Why Money Market Fund NAV Stays at $1.00

A money market fund’s net asset value (NAV) stays fixed at $1.00 per share because the SEC requires these funds to use amortized-cost accounting and maintain strict constraints on credit quality, maturity, and portfolio composition. This artificial stability masks small changes in the fund’s actual economic value, but it delivers a familiar, cash-like return and shields investors from the daily mark-to-market swings that plague other bond funds.

What Is Amortized-Cost Accounting?

Amortized-cost accounting is the sleight of hand that keeps NAV at $1.00. Instead of revaluing the fund’s holdings daily to current market prices (mark-to-market), the SEC allows money market funds to adjust the cost basis of their securities over their life using a straight-line method — as if interest accrues predictably and the security will be held to maturity.

When you buy a Treasury bill for $99.50 with a face value of $100, maturing in 60 days, amortized-cost accounting spreads the $0.50 gain evenly over those 60 days. Each day, the value rises by a tiny fraction, accruing “interest” that shows up as a higher share price or reinvested dividend. By maturity, the security has climbed back to $100 (face value). The fund realizes the gain in stages, not all at once.

Under true mark-to-market accounting, that same bill would jump or fall daily with changes in short-term interest rates. If rates fell after purchase, the bill would be worth more than $99.50; if rates rose, it would be worth less. The fund’s NAV would fluctuate accordingly. But amortized-cost accounting smooths those moves away, keeping the NAV at $1.00.

Why the SEC Imposed This Rule

Money market funds historically marketed themselves as cash equivalents — liquid, safe, and stable. Investors expected $1 in, $1 out (plus interest). When interest rates moved, money market funds’ actual values changed, but funds wanted to avoid the PR nightmare and investor confusion of saying “your money-market investment is down 0.3% today because rates rose.”

The SEC codified amortized-cost accounting to preserve that cash-like image and prevent runs. If money market NAVs fluctuated like bond-fund NAVs, investors would redeem en masse during rate spikes, threatening the funds’ stability and the short-term credit markets they fund.

The 2008 financial crisis exposed a dangerous side effect: when credit risk rose sharply, some money market funds’ actual value fell below $1.00, and they “broke the buck.” This shattered the illusion and triggered panicked redemptions. To prevent a repeat, the SEC added reserve-fund rules: funds now maintain a buffer (typically 1–2% of assets) to absorb losses and prevent NAV from falling below $1.00.

Portfolio Constraints That Support Stability

To keep a stable $1.00 NAV credible, the SEC imposes strict rules on what money market funds can buy:

Maturity limits: No single security can have a maturity longer than 397 days (roughly 13 months); the fund’s portfolio weighted-average maturity cannot exceed 60 days. This discipline ensures that interest-rate movements have minimal impact on NAV — if all holdings mature within 60 days, a 1% change in yields moves the overall value only a few basis points.

Credit quality: Funds must invest primarily in investment-grade securities — Treasury bills, highly rated commercial paper, repurchase agreements, and agency debt. Default risk is minimal.

Concentration limits: No single issuer can account for more than 5% of assets (with narrow exceptions), limiting idiosyncratic risk.

These constraints create a “boring” portfolio by design. But boring is the point. A money market fund’s job is to provide a low-volatility parking place for cash, not to hunt for yield. The yield it does produce comes from the spread between the rate the fund earns on its holdings and the rate it pays investors.

How Returns Happen (and Why They’re Real)

Amortized-cost accounting doesn’t make returns disappear — it just records them differently. When a money market fund holds a Treasury bill bought at $99.50 and redeemed at $100, the $0.50 gain is real. Under amortized-cost accounting, it’s booked as accrued interest over the holding period, showing up as a small daily increase in NAV (if reinvested) or as a dividend payment.

A fund’s yield comes from:

• Coupon payments on short-term notes and bonds
• The spread between the purchase price and face value on discounted securities
• Reinvestment of maturing proceeds at market rates

If you buy a money market fund yielding 5% while investing elsewhere yields 3%, you’re capturing real economic value — amortized-cost accounting just hides the daily volatility that would normally accompany a higher-yielding, longer-duration portfolio.

The NAV-Fixing Mechanism: What Happens on Trouble Days

On days when credit fear spikes or interest rates jump sharply, amortized-cost accounting faces a test. If the fund’s actual economic value (mark-to-market) falls below $1.00, the gap must be covered somehow.

The primary buffer is the reserve fund — assets earmarked to absorb losses without breaching the $1.00 NAV. Funds are required to hold this reserve up to 1–2% of assets (depending on fund risk profile). If a credit event hits, the reserve is tapped first. Investors redeem at $1.00, but the fund’s economics take the loss.

If the reserve is depleted, the fund faces a few options:

• Cease operations and liquidate
• Ask regulators for permission to allow NAV to float below $1.00 (a “break the buck” event)
• Be absorbed by another fund

The 2008 crisis proved that even strict rules don’t guarantee stability if credit stress is severe enough. Since then, the regulatory regime has tightened further, and money market funds have begun offering different share classes: some with stable $1.00 NAV (for retail investors who prize certainty), and others with floating NAV (for institutions willing to tolerate daily price moves in exchange for greater transparency and lower losses in a panic).

Who Benefits and Why It Matters

Retail investors and corporations with surplus cash prefer stable-NAV money market funds. The simplicity and familiarity — $1 in, $1.00x out — are valuable. You’re not tracking a fluctuating price; you’re earning a known yield on what amounts to a ultra-safe deposit.

Financial systems benefit because money market funds are a critical source of short-term funding for banks, corporations, and governments. If money market funds became volatile and unreliable, the entire plumbing of short-term credit would suffer.

Fund managers benefit because stable NAV avoids the political and psychological friction of marking-to-market. But the cost is regulatory burden: the constraints on maturity, credit quality, and diversification make it hard to generate exceptional returns. Money market funds compete almost entirely on fees and brand trust, not investment acumen.

The Trade-Off: Stability vs. Transparency

Amortized-cost accounting creates a comfortable fiction. Your NAV never moves, so you sleep at night. But the actual economic value of the fund — what you’d get if you forced it to liquidate and sell everything at current market prices — may differ from $1.00. On good days, it’s above $1.00; on stress days, it’s below.

The financial crisis showed that prolonged stress can exhaust the reserve fund. Some experts argue for floating NAV (or widening use of it), which would force daily transparency: if your share is worth $0.998, you see it immediately, and panic-driven runs become less likely because the loss is already priced in.

Today’s rule is a compromise: stable-NAV funds for those who value certainty, and floating-NAV options (more common since 2014 regulatory changes) for those who prefer true mark-to-market pricing and are less prone to run behavior.

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