Size Premium
Executive Summary
Key Takeaways
- ✓ The size premium is an additional cost of equity adjustment for smaller companies, layered on top of the standard CAPM build.
- ✓ It reflects the empirical observation that small-cap stocks have historically earned higher average returns than beta alone would predict.
- ✓ Size premium is typically sourced from published studies bucketed by market capitalization decile.
- ✓ The size premium's magnitude and even its ongoing validity are debated in academic and practitioner literature, so its use should be explicitly disclosed.
- ✓ Size premium is one of several possible additive cost of equity adjustments alongside country risk premium, and each should be separately identified.
Definition¶
The size premium is an additional premium sometimes added to cost of equity for smaller companies, reflecting the empirical observation that small-capitalization stocks have historically earned higher average returns than a single-factor CAPM model, using beta alone, would predict.
Rationale¶
CAPM assumes that beta fully captures a stock's systematic risk. Empirical research into historical stock returns has found that smaller companies, as a group, have earned returns in excess of what their measured betas alone would justify. Proposed explanations for this observed size effect include lower liquidity, less diversified operations and customer bases, greater sensitivity to economic downturns, and higher information and transaction costs associated with smaller companies — risk factors that a single-factor beta does not fully reflect.
Application¶
The size premium is layered on top of the standard CAPM cost of equity build:
Cost of Equity = Rf + β × ERP + Size Premium
Size premium figures are typically sourced from published studies that bucket historical returns by market capitalization decile, with smaller deciles carrying larger premiums. The subject company's estimated market capitalization, or an equivalent proxy for private companies and specific projects, determines which bucket's premium is applied.
A Debated Adjustment¶
Unlike the base equity risk premium, the size premium's magnitude, persistence, and even its underlying theoretical validity are actively debated in academic and practitioner literature. Some research suggests the historical size effect has diminished, been arbitraged away, or reversed in more recent data periods. Given this lack of consensus, institutional practice requires the use and source of any size premium to be explicitly disclosed rather than silently embedded in the discount rate.
Relationship to Other Adjustments¶
Size premium is one of several possible additive adjustments to CAPM's baseline cost of equity, alongside country risk premium for sovereign or political risk. Where multiple adjustments are used in the same valuation, each should be separately identified and sourced rather than combined into a single unexplained cost-of-equity uplift.
Audit Considerations¶
- Confirm whether a size premium has been applied and, if so, that its source, market-capitalization bucket, and magnitude are disclosed
- Assess whether the subject's size genuinely falls within the range the source study intends to address
- Confirm size premium is not double-counted with other adjustments addressing overlapping risk factors, such as an already-elevated beta from small, thinly traded comparables
- Where a size premium is applied, confirm the model or report notes the debated nature of the adjustment rather than presenting it as an uncontested convention
Common Errors¶
| Error | Description | Risk |
|---|---|---|
| Undisclosed size premium | Premium applied without stating source or magnitude | Cannot be independently assessed or replicated |
| Double-counting with beta | Size-related risk already reflected in an elevated beta from small comparables, with a size premium added on top | Overstates cost of equity |
| Applying size premium to large companies | Premium applied to companies well outside the small-cap range the source study addresses | Unjustified, unsupported uplift to the discount rate |
Continue Reading¶
Prerequisites¶
- CAPM (Capital Asset Pricing Model)
- Discounted Cash Flow (DCF) Valuation — the parent pillar
Related Glossary¶
Related Technical Guides¶
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Frequently Asked Questions
Why is a size premium added on top of CAPM?
Because empirical studies have found that small-capitalization stocks have historically earned higher average returns than a single-factor CAPM model, using beta alone, would predict, suggesting beta does not fully capture size-related risk factors such as lower liquidity and less diversified operations.
How is the size premium typically sourced?
From published size-premium studies that bucket historical stock returns by market capitalization decile and measure the excess return unexplained by CAPM for each bucket. The subject company's estimated market capitalization or asset value determines which bucket's premium is applied.
Is the size premium universally accepted?
No. Its magnitude, persistence over time, and even its theoretical validity are debated among academics and practitioners. Some argue the historical size effect has diminished or reversed in more recent data. Given this, its use and source should always be explicitly disclosed.
Does size premium apply to large, listed companies?
Typically not, or only marginally — size premium is primarily relevant for small and micro-cap companies, and for private companies and specific projects that are small relative to the listed comparable universe used to build their discount rate.
How does size premium interact with country risk premium?
They are separate, additive adjustments addressing different risk factors — size premium addresses company-specific size-related risk, while country risk premium addresses sovereign or political risk. Both may apply to the same valuation and should each be separately identified rather than blended into a single adjustment.
Related Articles
CAPM (Capital Asset Pricing Model)
The Capital Asset Pricing Model (CAPM) is the standard methodology for estimating the cost of equity — the return equity investors require to hold a company's stock, given its systematic risk relative to the broader market. CAPM expresses cost of equity as the risk-free rate plus the company's beta multiplied by the equity risk premium (the excess return the market as a whole is expected to earn over the risk-free rate). CAPM is the most widely used cost-of-equity methodology in institutional valuation practice and is the standard input to the cost-of-equity component of WACC.
Cost of Equity
Cost of equity is the rate of return equity investors require to compensate them for the risk of holding a company's stock, given its systematic risk relative to the broader market. It is most commonly estimated using the Capital Asset Pricing Model (CAPM), which expresses cost of equity as the risk-free rate plus a beta-adjusted equity risk premium. Cost of equity serves two roles in a DCF valuation: it is one of the two components blended into WACC (alongside the after-tax cost of debt), and it is used as the sole discount rate when valuing a levered cash flow (FCFE) directly.
Equity Risk Premium (ERP)
The equity risk premium (ERP) is the additional return equity investors require, above the risk-free rate, for bearing the risk of holding equities as an asset class rather than a risk-free instrument. ERP is not directly observable and must be estimated, typically from long-run historical average equity returns in excess of government bond yields, from implied ERP models that back the premium out of current market prices, or from surveys of practitioner expectations. ERP is a required input to the Capital Asset Pricing Model (CAPM), where it is multiplied by beta to determine the equity-risk component of cost of equity. Because reasonable ERP estimates can differ materially between sources, the ERP figure used in a valuation should always be disclosed alongside its source and date.
Beta
Beta is a measure of a stock's systematic risk — the portion of its return volatility that is correlated with movements in the broader market and cannot be diversified away. A beta of 1.0 moves in line with the market; a beta above 1.0 indicates higher-than-market sensitivity, and a beta below 1.0 indicates lower sensitivity. Beta is the key input to the Capital Asset Pricing Model (CAPM), which is used to estimate the cost of equity component of the discount rate in a DCF valuation. Beta can be sourced from a regression of a company's historical stock returns against a market index, or taken from published data services, and for private companies or specific projects is typically derived from a set of unlevered comparable betas re-levered to the subject's target capital structure.
Country Risk Premium (CRP)
The country risk premium (CRP) is an additional premium added to the cost of equity for cash flows or assets exposed to a specific country's sovereign or political risk, beyond the general equity risk premium applicable in mature, well-diversified markets. CRP is relevant whenever a DCF valuation involves cash flows exposed to a country carrying meaningfully higher sovereign risk than the base market used to estimate the equity risk premium, commonly proxied using sovereign credit default swap spreads, sovereign bond yield spreads over a risk-free benchmark, or published country risk ratings. CRP should be applied transparently and only once, since double-counting country risk (for example, in both the discount rate and the cash flow forecast) is a common and material valuation error.