Definition
A money market fund is a type of mutual fund that invests in very short-term, high-quality debt — things like U.S. Treasury bills, repurchase agreements ("repos"), and, for some funds, top-rated commercial paper issued by large corporations. The holdings mature in days to a few months, which keeps the fund's value extremely stable.
The defining feature is the stable $1.00 share price. Most money market funds are managed so that one share is always worth one dollar. You buy in at $1.00, you sell out at $1.00, and the return you earn shows up as interest that accrues and is typically paid to you monthly. Because the price barely moves, the fund behaves much like a cash account that pays a yield — which is exactly why investors use it as a place to park money they are not ready to invest.
That $1.00 target is a design goal, not a legal guarantee. The fund's true per-share value is still a net asset value (NAV) like any other fund's — it just sits so close to $1.00 that it is quoted as a constant. In rare, severe market stress a money fund's real value can slip below $1.00, an event the industry calls "breaking the buck." It is uncommon, but it is the reason a money market fund is not the same thing as cash in a bank.
The standard way funds quote their income is the 7-day SEC yield — a close cousin of the 30-day SEC yield used for bond ETFs. It annualizes the income the fund earned over the most recent seven days, net of the fund's expenses. Because the holdings turn over so quickly, a 7-day window is fresh enough to reflect what the fund is earning right now.
Why It Matters
For an income investor, a money market fund solves a specific problem: where do I keep cash that I want to stay safe, stay liquid, and still earn something? Before yields on cash rose, the answer was often "a checking account earning nothing." A money market fund lets that same cash earn a yield that tracks short-term interest rates, while remaining available to withdraw — usually the same day or the next day.
There are three broad types, and the differences matter:
- Government money market funds hold only U.S. Treasury bills, government agency debt,
and repos backed by those securities. They are the most conservative and the most common. VMFXX, Vanguard's flagship federal money market fund, is a well-known example.
- Prime money market funds can also hold corporate commercial paper and other
short-term private debt. They usually yield a little more than government funds because they take slightly more credit risk, and some carry redemption rules that government funds do not.
- Municipal (tax-exempt) money market funds hold short-term debt issued by state and
local governments. Their income is generally exempt from federal income tax, which can make their lower headline yield competitive once you account for taxes.
The single most important thing to understand is that a money fund's yield tracks the Federal Reserve and short-term rates — and can move fast. When the Fed raises rates, money fund yields climb within weeks as the fund rolls its maturing bills into higher- yielding ones. When the Fed cuts, the yield falls just as quickly. A money market fund is therefore a *floating-rate* place for cash: today's attractive 7-day SEC yield is not locked in and can be meaningfully lower a few months later.
Key point: a money market fund is not FDIC-insured. A bank savings account is insured up to $250,000 per depositor; a money fund is a securities investment. The risk is low, but it is not zero, and it is a different kind of protection.
Example
The figures below are illustrative and chosen to show the relationships — they are not live quotes. Real yields move with the Fed, sometimes week to week, so look each option up for current numbers. What matters is the *pattern*, not the exact percentages.
| Vehicle | Yield basis | Liquidity | Main risk | Tax angle |
|---|---|---|---|---|
| Government money market fund | 7-day SEC yield (~4.8%) | Same/next-day, no fixed price move | Rate falls fast; not FDIC-insured | Treasury portion often state-tax-exempt |
| T-bill ETF (SGOV, BIL) | 30-day SEC yield (~4.9%) | Trades intraday; settles T+1 | Tiny price wiggle intraday | Treasury interest often state-tax-exempt |
| High-yield savings (HYSA) | Stated APY (~4.5%) | Same-day at the bank | Bank can cut APY anytime | Fully taxable at ordinary rates |
Read across the rows and the trade-offs come into focus. A government money market fund and a T-bill ETF like SGOV or BIL are close cousins: both hold short-dated Treasuries and both quote an SEC yield, so their returns tend to land in the same neighborhood. The main mechanical difference is *how* you hold them. The money fund keeps a flat $1.00 price and settles inside your brokerage as cash-like; the ETF trades on an exchange at a price that drifts a few cents up over the month and drops on the ex-dividend date when it pays out.
A high-yield savings account is the banking-world alternative. Its headline APY may look similar, but two things set it apart: it is FDIC-insured up to the limit, and its interest is fully taxable at your ordinary income rate. The money fund and the T-bill ETF, by contrast, earn much of their income from U.S. Treasury securities, and Treasury- backed interest is generally exempt from state and local income tax. For an investor in a high-tax state, that exemption can quietly tip the after-tax comparison in favor of the Treasury-heavy options even when the pre-tax yields look identical.
How They Compare To Cash ETFs
If money market funds sound a lot like the "cash" and "ultra-short Treasury" ETFs you may have seen, that is because they overlap heavily — they just live in different wrappers.
- Same underlying assets. A government money fund and a T-bill ETF such as
SGOV both own short-dated U.S. Treasuries. Their yields rise and fall together with the same short-term rates.
- Different price behavior. A money fund holds a constant $1.00 NAV. A T-bill ETF has
a real, moving market price — it typically ticks up a little each day as interest accrues, then steps down on the ex-dividend date. Over a full month the total return is similar; the ETF just shows it as price-plus-distribution rather than a flat price.
