MMIBX vs. MUB

Last updated Jun 3, 2026

MMIBX (MFS Municipal Income Fund) and MUB (iShares National AMT-Free Muni Bond ETF) are both Municipal Bonds funds. Over the past 10 years, MMIBX returned 1.50%/yr vs 2.00%/yr for MUB. A 0.58 correlation means they provide meaningful diversification when combined. MMIBX charges 1.45%/yr vs 0.07%/yr for MUB.

Performance

MMIBX vs. MUB - Performance Comparison

The chart below illustrates the hypothetical performance of a $10,000 investment in MFS Municipal Income Fund (MMIBX) and iShares National AMT-Free Muni Bond ETF (MUB). The values are adjusted to include any dividend payments, if applicable.

Returns By Period

The year-to-date returns for both stocks are quite close, with MMIBX having a 1.38% return and MUB slightly lower at 1.32%. Over the past 10 years, MMIBX has underperformed MUB with an annualized return of 1.50%, while MUB has yielded a comparatively higher 2.00% annualized return.

MMIBX

• 1D: 0.00%
• 1M: 0.51%
• YTD: 1.38%
• 6M: 1.63%
• 1Y: 6.69%
• 3Y*: 3.55%
• 5Y*: -0.01%
• 10Y*: 1.50%

MUB

• 1D: 0.15%
• 1M: 0.53%
• YTD: 1.32%
• 6M: 1.88%
• 1Y: 7.06%
• 3Y*: 3.46%
• 5Y*: 0.91%
• 10Y*: 2.00%

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

MMIBX vs. MUB - Yearly Performance Comparison

	2026 (YTD)	2025	2024	2023	2022	2021	2020	2019	2018	2017
MMIBX MFS Municipal Income Fund	1.38%	3.56%	2.42%	5.45%	-12.27%	2.35%	3.28%	7.40%	0.38%	5.00%
MUB iShares National AMT-Free Muni Bond ETF	1.32%	3.78%	1.26%	5.56%	-7.34%	1.02%	5.12%	7.06%	0.93%	4.72%

Correlation

The correlation between MMIBX and MUB is 0.71, which is moderate. They share some common price drivers but move independently often enough to provide real diversification benefit when combined.

	Correlation
Correlation (1Y) Calculated over the trailing 1-year period	0.71
Correlation (3Y) Calculated over the trailing 3-year period	0.74
Correlation (5Y) Calculated over the trailing 5-year period	0.72
Correlation (10Y) Calculated over the trailing 10-year period	0.68
Correlation (All Time) Calculated using the full available price history since Sep 11, 2007	0.58

The correlation between MMIBX and MUB shifts across timeframes, from 0.58 (all time) to 0.74 (3 years), reflecting how their relationship changes across market environments.

Return for Risk

MMIBX vs. MUB — Risk / Return Rank

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

MMIBX

MMIBX Risk / Return Rank: 48
MMIBX Sharpe Ratio Rank: 47
MMIBX Sortino Ratio Rank: 57
MMIBX Omega Ratio Rank: 74
MMIBX Calmar Ratio Rank: 32
MMIBX Martin Ratio Rank: 32

MUB

MUB Risk / Return Rank: 67
MUB Sharpe Ratio Rank: 74
MUB Sortino Ratio Rank: 78
MUB Omega Ratio Rank: 84
MUB Calmar Ratio Rank: 48
MUB Martin Ratio Rank: 50

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.

MMIBX vs. MUB - Risk-Adjusted Trends Comparison

This table presents a comparison of risk-adjusted performance metrics for MFS Municipal Income Fund (MMIBX) and iShares National AMT-Free Muni Bond ETF (MUB). 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.

	MMIBX	MUB	Difference
Sharpe ratio Return per unit of total volatility	2.03	2.43	-0.40
Sortino ratio Return per unit of downside risk	3.18	3.55	-0.37
Omega ratio Gain probability vs. loss probability	1.48	1.51	-0.03
Calmar ratio Return relative to maximum drawdown	2.18	2.42	-0.25
Martin ratio Return relative to average drawdown	7.36	8.59	-1.24
Data is calculated on a 1-year rolling basis and updated daily. The trend shows the change in the indicator over the past month.

