The differences between Basel I, II, and III come down to what each framework counts as capital, which risks it forces banks to hold capital against, and what safeguards it adds beyond the capital ratio itself. Basel I (1988) set a flat 8% capital requirement against credit risk using four crude risk buckets. Basel II (2004) kept the 8% but made risk measurement far more sensitive, added market and operational risk, and built the framework around three regulatory “pillars.” Basel III (phased in from 2013 and still being finalized) kept the 8% headline again but demanded that most of it be genuine common equity, layered on capital buffers, and introduced the first-ever global liquidity and leverage rules.1Bank for International Settlements. The Basel Committee – Overview
Why There Are Three Accords
The Basel Committee on Banking Supervision was created in 1974 after the failure of Bankhaus Herstatt exposed how badly cross-border banking had outgrown national regulation. European banks had paid Deutsche marks to Herstatt on the day German authorities shut it down, then lost everything when the corresponding dollar payments never came through from New York. Settlement volumes in New York dropped an estimated 60% over the following three days.2Bank for International Settlements. Settlement Risk in Foreign Exchange Markets and CLS Bank Central bank governors from the Group of Ten responded by setting up a committee at the Bank for International Settlements in Basel to write shared rules.3Bank for International Settlements. History of the Basel Committee Each version of the accord has been a response to the failures of the previous one, which is why the three frameworks look progressively more elaborate.
Basel I: A Flat 8% and Four Risk Buckets
The 1988 Basel Capital Accord established a single global rule: banks must hold capital equal to at least 8% of their risk-weighted assets.3Bank for International Settlements. History of the Basel Committee Qualifying capital was split into two tiers. Tier 1 was common stock and disclosed reserves, the most permanent form of funding. Tier 2 covered general loss provisions and long-term subordinated debt with original maturities over five years. At least half of the 8% had to be Tier 1.4Bank for International Settlements. International Convergence of Capital Measurement and Capital Standards
To calculate risk-weighted assets, Basel I sorted every exposure into one of four buckets:
- 0% weight for cash and government bonds from stable countries
- 20% weight for claims on public-sector entities and multilateral development banks
- 50% weight for residential mortgages
- 100% weight for corporate loans and most other commercial lending
A $1 million corporate loan therefore consumed $80,000 of capital, while the same amount in residential mortgages consumed only $40,000. For the first time, a bank stacking up risky loans against thin capital had a problem regulators everywhere could see and measure the same way.
The framework’s weakness was its bluntness. Four buckets meant a loan to a blue-chip company sat in the same 100% category as a loan to a failing startup. Market risk (losses on trading positions) and operational risk (losses from fraud or system failures) were ignored entirely. Banks learned to move risk off their balance sheets through securitization while keeping the same thin cushion. Those gaps are what Basel II tried to close.
Basel II: Three Pillars and Risk-Sensitive Models
Published in 2004, Basel II kept the 8% minimum but rebuilt what went into the denominator.5Bank for International Settlements. Basel II – International Convergence of Capital Measurement and Capital Standards – A Revised Framework The framework was organized around three pillars.
Pillar 1: Minimum Capital Requirements
Pillar 1 expanded the capital charge to cover market risk and operational risk in addition to credit risk.6European Central Bank. ECB Financial Stability Review December 2004 For credit risk, banks could choose between two methods. The standardized approach used external credit ratings from agencies like Moody’s or S&P to assign risk weights. The internal ratings-based (IRB) approach let banks use their own statistical models to estimate default probabilities and loss severities.5Bank for International Settlements. Basel II – International Convergence of Capital Measurement and Capital Standards – A Revised Framework Large banks moved to IRB because it produced more granular estimates and, often, lower capital requirements.