- Different access. A money fund is bought and sold at the end-of-day price directly
in your brokerage, often usable as a cash sweep. An ETF trades intraday like a stock, which some investors prefer for flexibility.
- Watch the quote you compare. A money fund quotes a 7-day SEC yield; an ETF
usually quotes a 30-day SEC yield. Both are net of fees and comparable in spirit, but they are measured over different windows, so treat small differences as noise.
Neither wrapper is universally "better." The money fund wins on simplicity and a rock- steady price; the ETF wins on intraday trading and, sometimes, a lower expense ratio.
Common Mistakes
- Assuming a money market fund is FDIC-insured. It is not. A money market *deposit
account* at a bank is insured; a money market *fund* is a securities investment. The risk is low, but confusing the two is the most common and most important error.
- Confusing the stable $1.00 price with "no risk." The stable price is a management
goal, not a guarantee. In extreme stress a fund can "break the buck" and fall below $1.00. It is rare, but it means the fund is not literally the same as cash.
- Treating today's yield as permanent. A money fund's yield floats with the Fed. The
attractive 7-day SEC yield you see today can drop sharply within months if short rates fall. If you need a locked-in rate, a money fund is the wrong tool.
- Ignoring the tax angle. A prime fund's slightly higher pre-tax yield can lose to a
Treasury-heavy government fund after taxes, because Treasury interest is often exempt from state tax while corporate-paper income is not. Compare after-tax yields.
- Parking long-term money in cash ("cash drag"). A money fund is excellent for an
emergency fund or money you will spend soon. But cash earning ~4-5% badly lags stocks and dividend ETFs over long horizons. Money you will not touch for years usually should not sit in a money fund — that opportunity cost is the "cash drag" on a portfolio.
- Comparing the wrong yield numbers. Lining a money fund's 7-day SEC yield up against
a bank's APY, an ETF's 30-day SEC yield, and a bond fund's distribution rate is apples to oranges. Know which figure each product is quoting before you decide one "yields more."
FAQ
Are money market funds safe?
Money market funds are among the lowest-risk investments available, but "low risk" is not the same as "no risk." They hold very short-term, high-quality debt and aim for a stable $1.00 share price, so day-to-day losses are extremely unlikely. However, they are not FDIC-insured, and in rare, severe market stress a fund can "break the buck" and fall below $1.00. Government money market funds, which hold only Treasuries and government- backed repos, are considered the safest type. For most investors a money fund is a very safe place for cash — just understand it is a securities investment, not an insured bank deposit.
Money market fund vs high-yield savings account?
They are close competitors for the same job: earning a yield on cash you want to keep liquid. A high-yield savings account (HYSA) is a bank product, FDIC-insured up to $250,000 per depositor, with interest taxed fully at ordinary rates. A money market fund is a brokerage investment, not FDIC-insured, whose yield tracks short-term rates and whose Treasury-derived income is often exempt from state tax. In a high-tax state, the money fund's after-tax yield can win even when the pre-tax numbers look similar. If FDIC insurance is your priority, choose the HYSA; if you want a Treasury-backed yield inside your brokerage and a possible state-tax break, the money fund is compelling.
What is the 7-day SEC yield?
The 7-day SEC yield is the standardized figure money market funds use to quote their income. It takes the income the fund earned over the most recent seven days, subtracts the fund's expenses, and annualizes the result. Because a money fund's holdings mature so quickly, a seven-day window is recent enough to reflect what the fund is earning right now. It is the money-fund equivalent of the 30-day SEC yield used for bond ETFs, and because every fund calculates it the same way, it lets you compare two money funds on a level field.
Government vs prime vs municipal money market funds — what's the difference?
Government funds hold only Treasuries, agency debt, and repos backed by them; they are the most conservative and most common. Prime funds also hold corporate commercial paper and other short-term private debt, so they usually yield a bit more in exchange for slightly higher credit risk and sometimes extra redemption rules. Municipal (tax- exempt) funds hold short-term state and local government debt, and their income is generally exempt from federal income tax — which can make a lower headline yield competitive after taxes, especially for high earners. Match the type to how much risk and which tax treatment you want.
How do money market funds compare to T-bill ETFs like SGOV or BIL?
They are close cousins. A government money fund and a T-bill ETF such as SGOV or BIL both hold short- dated U.S. Treasuries, so their yields move together and both offer Treasury interest that is often state-tax-exempt. The main difference is the wrapper: the money fund keeps a flat $1.00 price and settles as cash inside your brokerage, while the ETF trades intraday at a price that drifts up over the month and steps down on the ex-dividend date. Total returns tend to be similar; pick the money fund for simplicity or the ETF for intraday trading and sometimes a lower fee.
Why did my money market fund's yield drop?
Because money market fund yields float with short-term interest rates. The fund constantly rolls maturing bills into new ones, so when the Federal Reserve cuts rates, the fund's new holdings earn less and its 7-day SEC yield falls within weeks. The reverse happens when the Fed raises rates. This is normal and expected — a money fund never locks in a rate. If a steady, guaranteed yield matters to you, a money fund is the wrong tool; consider a CD or a longer-dated bond instead.