Guide

Money market funds explained

A money market fund (MMF) is a mutual fund that invests in very short-term, high-credit-quality debt instruments — Treasury bills, repurchase agreements, commercial paper, and certificates of deposit — with the goal of preserving principal and paying a modest yield. Unlike most mutual funds, retail MMFs traditionally quote a stable $1.00 net asset value (NAV) per share; your balance looks like a bank balance even though you own fund shares. Millions of investors encounter MMFs through brokerage sweep accounts that automatically park idle cash overnight. This guide explains how MMFs work, the government vs prime vs municipal types, what "breaking the buck" means, how yields compare to T-bills and high-yield savings, and where MMFs fit in an emergency fund or cash allocation sleeve.

What money market funds actually hold

Under SEC Rule 2a-7, money market funds must maintain a portfolio of short-maturity, liquid instruments. Typical holdings include:

U.S. Treasury bills and agency debt — the safest slice, common in government MMFs.
Repurchase agreements (repos) — overnight collateralized loans, often backed by Treasuries.
Commercial paper — unsecured short-term corporate IOUs; higher yield, higher credit risk (prime funds).
Certificates of deposit and time deposits — bank-issued short-term debt.
Municipal short-term notes — tax-exempt debt for state and local governments (muni MMFs).

Weighted average maturity is capped (generally 60 days or less for the strictest categories), and funds must hold minimum liquidity buffers so redemptions can be met even when markets freeze. The portfolio looks like the short end of fixed income — but packaged for daily liquidity rather than holding individual bonds to maturity.

How the stable $1 NAV works

Most retail MMFs use amortized cost accounting to round share prices to $1.00, so small interest-rate moves do not show up as penny-by-penny NAV fluctuations. Instead, yield accrues as additional shares or as a stated seven-day SEC yield that compounds inside the fund. Institutional "prime" funds that invest in corporate paper can still use the stable NAV for government and retail share classes, but post-2008 reforms created floating-NAV institutional share classes for certain prime holdings — a distinction most individual investors never touch.

The stable NAV is a convenience, not a guarantee. If portfolio losses exceed a fund's cushion, the NAV can fall below $1.00 — known as breaking the buck. The Reserve Primary Fund broke the buck in September 2008 after Lehman Brothers commercial paper defaulted, triggering a broader run on prime MMFs. The 2020 pandemic shock caused the Federal Reserve to backstop MMFs again when institutional prime funds faced heavy outflows. Government MMFs — limited to Treasuries, agencies, and repos collateralized by government securities — have a far cleaner history for retail savers.

Government, prime, and municipal MMFs

Not all money market funds take the same risk. The three main retail categories:

Government money market funds

Invest at least 99.5% of assets in government securities — Treasuries, agency debt, and government repos. Credit risk is minimal; yields track short Treasury rates closely. These are the default choice for conservative cash parking, brokerage sweeps at major firms, and many 401(k) stable-value adjacent options. When headlines mention "money market yields," government fund SEC yields are usually the cleanest benchmark.

Prime money market funds

Hold a mix of government paper and corporate commercial paper, CDs, and bank debt. Prime funds historically paid more than government funds because corporates carry spread over Treasuries. They also broke the buck in 2008 and saw stress in 2020. Post-reform rules impose liquidity fees and redemption gates when weekly liquid assets fall below thresholds — meaning you might pay to exit or wait during a crisis. Many retail investors have migrated to government funds; prime remains more common in institutional treasury management.

Municipal (tax-exempt) money market funds

Invest in short-term municipal debt. Interest is generally federal income-tax exempt and may be state-tax exempt if you buy a fund focused on your state's issuers. The stated yield looks lower than taxable government funds, but the tax-equivalent yield can win for investors in high marginal brackets. Compare after-tax returns using your actual federal and state rates — not a generic example.

Sweep accounts: where most people meet MMFs

When you sell stock or receive a dividend at a brokerage, cash often lands in a sweep account — an automatic transfer into a partner money market fund (or, at some banks, FDIC-insured bank sweep programs). You earn yield on idle cash without manually buying a fund each day. Sweep rates change with the fund's portfolio yield minus any platform spread; they are not always the highest available rate on the platform's fund menu.

Check three details in your brokerage settings: which fund is the default sweep, whether you can elect a higher-yield government fund manually, and whether uninvested cash below a minimum sits at 0% instead of sweeping. Active traders sometimes leave large cash balances in sweep vehicles between trades — reasonable for liquidity, but worth comparing to direct T-bill ladders if the balance is large and persistent.

