Core capital is the highest-quality layer of a bank’s regulatory capital, the money that stands first in line to absorb losses while the bank is still operating. Under the Basel III framework, core capital centers on Common Equity Tier 1 (CET1), which must equal at least 4.5% of a bank’s risk-weighted assets. Effective requirements run considerably higher once buffers are added, and a bank that slips into buffer territory faces automatic restrictions on dividends, buybacks, and discretionary bonuses.
The reason regulators care so much about this particular slice of capital is simple. Common equity has no maturity date and no required payments. A bank can burn through it to cover bad loans and keep operating. Debt, by contrast, eventually has to be repaid, and hybrid instruments only absorb losses under specific triggers. When supervisors stress-test a balance sheet, CET1 is the number they watch most closely because it tells them how much punishment the bank can take before depositors or taxpayers are exposed.
What Counts as Common Equity Tier 1
CET1 is built from items that sit fully subordinate to every other obligation the bank carries. The major components are common stock (including share premium above par value), retained earnings, and certain accumulated other comprehensive income items. These represent money that belongs to shareholders last in a liquidation, which is precisely why regulators count it first for capital purposes.
Common stock is the most direct loss absorber. When a bank’s assets fall in value, the loss lands on shareholders’ equity. Retained earnings work the same way: accumulated profits the bank chose not to pay out, sitting on the balance sheet ready to cushion future losses. Together, these two items make up the bulk of CET1 at most large banks.
Regulatory Deductions
Banks cannot simply total their equity accounts and call the result CET1. Regulators require a series of deductions to strip out items that look like capital on paper but would evaporate in a crisis. The most significant are goodwill and other intangibles, deferred tax assets that depend on future profitability, and investments in unconsolidated financial institutions.
Goodwill represents the premium a bank paid to acquire another company above the target’s book value. It sits on the balance sheet as an asset, but you cannot sell goodwill to cover depositor losses. Stripping it out prevents banks from inflating their capital ratios with items that have little liquidation value. Deferred tax assets that rely on future profits face a similar problem: if the bank is losing money, those tax benefits may never materialize.
Some deductions use a threshold approach rather than a full write-off. Significant investments in the common shares of unconsolidated financial companies, mortgage servicing rights, and deferred tax assets from temporary differences each receive limited recognition. Each item is individually capped at 10% of adjusted common equity, and the three together cannot exceed 15% of CET1. Anything above those limits gets deducted.
Additional Tier 1 and the Broader Tier 1 Number
The phrase “core capital” is often used interchangeably with Tier 1 capital, which is broader than CET1 alone. The second component is Additional Tier 1 (AT1), which also absorbs losses while the bank is a going concern but uses instruments that fall short of the CET1 criteria. The most common AT1 instruments are perpetual contingent convertible bonds, often called CoCos. These are debt-like securities that automatically convert into common equity or get written down when capital drops to a specified trigger.
AT1 instruments must be perpetual, and coupon payments must be fully discretionary; the bank can skip a coupon without triggering default. All AT1 instruments must also be capable of conversion or write-down at the point of non-viability, meaning the moment the regulator determines the bank would fail without intervention or when public funds are injected to prevent failure.
The distinction matters for the ratios. The 4.5% CET1 minimum has to be met with common equity alone. The broader 6% Tier 1 minimum can include both CET1 and AT1. In practice, most large banks hold far more CET1 than the minimum, so AT1 sits as a supplementary layer rather than the main defense.
How the Ratio Actually Works
Capital ratios would be meaningless without a denominator that reflects the actual risk on a bank’s books. That denominator is Risk-Weighted Assets. The formula is straightforward: CET1 Ratio = CET1 Capital รท Risk-Weighted Assets. The complexity lives inside RWA, which assigns every asset a weight based on how likely it is to generate losses.
Cash held at the bank carries a 0% risk weight, requiring no capital backing. Government bonds from highly rated sovereigns (AAA to AA-) also receive 0%. As credit quality drops, the weights climb: A-rated sovereigns carry 20%, BBB sovereigns sit at 50%, and unrated corporate loans generally receive 100%. Residential mortgages fall in between and vary by loan-to-value ratio; under the Basel III standardized approach, a mortgage with an LTV of 50% or less carries a 20% weight, while one where the borrower owes more than the home is worth carries 70%.