MMIBX vs. MUB - Sharpe Ratio Comparison

The current MMIBX Sharpe Ratio is 2.03, which is comparable to the MUB Sharpe Ratio of 2.43. The chart below compares the historical Sharpe Ratios of MMIBX and MUB, 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

	MMIBX	MUB	Difference
Sharpe Ratio (1Y) Calculated over the trailing 1-year period	2.03	2.43	-0.40
Sharpe Ratio (5Y) Calculated over the trailing 5-year period	-0.00	0.23	-0.23
Sharpe Ratio (10Y) Calculated over the trailing 10-year period	0.35	0.41	-0.06
Sharpe Ratio (All Time) Calculated using the full available price history	0.82	0.58	+0.24

Drawdowns

MMIBX vs. MUB - Drawdown Comparison

The maximum MMIBX drawdown since its inception was -17.40%, which is greater than MUB's maximum drawdown of -13.68%. Use the drawdown chart below to compare losses from any high point for MMIBX and MUB.

Drawdown Indicators

	MMIBX	MUB	Difference
Max Drawdown Largest peak-to-trough decline	-17.40%	-13.68%	-3.72%
Max Drawdown (1Y) Largest decline over 1 year	-3.01%	-2.79%	-0.22%
Max Drawdown (3Y) Largest decline over 3 years	-7.23%	-5.34%	-1.89%
Max Drawdown (5Y) Largest decline over 5 years	-17.19%	-11.88%	-5.31%
Max Drawdown (10Y) Largest decline over 10 years	-17.19%	-13.68%	-3.51%
Current Drawdown Current decline from peak	-1.25%	-0.62%	-0.63%
Average Drawdown Average peak-to-trough decline	-2.81%	-2.23%	-0.58%
Ulcer Index Depth and duration of drawdowns from previous peaks	0.89%	0.79%	+0.10%

Volatility

MMIBX vs. MUB - Volatility Comparison

MFS Municipal Income Fund (MMIBX) has a higher volatility of 1.22% compared to iShares National AMT-Free Muni Bond ETF (MUB) at 0.98%. This indicates that MMIBX's price experiences larger fluctuations and is considered to be riskier than MUB based on this measure. The chart below showcases a comparison of their rolling one-month volatility.

Volatility by Period

	MMIBX	MUB	Difference
Volatility (1M) Calculated over the trailing 1-month period	1.22%	0.98%	+0.24%
Volatility (6M) Calculated over the trailing 6-month period	2.34%	2.23%	+0.11%
Volatility (1Y) Calculated over the trailing 1-year period	3.18%	2.93%	+0.25%
Volatility (5Y) Calculated over the trailing 5-year period, annualized	4.41%	4.06%	+0.35%
Volatility (10Y) Calculated over the trailing 10-year period, annualized	4.34%	4.92%	-0.58%

MMIBX vs. MUB - Expense Ratio Comparison

MMIBX has a 1.45% expense ratio, which is higher than MUB's 0.07% expense ratio.

Dividends

MMIBX vs. MUB - Dividend Comparison

MMIBX's dividend yield for the trailing twelve months is around 2.96%, less than MUB's 3.17% yield.

Position	TTM	2025	2024	2023	2022	2021	2020	2019	2018	2017	2016	2015
MMIBX MFS Municipal Income Fund	2.96%	3.81%	2.51%	2.05%	1.42%	1.45%	1.86%	2.45%	2.68%	2.67%	2.87%	2.98%
MUB iShares National AMT-Free Muni Bond ETF	3.17%	3.14%	3.01%	2.65%	2.11%	1.81%	2.11%	2.42%	2.46%	2.26%	2.21%	2.51%

Frequently Asked Questions

Can I hold both MMIBX and MUB in my portfolio?

MMIBX and MUB have a correlation of 0.71, meaning they provide meaningful diversification benefit when combined. Depending on your allocation goals, holding both could reduce overall portfolio risk.

Which is riskier, MMIBX or MUB?

MMIBX has higher volatility (1.22%) compared to MUB (0.98%). In terms of maximum drawdown, MMIBX dropped -17.40% vs MUB's -13.68%.

Which has better risk-adjusted returns?

MUB currently has the higher Sharpe Ratio (2.43 vs 2.03), 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.