MMFs vs high-yield savings vs T-bills vs CDs

All four are "cash-like," but the protections and mechanics differ:

High-yield savings accounts (HYSA) — Bank deposits insured by FDIC (or NCUA for credit unions) up to $250,000 per depositor per institution. Rate is set by the bank and can change anytime. Best for emergency funds where insurance clarity matters.
Money market funds — Not FDIC insured. You own mutual fund shares; risk is credit and liquidity risk of the underlying portfolio (low for government funds, higher for prime). Often slightly higher yield than HYSA at brokers; easy integration with investment accounts.
Treasury bills — Direct obligations of the U.S. government. Bought at auction or via ETFs; state-tax exempt on interest at the federal level. Slightly more operational friction than a one-click MMF but maximal credit clarity. See the dedicated T-bill guide for auction mechanics and laddering.
Certificates of deposit (CDs) — Time deposits with a fixed term and early-withdrawal penalties. Higher yield than demand deposits when you lock up funds you will not need until maturity.

A common retail stack: HYSA or government MMF for the first tier of emergency liquidity, T-bill ladder or government MMF for larger brokerage cash, and CDs only for known future expenses with a firm date. The right mix depends on insurance preference, tax bracket, and whether cash lives at a bank or a brokerage.

Reading the yield quote

MMFs publish a seven-day SEC yield — the annualized income the fund earned over the past seven days, net of expenses, assuming that rate continued for a year. It moves daily with short rates. Compare SEC yield minus expense ratio across funds; a 0.10% expense difference matters on a $100,000 balance ($100/year). Unlike compound growth in long-term equities, MMF yield is almost entirely income — reinvest distributions if your goal is to keep pace with inflation on cash, not to build wealth.

Taxes, expenses, and 401(k) stable value

Taxable MMF distributions are generally ordinary income — not qualified dividend rates. Municipal MMF income is usually federal-tax exempt; state treatment varies. Hold taxable government MMFs in taxable brokerage accounts; hold tax-exempt muni MMFs only when your bracket justifies the lower pre-tax yield.

Expense ratios on government MMFs from major providers often range from 0.09% to 0.40%; some brokerages waive fees on their proprietary sweep fund. In employer 401(k) plans, a "stable value" or money market option may be the default for contributions until you elect stock and bond funds — understand that idle 401(k) cash is not earning equity returns and may lag inflation over decades if you never rebalance.

Risks worth remembering

Not bank insurance. MMFs are securities. Government fund risk is tiny but not identical to FDIC.
Prime fund credit and gate risk. Prefer government funds unless you explicitly need the extra spread and understand liquidity fees.
Rate risk is minimal but nonzero. When the Fed cuts rates, MMF yields fall within weeks — unlike a fixed-rate CD.
Inflation erosion. Cash vehicles preserve nominal dollars; they rarely beat long-run inflation after tax. MMFs are parking, not investing.
Platform risk. Brokerage failure is separate from fund failure; SIPC protects securities at member brokers, not the MMF's underlying credit losses.

Production checklist for retail cash allocation

Classify the cash: emergency liquidity, near-term spending, or dry powder between trades.
Default to government MMF or FDIC HYSA for emergency tiers — skip prime unless you understand gates and credit spread.
Compare seven-day SEC yield net of expense ratio across your broker's sweep fund and standalone options.
For large persistent balances, model tax-equivalent yield: muni MMF vs taxable government MMF vs T-bill ETF.
Confirm sweep settings so idle cash is not earning 0% at the brokerage.
Revisit allocation when the Fed shifts policy — MMF yields follow short rates, not your memory of last year's quote.
Do not confuse MMF yield with total portfolio return; keep equity and bond sleeves sized per your allocation plan.

Key takeaways

Money market funds are mutual funds holding short-term debt, designed for daily liquidity and stable $1 share prices on retail share classes.
Government MMFs are the conservative default; prime funds add corporate credit risk and crisis-era gate history.
Sweep accounts automatically park brokerage cash in MMFs — verify which fund and what yield you actually receive.
MMFs are not FDIC insured; HYSA and T-bills trade different protections for similar economic roles.
SEC seven-day yield is the apples-to-apples comparison metric — subtract expense ratios before choosing.
MMFs are for preserving cash, not building wealth — pair with equities and bonds for long-term goals.