It’s important to understand that residual income doesn’t have the same meaning for equity valuation as it does for personal finance. That’s because it doesn’t entail leftover cash, or so-called disposable income, but it has to do with the income that a company generates, after accounting for capital costs, according either to CAPM (Capital Asset Pricing Model), or APT (Arbitrage Pricing Theory). In other words, residual income valuation is the money the company is likely to be left with, once it covers opportunity costs (or risk costs), relative to the book value of Shareholders’ equity. Bear in mind that firms have no legal obligation to compensate shareholders, like they do for bondholders. However, without this form of compensation for the risk they’re taking with their investment, it’s unlikely they will attract investors. Here’s all of the above, laid out in a plain math formula:

Residual income is calculated as net income less a charge for the cost of capital. The charge is known as the equity charge and is calculated as the value of equity capital multiplied by the cost of equity or the required rate of return on equity. Given the opportunity cost of equity, a company can have positive net income but negative residual income.