PMMF vs. SPAXX

Last updated Jun 3, 2026

PMMF (iShares Prime Money Market ETF) and SPAXX (Fidelity Government Money Market Fund) are both Money Market funds. Over the past year, PMMF returned 4.01% vs 3.66% for SPAXX. At a 0.01 correlation, their price movements are largely independent.

Performance

PMMF vs. SPAXX - Performance Comparison

The chart below illustrates the hypothetical performance of a $10,000 investment in iShares Prime Money Market ETF (PMMF) and Fidelity Government Money Market Fund (SPAXX). The values are adjusted to include any dividend payments, if applicable.

Returns By Period

In the year-to-date period, PMMF achieves a 1.51% return, which is significantly higher than SPAXX's 1.37% return.

PMMF

• 1D: 0.01%
• 1M: 0.31%
• YTD: 1.51%
• 6M: 1.82%
• 1Y: 4.01%
• 3Y*: —
• 5Y*: —
• 10Y*: —

SPAXX

• 1D: 0.00%
• 1M: 0.28%
• YTD: 1.37%
• 6M: 1.67%
• 1Y: 3.66%
• 3Y*: 2.42%
• 5Y*: 1.45%
• 10Y*: —

*Multi-year figures are annualized to reflect compound growth (CAGR)

PMMF vs. SPAXX - Yearly Performance Comparison

	2026 (YTD)	2025
PMMF iShares Prime Money Market ETF	1.51%	3.85%
SPAXX Fidelity Government Money Market Fund	1.37%	3.61%

Correlation

The correlation between PMMF and SPAXX is -0.01, meaning there is essentially no relationship between their price movements. Each responds to its own set of market drivers, making them strong candidates for combining in a diversified portfolio.

	Correlation
Correlation (1Y) Calculated over the trailing 1-year period	-0.01
Correlation (All Time) Calculated using the full available price history since Feb 7, 2025	0.01

Return for Risk

PMMF vs. SPAXX — Risk / Return Rank

Compare risk-adjusted metric ranks to identify better-performing investments over the past 12 months.

PMMF

PMMF Risk / Return Rank: 100
PMMF Sharpe Ratio Rank: 100
PMMF Sortino Ratio Rank: 100
PMMF Omega Ratio Rank: 100
PMMF Calmar Ratio Rank: 100
PMMF Martin Ratio Rank: 100

The rank (0–100) shows how this investment's returns compare to the risk taken. Higher = better. Based on the past 12 months of data, combining Sharpe, Sortino, and other metrics used by quantitative funds and institutional investors.

PMMF vs. SPAXX - Risk-Adjusted Trends Comparison

This table presents a comparison of risk-adjusted performance metrics for iShares Prime Money Market ETF (PMMF) and Fidelity Government Money Market Fund (SPAXX). Risk-adjusted metrics are performance indicators that assess an investment's returns in relation to its risk, enabling a more accurate comparison of different investment options.

	PMMF	SPAXX	Difference
Sharpe ratio Return per unit of total volatility	19.78	3.65	+16.14
Sortino ratio Return per unit of downside risk	95.43	—	—
Omega ratio Gain probability vs. loss probability	38.46	—	—
Calmar ratio Return relative to maximum drawdown	162.38	—	—
Martin ratio Return relative to average drawdown	1,502.39	—	—
Data is calculated on a 1-year rolling basis and updated daily. The trend shows the change in the indicator over the past month.

PMMF vs. SPAXX - Sharpe Ratio Comparison

The current PMMF Sharpe Ratio is 19.78, which is higher than the SPAXX Sharpe Ratio of 3.65. The chart below compares the historical Sharpe Ratios of PMMF and SPAXX, calculated using daily returns over the previous 12 months. A higher Sharpe Ratio indicates better risk-adjusted performance relative to the risk-free rate.

Sharpe Ratios by Period

	PMMF	SPAXX	Difference
Sharpe Ratio (1Y) Calculated over the trailing 1-year period	19.78	3.65	+16.14
Sharpe Ratio (5Y) Calculated over the trailing 5-year period	—	2.13	—
Sharpe Ratio (All Time) Calculated using the full available price history	11.98	2.13	+9.85

Drawdowns

PMMF vs. SPAXX - Drawdown Comparison

The maximum PMMF drawdown since its inception was -0.13%, which is greater than SPAXX's maximum drawdown of 0.00%. Use the drawdown chart below to compare losses from any high point for PMMF and SPAXX.

Drawdown Indicators

	PMMF	SPAXX	Difference
Max Drawdown Largest peak-to-trough decline	-0.13%	0.00%	-0.13%
Max Drawdown (1Y) Largest decline over 1 year	-0.02%	0.00%	-0.02%
Max Drawdown (3Y) Largest decline over 3 years	—	0.00%	—
Max Drawdown (5Y) Largest decline over 5 years	—	0.00%	—
Current Drawdown Current decline from peak	0.00%	0.00%	0.00%
Average Drawdown Average peak-to-trough decline	-0.00%	0.00%	0.00%
Ulcer Index Depth and duration of drawdowns from previous peaks	0.00%	0.00%	0.00%

Volatility

PMMF vs. SPAXX - Volatility Comparison

The current volatility for iShares Prime Money Market ETF (PMMF) is 0.05%, while Fidelity Government Money Market Fund (SPAXX) has a volatility of 0.28%. This indicates that PMMF experiences smaller price fluctuations and is considered to be less risky than SPAXX based on this measure. The chart below showcases a comparison of their rolling one-month volatility.

Volatility by Period

	PMMF	SPAXX	Difference
Volatility (1M) Calculated over the trailing 1-month period	0.05%	0.28%	-0.23%
Volatility (6M) Calculated over the trailing 6-month period	0.12%	0.72%	-0.60%
Volatility (1Y) Calculated over the trailing 1-year period	0.20%	1.03%	-0.83%
Volatility (5Y) Calculated over the trailing 5-year period, annualized	0.34%	0.69%	-0.35%
Volatility (10Y) Calculated over the trailing 10-year period, annualized	0.34%	0.69%	-0.35%

Dividends

PMMF vs. SPAXX - Dividend Comparison

PMMF's dividend yield for the trailing twelve months is around 3.76%, more than SPAXX's 3.59% yield.

Position	TTM	2025	2024	2023
PMMF iShares Prime Money Market ETF	3.76%	3.59%	0.00%	0.00%
SPAXX Fidelity Government Money Market Fund	3.59%	3.88%	1.53%	0.41%

Frequently Asked Questions

Can I hold both PMMF and SPAXX in my portfolio?

PMMF and SPAXX have a correlation of -0.01, meaning they provide meaningful diversification benefit when combined. Depending on your allocation goals, holding both could reduce overall portfolio risk.

Which is riskier, PMMF or SPAXX?

SPAXX has higher volatility (0.28%) compared to PMMF (0.05%). In terms of maximum drawdown, PMMF dropped -0.13% vs SPAXX's 0.00%.

Which has better risk-adjusted returns?

PMMF currently has the higher Sharpe Ratio (19.78 vs 3.65), meaning it's delivered slightly more return per unit of risk over the trailing 12 months. However, this ranking shifts over time - use the Risk/Return Score above for a more comprehensive view that combines Sharpe, Sortino, and other measures used by quantitative funds.