Definition of the Barclays U.S. Aggregate Bond Index

The Barclays U.S. Aggregate Bond Index is the former name of what is now known as the Bloomberg U.S. Aggregate Bond Index, commonly called the “U.S. Aggregate” or “Agg.”

It is a broad-based benchmark that measures the performance of the U.S. investment-grade, U.S. dollar-denominated, fixed-rate taxable bond market.

The index includes several major categories of fixed-income securities, including U.S. Treasury securities, government-related bonds, corporate bonds, mortgage-backed securities (MBS), commercial mortgage-backed securities (CMBS), and asset-backed securities (ABS).

Investors and investment managers commonly use the index to evaluate the performance of U.S. investment-grade bond portfolios.

For example, a U.S. bond fund may compare its performance with the Bloomberg U.S. Aggregate Bond Index to evaluate how the portfolio performed relative to the broader investment-grade fixed-income market.

Purpose of the Barclays U.S. Aggregate Bond Index

The Bloomberg U.S. Aggregate Bond Index serves several important functions in fixed-income investing:

1. Benchmarking Performance – Provides a reference point for evaluating actively and passively managed bond portfolios.
2. Representing the Bond Market – Measures performance across several major segments of the U.S. investment-grade fixed-income market.
3. Supporting Portfolio Analysis – Helps investors compare portfolio characteristics and performance with a broad bond-market benchmark.
4. Supporting Fund and ETF Strategies – Some mutual funds and ETFs seek to track the index or use it as a performance benchmark.
5. Evaluating Fixed-Income Risk – Index characteristics such as duration, yield, and credit composition can help investors understand interest-rate and credit exposure.

Components of the Index

Treasury Securities

The index includes eligible marketable debt securities issued by the U.S. Treasury.

Treasury securities generally carry very low credit risk because they are backed by the full faith and credit of the U.S. government, though their market values remain sensitive to interest-rate changes.

Government-Related Bonds

This category includes eligible securities associated with government agencies and other qualifying government-related issuers.

It can also include certain sovereign and supranational securities that meet the index's eligibility requirements.

Corporate Bonds

The index includes eligible U.S. dollar-denominated investment-grade corporate bonds.

These securities introduce corporate credit risk in addition to interest-rate risk.

Mortgage-Backed Securities (MBS)

Mortgage-backed securities represent a significant segment of the index.

The index includes eligible agency mortgage pass-through securities backed by pools of residential mortgages.

MBS can involve prepayment and extension risk because homeowners may repay mortgages at different rates as interest rates and economic conditions change.

Commercial Mortgage-Backed Securities (CMBS)

The index also includes eligible securities backed by pools of commercial mortgage loans.

CMBS can provide exposure to financing associated with properties such as offices, retail centers, hotels, industrial facilities, and multifamily housing.

Asset-Backed Securities (ABS)

Eligible asset-backed securities are also represented.

ABS are generally backed by pools of financial assets such as auto loans, credit card receivables, or other consumer and business obligations.

Key Eligibility Requirements

The Bloomberg U.S. Aggregate Bond Index follows defined rules determining which securities qualify for inclusion.

In general, eligible securities must meet requirements relating to factors such as:

• Currency – Securities must generally be denominated in U.S. dollars.
• Credit Quality – Securities must generally qualify as investment grade under the index methodology.
• Coupon Structure – The benchmark primarily covers fixed-rate securities.
• Tax Status – The index focuses on taxable bonds.
• Amount Outstanding – Securities must meet minimum issue-size requirements applicable to their sector.
• Remaining Maturity – Eligible securities generally must have at least one year remaining until final maturity.

Because securities mature, are issued, receive rating changes, or otherwise cease to meet eligibility requirements, the index composition changes over time.

Advantages and Disadvantages of the Index

Advantages

• Broad Market Representation – Covers several major sectors of the U.S. investment-grade bond market.
• Widely Used Benchmark – Provides a common reference point for evaluating fixed-income portfolios.
• Rules-Based Methodology – Includes securities based on established eligibility criteria.
• Diversification Across Bond Sectors – Includes government, corporate, mortgage-backed, and asset-backed securities.
• Accessible Through Investment Funds – Investors can obtain exposure through mutual funds and ETFs designed to track broad U.S. bond indexes.

Disadvantages

• No High-Yield Corporate Bonds – Below-investment-grade corporate debt is generally outside the benchmark.
• Interest-Rate Risk – The value of bonds in the index can decline when market interest rates rise.
• Credit Risk – Corporate and certain securitized holdings can be affected by changes in issuer or borrower credit conditions.
• Mortgage Prepayment Risk – Mortgage-backed securities can behave differently as borrowers refinance or repay mortgages.
• Not the Entire U.S. Bond Market – Municipal securities and several other fixed-income categories are outside the benchmark's scope.

Barclays U.S. Aggregate Bond Index vs. S&P 500

The two indexes measure different asset classes and should not be interpreted as direct substitutes for one another.

Related Terms

• Bloomberg U.S. Aggregate Bond Index: The current name of the benchmark formerly associated with Barclays and Lehman Brothers.
• Bond ETF: An exchange-traded fund that invests primarily in bonds and may seek to track a fixed-income index.
• Investment-Grade Bond: A bond meeting applicable credit-quality standards for investment-grade classification.
• Duration: A measure commonly used to evaluate a bond or bond portfolio's sensitivity to interest-rate changes.
• Mortgage-Backed Security (MBS): A security backed by a pool of mortgage loans.
• Fixed-Income Benchmark: An index used as a reference for measuring and comparing bond-market performance.

Interesting Fact

Did you know? The index now called the Bloomberg U.S. Aggregate Bond Index has changed names several times during its history. Its lineage includes Lehman Brothers, Barclays, and Bloomberg, which is why investors may still encounter older names such as the Lehman Aggregate or Barclays Aggregate.

Statistic

As of August 31, 2026, the Bloomberg U.S. Aggregate Bond Index contained 13,774 securities with a total market value of approximately $30.8 trillion. U.S. Treasury securities represented approximately 46.8% of the index's market value, making them its largest major sector.

Frequently Asked Questions (FAQ)

1. What is the Barclays U.S. Aggregate Bond Index used for?

It is primarily used as a benchmark for evaluating the performance of broad U.S. investment-grade bond portfolios. Funds may also seek to replicate its performance.

2. Is it still called the Barclays U.S. Aggregate Bond Index?

No. Its current official name is the Bloomberg U.S. Aggregate Bond Index. The Barclays name remains common in older documents and discussions because Barclays previously administered the benchmark.

3. Can investors invest directly in the index?

No. An index is a measurement benchmark rather than an investment product. Investors can obtain similar exposure through mutual funds, ETFs, and other products designed to track or approximate the index.

4. What types of bonds are excluded from the index?

The index does not represent every segment of the bond market. For example, it generally excludes below-investment-grade corporate bonds, municipal bonds, inflation-linked Treasury securities, and securities that fail to meet its maturity, size, currency, or other eligibility requirements.

5. Why is the Bloomberg U.S. Aggregate Bond Index important?

The index provides a broad measure of the U.S. investment-grade taxable bond market. Its broad coverage makes it useful for benchmarking bond portfolios, evaluating fixed-income performance, and analyzing the broader U.S. bond market.