I’ve been thinking lately about something many investors overlook when they start analyzing stocks: the difference between what a share costs and what it is truly worth according to its accounting records. That’s where net book value comes in—a concept that can completely change how you see stock-market opportunities.

Basically, when we talk about net book value, we mean the company’s equity attributable to each share. In other words, the company’s actual net worth divided by the number of shares. Unlike par value, which is set at the time of issuance, net book value reflects the company’s current situation by taking into account its assets, liabilities, and accumulated reserves. Many people also call it book value, and it’s the fundamental tool of value investing—an approach that aims to find companies trading below what they’re really worth.

Now, to calculate a company’s net book value, you need access to its financial statements. Fortunately, publicly traded companies are required to publish their quarterly and annual reports. The formula is simple: you subtract total liabilities from total assets, and then divide the result by the number of shares outstanding. This gives you the net book value per share. Let’s imagine a company with 3.2 billion in assets, 620 million in liabilities, and 12 million shares. The calculation is straightforward: 3,200 minus 620 equals 2,580; divided by 12 million, that comes to approximately 215 euros per share.

What’s interesting is when you compare that figure with the market price. This is where the ratio Precio/Valor Contable, or P/VC, as analysts call it, comes in. If you divide the market price by the net book value and get a result greater than 1, it means the stock is expensive relative to its books. Below 1, it’s cheap. It sounds simple, but it’s surprising how this tool can expose market inefficiencies.

I have to be honest: the concept has important limitations. Net book value does not take intangible assets into account, which is especially problematic for technology companies. A software startup may have relatively low development costs but be incredibly profitable—something that doesn’t show up in the books. That’s why you often see the P/VC ratio in the tech sector generally much higher than in other sectors, not necessarily because they’re overvalued, but because the tool simply doesn’t capture all of their real value.

Also, this method has little impact on small companies, which are often newly created and trade based on future expectations rather than on their current equity. And there’s another serious problem: creative accounting. Some accountants use legal techniques to “dress up” results, overstating assets and understating liabilities. The books can be completely falsified.

The case of Bankia is the perfect example of why you shouldn’t blindly trust this metric. In 2011, it went public with a 60 percent discount to its book value, which seemed like an incredible opportunity. But afterward, it had a disastrous performance and ended up being absorbed by Caixabank in 2021. This shows that calculating net book value is only the first step, not the definitive solution.

That said, when you’re choosing between two companies and both seem interesting, knowing their net book value can be the differentiating factor. It’s especially useful in fundamental analysis—the approach that looks beyond charts and focuses on the company’s real data: its balance sheets, its sector, its outlook, and its management.

The point is that net book value is a useful support for your decisions, not a magic answer. The real investment opportunities emerge when you combine this metric with a comprehensive analysis of the company, its competitive advantages, and the macroeconomic context. Use it as a compass, not as a map.