Credit Ratings: S&P, Moody's, and Fitch Scales

Key Takeaways

• Investment grade runs from AAA/Aaa to BBB-/Baa3; anything below is speculative or high yield.
• Ratings are ordinal rankings of creditworthiness, not precise default probabilities.
• Agencies use an issuer-pays model that creates a structural conflict of interest investors should factor in.
• A downgrade below investment grade can trigger forced selling by index funds and constrained institutional investors.

What It Is

Credit ratings are forward-looking opinions about creditworthiness. They are ordinal rankings, meaning AAA is stronger than AA and AA is stronger than A, but the scale does not tell you by how much. Ratings are issued on individual debt instruments (issue ratings) and on the entities that issue them (issuer ratings), and the two can differ when specific bonds have collateral, seniority, or guarantees that change the payoff profile.

Ratings are paid for by the issuer under the dominant "issuer-pays" business model. That arrangement creates a structural conflict of interest the agencies manage through analytical walls, disclosure, and post-crisis regulatory oversight, but the conflict is real and worth remembering.

The Intuition

Most investors cannot review the full financials, debt covenants, industry outlook, and management history for every bond in the market. Agencies exist to compress that mountain of information into a single comparable grade.

The grade is not a probability. It is a peer ranking. An AA-rated issuer is judged to have a meaningfully lower chance of default than a BB issuer over a relevant horizon, but the agencies do not claim the step from AA to A is the same size as the step from A to BBB. Historical default rate studies published by each agency show the relationship and it is highly non-linear: default frequencies rise slowly through the investment-grade tiers and then accelerate sharply once you cross into speculative territory.

Ratings also shape demand. Many institutional investors, including insurance companies and pension funds, are constrained by policy or regulation to hold mostly investment-grade paper. A downgrade across the investment-grade line can force mechanical selling, which is why the boundary is economically meaningful beyond the label itself.

How It Works

The three agencies use different notation for the same hierarchy.

The plus and minus suffixes at S&P and Fitch, and the 1, 2, 3 numerical modifiers at Moody's, mark the upper, middle, and lower thirds of a broader letter category.

Most ratings come with an outlook (positive, stable, or negative) indicating a likely direction over six months to two years, and sometimes with a watch or rating review flag that signals an imminent action. A rating is reviewed on a regular schedule and also after material events such as earnings, mergers, or covenant breaches.

Worked Example

Consider a hypothetical industrial bond rated BBB- by S&P, Baa3 by Moody's, and BBB- by Fitch. That is the lowest notch of investment grade across all three scales.

If the issuer's leverage rises and S&P downgrades to BB+, the bond has crossed into speculative grade on that scale. Even though Moody's and Fitch have not moved yet, major IG bond indices, which typically require investment-grade status from at least two of three agencies, might still include the bond. If a second agency follows, the bond becomes a fallen angel and gets ejected from the IG index, forcing index-tracking funds to sell. Price often drops in anticipation of the move, not on the downgrade announcement itself.

This example shows why the exact ratings of all three agencies and the index inclusion rule matter, not just a single letter grade.

Common Mistakes

1. Treating ratings as default probabilities. They are not. A BB rating from S&P has historically corresponded to a wide range of one-year default rates depending on the vintage and the economic cycle. Use the agency's published cumulative default studies if you need an actual number, and accept that the number is a historical average, not a forecast.

2. Not distinguishing issue ratings from issuer ratings. A company rated BB overall can have senior secured bonds rated BB+ (uplift for collateral) and subordinated bonds rated B (notched down for lower recovery). Always read the rating on the specific CUSIP you own.

3. Ignoring downgrade lags. Agencies are often criticized for reacting after the market has already moved. By the time a downgrade is announced, spreads have usually widened to price in the new level. Treat ratings as a lagging indicator of credit stress, not a leading one.

4. Treating "investment grade" as risk-free. Investment grade simply means low default probability; it does not mean zero. The lowest IG tier (BBB / Baa) is where a meaningful share of fallen angels originate, and IG bonds can still lose value through spread widening, duration, or credit migration.

5. Comparing S&P BBB to Moody's Baa literally. The notation maps 1-to-1 across scales as a convention, but the agencies' underlying criteria are not identical, and at any given moment the same issuer can be rated slightly differently by each agency. Use the split-rating difference as information, not noise.

Frequently Asked Questions

Who pays for credit ratings, and why does that matter? Under the dominant issuer-pays model, the company or government seeking a rating pays the agency's fee. This creates a potential conflict of interest, as the agency could be tempted to give favorable ratings to retain client relationships. Post-2008 reforms added more oversight and required disclosure of rating methodology, but the structural conflict remains and sophisticated investors monitor it.

What is a "split rating" and how should investors handle it? A split rating occurs when two agencies assign different grades to the same bond, such as BBB+ from S&P and BB+ from Moody's. Bond indices often apply a "lower of two" or "average" rule for index inclusion. Investors should investigate the cause of the split rather than averaging the grades, since the disagreement often reflects a genuine analytical difference about the issuer's risk.

How quickly do agencies act after an issuer's credit deteriorates? Agencies have historically been criticized for lagging the market. Credit spreads frequently widen weeks or months before a formal downgrade, as markets price in credit deterioration faster than the agency review cycle allows. Investors using ratings as a primary credit signal may act too late; pairing ratings with spread analysis provides a more timely picture.

Can a country's government bonds be rated differently from its corporate bonds? Yes. Government bonds receive a sovereign rating, which typically acts as a ceiling for corporate bonds issued by entities in the same country. A corporation rarely receives a higher rating than its home government because a sovereign default usually impairs the corporate sector too. There are exceptions for corporates with significant foreign revenue or operations.

What happens to bond fund flows when a large issuer is downgraded to junk? Fallen angel downgrades force investment-grade bond funds that cannot hold below-BBB/Baa3 to sell, regardless of price. This mechanical selling pressure often depresses the price before and shortly after the downgrade announcement. High-yield funds, anticipating the addition to their benchmark index, sometimes buy in advance. The combination can create temporary price dislocation around the downgrade date.

Sources

1. S&P Global Ratings. "Understanding Credit Ratings." https://www.spglobal.com/ratings/en/about/understanding-credit-ratings
2. Moody's Investors Service. "Ratings Scale and Definitions." https://ratings.moodys.com/ratings
3. Fitch Ratings. "Rating Definitions." https://www.fitchratings.com/products/rating-definitions
4. Fidelity Learning Center. "Bond Ratings." https://www.fidelity.com/learning-center/investment-products/fixed-income-bonds/bond-ratings
5. SEC Investor.gov. "Bonds." https://www.investor.gov/introduction-investing/investing-basics/investment-products/bonds-or-fixed-income-products-0

Disclaimer

This article is educational content only and is not financial advice. Nothing here is a recommendation to buy, sell, or hold any security. Consult a licensed advisor before making investment decisions.