limited the interest banks could pay on deposits. The funds let ordinary savers earn market rates, and assets grew quickly once rates rose later in the decade. Early funds leaned on government debt. Many later added commercial paper and bank obligations for higher yields, which also added credit risk.

1994: the first loss. The Community Bankers U.S. Government Money Market Fund, a small fund sold mainly to institutions, was liquidated at about $0.96 per share after losses on derivatives, despite its “government” label.

2008: the Reserve Primary Fund. After Lehman Brothers filed for bankruptcy in September 2008, the Reserve Primary Fund, which held Lehman commercial paper, saw its NAV fall to $0.97. Heavy redemptions followed, the fund was liquidated, and investors pulled money from other prime funds as well. The U.S. Treasury responded with a temporary program guaranteeing money market fund shares held at that date, and the Federal Reserve set up emergency lending facilities.

2010 reforms. The SEC shortened maturities (including the 60-day WAM limit), tightened credit standards, introduced the first daily and weekly liquidity minimums, and allowed a fund’s board to suspend redemptions when liquidating a fund that had broken the buck.

2014 reforms. Besides requiring institutional prime and tax-exempt funds to float their NAV, the SEC let boards of non-government funds charge liquidity fees or temporarily suspend redemptions (“gates”) when weekly liquid assets dropped below set thresholds.

2020 and the 2023 reforms. In March 2020, investors redeemed heavily from prime funds as pandemic fears spread, and the Federal Reserve again created a liquidity facility. Regulators concluded that the fee and gate thresholds may have encouraged early redemptions. The 2023 reforms removed gates and the link between liquidity thresholds and fees, raised liquidity minimums to 25% daily and 50% weekly, and required institutional prime and tax-exempt funds to charge a liquidity fee when daily net redemptions exceed 5% of net assets, unless the cost of liquidity is minimal.

The low-rate years. From late 2008 through late 2015, the Federal Reserve kept its policy rate near zero and bought large amounts of securities through quantitative easing. Money market yields fell close to nothing, and many sponsors waived part of their fees to keep net yields from turning negative. Yields rose again once the Fed raised rates.

How a Money Market Fund Is Used

Money market funds fit short-term investment planning, where the priority is keeping money available and stable rather than growing it.

• A holding place between investments. Cash from a stock sale, a bond maturity or a bonus can wait here while you decide what to do next.
• Brokerage cash. Many brokers sweep uninvested cash into a money market fund or let you choose one as your settlement fund.
• Known near-term expenses. Money for taxes, tuition or a home down payment due within a year or two earns short-term rates without stock market exposure.
• Part of an emergency reserve. Some people keep part of an emergency fund in an insured account for immediate access and the rest in a money market fund.
• The cash slice of a portfolio. The fund often represents “cash” in an asset allocation, and its return is a common proxy for the risk-free rate in measures such as the Sharpe ratio.

Money market funds are not a good long-term holding. Over decades, short-term rates have offered far less growth than stocks, and in some periods they have fallen below the inflation rate, so money lost purchasing power even at $1.00 a share.

Advantages and Disadvantages

Advantages

• Low price volatility: short maturities and high credit quality keep the value very stable.
• Liquidity: shares can generally be redeemed on any business day, often with same-day or next-day settlement.
• Market-rate income: yields have often exceeded what standard savings accounts pay, especially when rates rise.
• Diversification and professional management for small balances.
• Low costs: many funds are no-load, so the main cost is the expense ratio.
• Tax options: municipal and Treasury-focused funds can reduce taxes for investors in higher brackets.

Disadvantages

• No FDIC insurance: principal is not guaranteed, and sponsors have no legal obligation to support the $1.00 price.
• Little capital appreciation: return is almost entirely interest.
• Interest rate sensitivity: income falls quickly when the central bank cuts rates.
• Inflation risk: real returns can be negative.
• Possible liquidity fees during heavy redemptions, especially at institutional prime and tax-exempt funds.
• Ordinary income taxation: dividends from taxable funds are not taxed at the lower qualified-dividend rates.

How Safe Is Money in a Money Market Fund?

Money market funds are among the lowest-risk investments available, but they are not risk-free. Three kinds of protection are worth separating.

FDIC insurance does not apply. FDIC and NCUA coverage protects deposits at banks and credit unions. A money market fund is a security, even when you buy it through a bank.

SIPC is narrower than it sounds. If your brokerage is a Securities Investor Protection Corporation member and fails with customer assets missing, SIPC can help restore your shares up to its limits. It does not cover a decline in the fund’s value, including a fund breaking the buck.

The fund’s own rules provide the real protection. Short maturities, diversification limits, liquidity minimums and credit standards keep losses unlikely and small. Past losses came from credit events (Lehman’s default) or holdings outside the usual strategy (derivatives in 1994).

Government funds carry the lowest credit risk in the category, because nearly all of their assets are Treasury or agency debt or repos backed by such debt. Corporate defaults do not reach them the way they reach prime funds. Their remaining risks are low yields when rates drop and the absence of any guarantee.

Money Market Fund vs. Similar Options

Money market fund vs. money market account

The names are similar, but a money market account is a bank or credit union deposit. It is insured by the FDIC or NCUA up to the applicable limits, often comes with checks or a debit card, may limit certain withdrawals per month, and pays a rate the bank can change at any time. A money market fund is an uninsured pooled investment whose yield comes from its portfolio. If your priority is a guaranteed balance, the account wins. If you want a yield tied directly to short-term market rates, or you are holding cash inside a brokerage account, the fund is often the more convenient choice.

Money market fund vs. high-yield savings account

A savings account is also an insured deposit with a rate the bank sets. Online savings rates can be competitive with money market fund yields, but they follow market rates at the bank’s discretion. Money market fund yields reflect the portfolio’s actual earnings.

Money market fund vs. Treasury bills

You can buy T-bills directly, either at auction or through a broker. This avoids a fund’s expense ratio and locks in a known yield until maturity. A government money market fund offers daily liquidity, automatic reinvestment and no need to manage individual maturity dates. In exchange, you pay a management fee and accept a yield that changes from day to day.

Money market fund vs. ultrashort bond fund

Ultrashort bond funds, sometimes sold as “enhanced cash,” hold slightly longer or lower-quality debt and do not have to follow Rule 2a-7. Their NAV floats freely and can fall noticeably when credit spreads widen or rates jump. They may yield more, but they are not a substitute where stability matters, so check their yield to maturity and duration before buying.

Money market fund vs. other mutual funds and ETFs

Stock and bond funds, whether mutual funds or ETFs, aim for growth or long-term income and accept price swings to get there. A money market fund gives up growth for stability. Some cash-like ETFs hold Treasury bills, but they trade at market prices during the day and do not maintain a $1.00 NAV.

FAQs

The Bottom Line

Money market funds suit investors parking cash they will need soon, such as brokerage balances, money between investments or funds for near-term bills. They offer stability and market-rate income, but they carry no FDIC insurance and can break the buck. Government funds carry the lowest credit risk, and long-term savings belong in growth investments because short-term yields can trail inflation.