The book value of a company is the amount of stockholders' equity, or owner's equity for an unincorporated business, reported on its balance sheet at a given date. It is calculated as total assets minus total liabilities, and it represents the accounting residual that would theoretically remain for owners if every asset were sold at its recorded value and every liability paid off exactly as recorded.
Why book value is not the same as market value
Because assets are generally carried at historical cost under the cost principle rather than current market value, and because valuable intangibles such as brand reputation, customer loyalty, and internally developed know-how are often not recorded as assets at all, a company's book value frequently diverges significantly from what the business would actually sell for, or what its shares trade for in the market. A company can have substantial book value and modest market value, or the reverse, depending on how far its accounting records have drifted from current economic reality.
Price-to-book as a valuation tool
Despite its limitations, investors still routinely compare a company's market capitalization to its book value, producing the price-to-book ratio, as one input among several in assessing whether a stock appears cheap or expensive relative to its accounting net worth. The ratio tends to be most meaningful for asset-heavy businesses such as banks and insurers, whose assets are closer to fair value by nature, and least meaningful for asset-light, intangible-heavy businesses such as software companies, where book value can understate true economic worth substantially.
Book value and negative equity
A company that has accumulated losses exceeding its original invested capital, or that has borrowed heavily and bought back large amounts of its own stock, can end up with negative book value, meaning its reported liabilities exceed its reported assets. This does not necessarily mean the company is worthless or insolvent in a practical sense, since market value can remain strongly positive even when accounting book value has turned negative, particularly for companies whose real value lies mostly in intangible assets the balance sheet never captured in the first place.
Why the measure still matters
Lenders, regulators, and analysts continue to track book value closely because it is grounded in audited, verifiable figures rather than market sentiment, and because certain regulatory capital requirements, loan covenants, and legal tests are explicitly defined in terms of book value rather than market value, making it a figure with real contractual and regulatory consequences even where it diverges substantially from what the business is actually worth.
A simple worked example
A company with $12 million in total assets and $7 million in total liabilities has a book value of $5 million. If that same company's shares trade on the market at a combined value of $9 million, its price-to-book ratio is 1.8, meaning investors are valuing the business at nearly twice its reported accounting net worth, often because the market is pricing in intangible value, such as brand strength or growth prospects, that never appears on the balance sheet itself.