Pillar 2: Supervisory Review
Pillar 2 gave national regulators authority to look past the numbers. Banks had to run their own internal capital adequacy assessment; regulators would review it and could require capital above the Pillar 1 minimum where they saw weaknesses in risk management, strategy, or governance.6European Central Bank. ECB Financial Stability Review December 2004 This turned the capital calculation into an ongoing conversation between banks and supervisors rather than a fixed arithmetic exercise.
Pillar 3: Market Discipline
Pillar 3 required detailed public disclosure of capital structure, risk exposures, and capital adequacy. Large internationally active banks had to make key disclosures quarterly, with other information on semi-annual or annual schedules.7Federal Reserve. Part 4 – The Third Pillar – Market Discipline The theory was that investors and counterparties would price a bank’s funding based on what they saw.
Why Basel II Failed in 2008
The IRB approach gave banks a strong incentive to build models that produced low risk weights, because lower weights meant lower capital. Lending concentrated in residential mortgages (reduced weight under every approach) and highly rated securitization tranches (almost negligible capital). When housing prices collapsed, those supposedly safe assets produced enormous losses. Market risk capital at major banks often represented less than 1% of trading book assets, even though trading books had grown to more than half of total assets at some institutions.
The framework also said nothing about liquidity. Banks were funding long-term mortgages with short-term wholesale borrowing that could vanish overnight, and when confidence collapsed in 2007 and 2008, apparently well-capitalized institutions couldn’t meet their obligations. Basel II also let banks count various hybrid debt instruments as regulatory capital, and those instruments proved useless at absorbing losses when it mattered.
Basel III: Better Capital, Plus Liquidity and Leverage
Basel III, developed between 2010 and 2017, is the direct answer to those failures. It raised both the quality and quantity of capital, and it introduced entirely new liquidity and leverage rules. Phase-in began in 2013, and the final package (sometimes called Basel 3.1 or the Basel III Endgame) took effect internationally from January 2023 with a five-year transition.8Financial Stability Board. Basel III – Implementation
The Capital Rules Got Stricter Inside the Same 8%
The headline number stayed at 8%, but the composition tightened dramatically. Common Equity Tier 1 (CET1) capital, the only form that absorbs losses while a bank keeps operating, must be at least 4.5% of risk-weighted assets. Total Tier 1 (CET1 plus Additional Tier 1) must reach 6%, and total capital including Tier 2 must hit 8%.9Bank for International Settlements. Definition of Capital in Basel III – Executive Summary Under Basel I the 8% could be half Tier 2; under Basel III most of it has to be genuine common equity.
Capital Buffers Stack on Top of the Minimum
Basel III added several buffers above the minimum requirements. The capital conservation buffer is 2.5% of CET1; a bank that dips into it faces automatic restrictions on dividends, share buybacks, and discretionary bonuses.10U.S. Securities and Exchange Commission. Regulatory Capital The countercyclical buffer runs from 0% to 2.5% and can be activated by national regulators during credit booms and released during downturns.11Bank for International Settlements. The Capital Buffers in Basel III – Executive Summary Global systemically important banks (G-SIBs) face an additional surcharge of 1% to 3.5% based on size, complexity, and interconnectedness.12eCFR. 12 CFR 217.403 – GSIB Surcharge
In practice a non-G-SIB targets a CET1 ratio of at least 7% (4.5% minimum plus 2.5% conservation buffer), while a G-SIB targets 8% to 9.5% or higher. As of late 2024, large internationally active banks held average CET1 ratios around 14%, well above the minimums, because falling into buffer territory triggers real restrictions.13Bank for International Settlements. Basel III Monitoring Report
Two New Liquidity Ratios
The Liquidity Coverage Ratio (LCR) requires banks to hold enough high-quality liquid assets (cash, government bonds, and similar instruments) to cover 30 days of net cash outflows under a stress scenario. This directly targets the short-term funding runs that destabilized institutions like Northern Rock and Bear Stearns.