The bank multiplies each asset’s exposure by its assigned weight and sums the results. A balance sheet loaded with government bonds and cash produces far lower RWA than the same total assets concentrated in corporate lending. That difference shows up directly in capital ratios, which is the point: banks taking more risk must hold more capital.
Minimums, Buffers, and the Effective Threshold
Basel III sets three minimum ratios, each measured against RWA. CET1 must be at least 4.5%. Total Tier 1 (CET1 plus AT1) must reach 6%. Total Capital (Tier 1 plus Tier 2) must hit 8%. These are hard floors, and breaching any of them triggers supervisory intervention.
The effective CET1 requirement is higher than 4.5% because of the Capital Conservation Buffer, set at 2.5% of RWA and met entirely with CET1. That brings the practical CET1 threshold to 7.0% for most banks. The buffer exists to be drawn down during stress, but a bank operating inside the buffer zone faces escalating restrictions on capital distributions.
The restrictions follow a graduated schedule. A bank with a CET1 ratio between 4.5% and 5.125% must conserve 100% of its earnings, paying nothing in dividends, buybacks, or discretionary bonuses. Between 5.125% and 5.75%, it must conserve 80%. The constraint loosens at each step until the bank clears 7.0%, at which point no restrictions apply.
Two further add-ons can raise the bar. National regulators can impose a Countercyclical Capital Buffer during periods of excessive credit growth, adding a further CET1 requirement on top of the conservation buffer. Global systemically important banks face a surcharge ranging from 1.0% to 3.5% of RWA, sorted into five buckets based on size, interconnectedness, cross-border activity, and complexity. The top bucket is intentionally left empty to discourage banks from growing more systemically important. For G-SIBs in the second-highest bucket, the surcharge is 2.5%, pushing their effective CET1 requirement to 9.5% or more before any countercyclical add-on.
What Happens When a Bank Falls Short
In the United States, the consequences go beyond buffer restrictions. The Prompt Corrective Action framework requires regulators to intervene with increasing severity as capital declines. An undercapitalized bank must submit a capital restoration plan within 45 days, and the regulator must act on that plan within 60 days of submission.
While operating under a shortfall, the bank faces hard constraints. It cannot let its average total assets grow beyond the prior quarter’s level unless the regulator has accepted its restoration plan and the growth is consistent with it. It cannot acquire interests in other companies, open new branches, or enter new lines of business without regulatory approval. Management fee payments to controlling entities are barred if paying them would leave the bank undercapitalized.
This is why banks maintain ratios well above the minimums. The cost of falling short is not only financial. A bank stuck under a capital restoration plan has effectively lost the ability to pursue any growth strategy, which often bites harder than the dividend restrictions alone. Capital ratios are also public: banks disclose them through quarterly Call Reports, so investors, analysts, and counterparties can see exactly where a bank stands. A CET1 ratio trending downward will draw questions from shareholders and higher funding costs long before regulators need to step in.
Where Tier 2 Fits
Tier 2 capital is not core capital, and the distinction matters. Tier 1, including both CET1 and AT1, is “going concern” capital that absorbs losses while the bank is alive and operating. Tier 2 is “gone concern” capital, meaning its loss-absorption features kick in primarily when the bank is failing or being wound down.
Tier 2 instruments include subordinated debt and certain hybrid securities. Unlike AT1, Tier 2 debt can have a maturity date, though it must carry an original maturity of at least five years and its recognized value amortizes in the final years before maturity. General loan-loss provisions also count toward Tier 2, subject to a cap. The less stringent criteria reflect its secondary role: Tier 2 protects depositors and senior creditors during resolution, but it is not the first line of defense.
The layered structure exists because not all capital is equally useful. A bank that meets the 8% total requirement by loading up on subordinated debt while holding thin common equity is far more fragile than one whose capital base is overwhelmingly CET1. That is the whole reason core capital sits at the center of the framework: it is the only capital that reliably does the job when losses actually arrive.