Basel III Capital Requirements: CET1, Buffers, and G-SIB Rules

Key Takeaways

• Basel III capital requirements set a 4.5% CET1 minimum, a 2.5% capital conservation buffer, and G-SIB surcharges of 1–3.5%, meaning the largest global banks must hold 10.5% CET1 before countercyclical add-ons.
• Capital is tiered by loss-absorbing quality: CET1 (common equity) absorbs losses first; AT1 (CoCos) converts or writes down on trigger; Tier 2 (subordinated debt) absorbs in gone-concern.
• A bank whose CET1 falls below 7% (minimum 4.5% plus conservation buffer 2.5%) faces automatic dividend and buyback restrictions even without regulatory intervention.
• The Basel IV output floor of 72.5% prevents IRB banks from producing RWA far below standardised results, closing the gap that let some banks appear well-capitalised on internal models.

What It Is

The Basel framework is published by the Basel Committee on Banking Supervision (BCBS) at the Bank for International Settlements. Basel III was released in 2010 and finalised through the 2017 post-crisis reforms package known informally as Basel IV. In the US, these rules are implemented through Federal Reserve Regulation Q.

At its core, Basel III says that every bank must hold enough high-quality capital to absorb unexpected losses across credit, market, and operational risk. The ratio that matters most is Common Equity Tier 1 (CET1) capital divided by Risk-Weighted Assets (RWA).

The Intuition

Before 2008, some banks met minimum ratios on paper using instruments that turned out not to absorb loss in a crisis. Basel III tightened the definition of what counts as capital, pushed loss absorption onto common equity first, and added conditional buffers that regulators can tighten in good times so banks have room to breathe in bad times.

The core idea is still simple: losses hit capital before they hit depositors or the taxpayer. The rules just make the capital stack harder, deeper, and more transparent.

How It Works

Capital is organised in tiers. CET1 is common equity plus retained earnings, minus regulatory deductions for goodwill, deferred tax assets, and certain intangibles. Additional Tier 1 (AT1) adds contingent convertible bonds (CoCos) that either write down or convert to equity on a trigger. Tier 2 includes subordinated debt.

The minimum ratios (expressed as a percentage of RWA) are:

CET1 minimum:           4.5%
Tier 1 minimum:         6.0%
Total capital minimum:  8.0%

Capital Conservation Buffer:  +2.5% (CET1)
Countercyclical Buffer:       0.0% to 2.5% (CET1, set nationally)
G-SIB Surcharge:              +1.0% to 3.5% (CET1, bucketed)

A global systemically important bank (G-SIB) in the highest bucket therefore faces a CET1 requirement of 4.5 + 2.5 + 3.5 = 10.5 percent before any countercyclical add-on, plus Pillar 2 supervisory capital set institution-by-institution.

RWA is calculated under one of two approaches. The Standardised Approach uses supervisory risk weights keyed to exposure class and external rating. The Internal Ratings-Based Approach lets approved banks use their own estimates of probability of default, loss given default, and exposure at default, subject to the output floor from Basel IV of 72.5 percent of the standardised RWA.

The capital conservation buffer is the one that matters daily. If CET1 falls into the buffer zone (below 7 percent but above 4.5 percent), the bank faces automatic restrictions on dividends, buybacks, and bonus payouts, scaled by how deep the breach is.

Worked Example

Consider a non-G-SIB US bank with 200 billion in Risk-Weighted Assets.

• CET1 capital: 18 billion
• Additional Tier 1: 3 billion
• Tier 2: 4 billion

Ratios:

CET1 ratio =     18 / 200 = 9.0%
Tier 1 ratio =   21 / 200 = 10.5%
Total ratio =    25 / 200 = 12.5%

The hard minimum plus conservation buffer for CET1 is 4.5 + 2.5 = 7.0 percent. The bank sits at 9.0 percent, with 2.0 points of "freeboard" above the buffer zone. That freeboard is what determines how much dividend and buyback it can commit without supervisory friction.