The Net Stable Funding Ratio (NSFR) takes a one-year view. A bank’s stable funding sources (capital, long-term debt, sticky retail deposits) must exceed the stable funding it needs given the liquidity profile of its assets. The NSFR was designed specifically against the pre-crisis practice of funding 30-year mortgages with overnight borrowing that had to be rolled every day.
A Leverage Ratio as a Backstop
Risk-weighted ratios can be flattered by aggressive modeling: if a bank’s models classify most assets as low-risk, the denominator shrinks and the ratio looks strong even when absolute leverage is huge. The Basel III leverage ratio ignores risk weights entirely. Tier 1 capital divided by total unweighted exposure, including off-balance-sheet items, must be at least 3%. G-SIBs face an additional leverage buffer equal to half their risk-based surcharge, so a G-SIB with a 2% surcharge needs a leverage ratio of at least 4%.14Bank for International Settlements. Leverage Ratio Requirements for Global Systemically Important Banks
An Output Floor on Internal Models
The 2017 finalization added an output floor: a bank’s internally modeled risk-weighted assets cannot fall below 72.5% of what the standardized approach would produce.15Bank for International Settlements. Finalising Basel III – In Brief The maximum capital benefit from a bank’s own models is therefore capped at 27.5%. This is the direct response to Basel II’s IRB problem.
Side-by-Side: What Changed at Each Step
Reading across the three accords, the pattern is consistent. Each version kept 8% as the headline capital requirement but changed what counts and what it covers.
- Risks covered: Basel I covered credit risk only. Basel II added market and operational risk. Basel III kept all three and added liquidity and leverage as separate requirements.
- Risk measurement: Basel I used four fixed buckets. Basel II allowed external ratings or internal models. Basel III keeps both but caps how far internal models can lower requirements through the 72.5% output floor.
- Quality of capital: Basel I let up to half the 8% be Tier 2. Basel II kept broadly similar tiers with hybrid instruments allowed. Basel III forces the core to be common equity (CET1 at 4.5%) and disallows most of the hybrid instruments that failed in 2008.
- Buffers: Basel I and II had none. Basel III stacks the 2.5% conservation buffer, the 0–2.5% countercyclical buffer, and the G-SIB surcharge on top of the minimums.
- Liquidity: Basel I and II were silent. Basel III introduced the LCR (30-day stress) and the NSFR (one-year stable funding).
- Leverage: Basel I and II relied solely on risk-weighted ratios. Basel III added a 3% unweighted leverage ratio, with more for G-SIBs.
Where Basel III Stands Now
Basel III is not a single document but a program still being finalized in national law. The European Union and several other jurisdictions have begun implementing the 2017 final reforms. The United States has taken a longer path: a 2023 proposal drew heavy industry criticism, and in March 2026 the Federal Reserve, OCC, and FDIC released a revised proposal split into three parts, one for the largest internationally active banks, another for smaller institutions, and a third from the Federal Reserve updating how systemic risk is measured for G-SIB surcharges.16Federal Reserve. Agencies Request Comment on Proposals to Modernize the Capital Framework The comment period closes in June 2026, with no final U.S. compliance deadline announced.
One notable feature of the 2026 U.S. re-proposal is a requirement, after a transition period, for certain large banks to reflect unrealized gains and losses on securities in regulatory capital. That provision responds to the 2023 collapse of Silicon Valley Bank, which appeared well-capitalized while holding large unrealized losses on its bond portfolio. Whether it survives the comment process is an open question.
The Basel Committee itself has no enforcement power. Its standards become binding only when national regulators write them into domestic law, and countries routinely go beyond the Basel minimums.17U.S. GAO. Bank Capital Reforms – US Agencies Participation in the Development of the International Basel Committee Standards The international leverage ratio floor is 3%; the United States requires 4% for all banks. Some countries define high-quality liquid assets more narrowly than Basel suggests. When comparing Basel I, II, and III, keep in mind that the accords set the international floor, and the rules a bank in any given country actually faces will usually be tougher.