The capital maintenance approach to net income measures profit by comparing the total amount of equity, or net assets, a business has at the end of a period against the amount it had at the beginning, with the difference, adjusted for any capital contributed or withdrawn during the period, treated as the period's net income or loss. It stands in contrast to the transaction approach, which is what ordinary financial accounting actually uses, where every individual transaction is recorded, sorted, and summarized to build up revenues, expenses, and net income directly.
A simple illustration of the logic
If a business owner had $1 million of net assets at the start of the year and $1.4 million at the end, having neither added nor withdrawn any capital during the year, the capital maintenance approach would conclude the business earned $400,000 of net income, without needing to examine a single individual sale or expense transaction to arrive at that figure. Any amounts the owner personally contributed or withdrew during the year would need to be added back or subtracted first, since those capital movements are not themselves income or loss.
Where this approach is actually used
Ordinary financial accounting relies almost exclusively on the transaction approach, since it provides far more useful detail about where revenue came from and where expenses were incurred, information a simple before-and-after equity comparison cannot provide on its own. The capital maintenance approach shows up most often in tax enforcement and legal contexts, where investigators reconstructing someone's income, often in a case involving unreported or illegally obtained funds, have no reliable transaction records to work from and instead estimate income indirectly by tracking how a person's net worth changed over time.
Limitations of the capital maintenance approach
Because it only produces a single net figure without any breakdown by source, the capital maintenance approach cannot distinguish operating income from investment gains, cannot separate recurring revenue from one-time windfalls, and is highly sensitive to how individual assets happen to be valued at the two measurement dates. A change in asset valuation method or a one-time appraisal adjustment can produce a large apparent swing in “income” under this approach that has nothing to do with the underlying business actually performing differently.
Why the distinction still matters conceptually
Even though the transaction approach dominates everyday accounting practice, understanding the capital maintenance approach helps clarify what net income is fundamentally meant to represent: the genuine increase in an entity's wealth over a period, after accounting for any capital the owners put in or took out, which is the same underlying economic concept the transaction approach is ultimately trying to measure through a very different, far more detailed, route.
Why accounting relies on the transaction approach instead
Despite its conceptual simplicity, the capital maintenance approach is rarely used as the primary basis for financial reporting, mainly because it cannot explain why income changed, only that it did. A business using this method alone would know its equity grew by $4 million but would have no breakdown of how much came from operating profit, financing activity, or a one-time gain, information investors and managers both need. The transaction approach that underlies actual GAAP financial statements instead records and classifies every individual revenue, expense, gain, and loss, arriving at the same bottom-line net income but with the full detail the capital maintenance shortcut leaves out entirely.