If the bank suffers a loss of 3 billion, CET1 falls to 15 billion, and the ratio drops to 7.5 percent. Still above the buffer floor, but the management buffer is almost gone. The bank will typically cut distributions voluntarily before waiting for the automatic brake. The buffer works partly by forcing that behaviour in advance.

Frequently Asked Questions

Q: What are Basel III capital requirements in simple terms? Basel III says banks must hold a minimum percentage of high-quality capital against their risk-weighted assets. The higher the risk, the more capital required. The rules ensure that losses hit the bank's own shareholders first, not depositors or taxpayers.

Q: How do Basel III capital requirements affect investment decisions? Higher capital requirements constrain return on equity for banks. Businesses that consume large amounts of capital, trading books, structured credit, must justify higher returns or shrink. The Supplementary Leverage Ratio also influences which low-risk activities banks will warehouse, affecting Treasury market liquidity.

Q: What is a real-world example of Basel III capital requirements in action? A non-G-SIB US bank with $18B CET1 and $200B RWA posts a 9.0% CET1 ratio. The hard minimum plus conservation buffer is 7.0%, leaving 2.0 points of freeboard. A $3B loss drops the ratio to 7.5%, nearly eliminating the buffer and prompting voluntary dividend cuts before automatic restrictions kick in.

Q: How can investors use bank capital ratios to assess financial institution risk? Look at CET1 ratio against the required minimum plus buffers. Check the trend over recent quarters. Compare RWA density (RWA/total assets) across banks, a very low density may signal aggressive internal model assumptions. Always check Pillar 2 disclosures for bank-specific capital add-ons.

Q: How are Basel III capital requirements different from leverage ratio rules? Risk-based requirements (CET1/RWA) scale capital to perceived risk. The leverage ratio (CET1/total exposure) ignores risk weights entirely. A bank can meet risk-based requirements while breaching the leverage ratio if it holds large amounts of low-risk assets like Treasuries. The two rules can bind in different situations.

Common Mistakes

1. Reading ratios without looking at RWA density. Two banks with the same CET1 ratio can carry very different economic risk if their RWA densities differ. Standardised approaches generally report higher RWA for the same exposures than IRB models, which is why the 72.5 percent output floor was introduced.

2. Ignoring Pillar 2. Pillar 1 is the published rulebook. Pillar 2 is the bank-specific overlay the supervisor applies based on risks not fully captured by Pillar 1, including interest rate risk in the banking book (IRRBB), concentration, and reputational risk. Published ratios can look comfortable while Pillar 2 is binding.

3. Treating AT1 and Tier 2 as equivalent to CET1. They are not. AT1 converts or writes down under stress, but only on predefined triggers. In March 2023 the Credit Suisse AT1 write-down shocked investors who had treated the instruments as high-grade debt. CET1 is what absorbs loss first and silently.

4. Forgetting the leverage ratio backstop. The Basel leverage ratio, and the US Supplementary Leverage Ratio (SLR) for advanced approaches banks, runs in parallel to RWA-based ratios. A bank can be well capitalised on RWA and still hit its SLR limit first.

5. Assuming G-SIB buffers never change. The G-SIB score is recalibrated annually. A bank can move bucket, raising or lowering its surcharge, based on size, interconnectedness, substitutability, complexity, and cross-jurisdictional activity.

Sources

1. Basel Committee on Banking Supervision. "Basel III: A global regulatory framework for more resilient banks and banking systems." BCBS 189. https://www.bis.org/publ/bcbs189.htm
2. Basel Committee on Banking Supervision. "Basel III: Finalising post-crisis reforms." BCBS d424. https://www.bis.org/bcbs/publ/d424.htm
3. Federal Reserve. "Regulatory Capital Rules: Regulation Q." https://www.federalreserve.gov/supervisionreg/reglisting.htm
4. Basel Committee on Banking Supervision. "Global systemically important banks: updated assessment methodology and the higher loss absorbency requirement." BCBS d445. https://www.bis.org/bcbs/publ/d445.htm

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.