The accounting equation

Everything a business has came from somewhere. That sentence, taken literally, is the whole of accounting: the things it holds must equal the claims on those things. Someone lent it, someone invested it, or the business earned it.

An asset is something the business controls that will produce future benefit — cash, a receivable, stock on a shelf, a van. A liability is an obligation to hand something over — a supplier's bill, an accrued cost, a deposit taken for goods not yet delivered. Equity is what is left over for the owners after every obligation is met. Not "the money in the bank", and not "what the company is worth" — the residual.

So on 1 September, the founders pay in $60,000. Assets go up by $60,000 in cash. Nobody outside is owed anything, so the whole claim belongs to the owners.

Those two definitions are narrower than the everyday words. "A present right to an economic benefit" excludes something you hope to get; "a present obligation to transfer an economic benefit" excludes something you intend to spend. Equity is defined last and defined as the leftover, which is why the equation is a definition rather than a discovery.

Now watch what the van does. Northwind buys it for $24,000 in cash. Assets fall by $24,000 and rise by $24,000 — cash became a van. Nothing on the right-hand side moves at all, because nothing about who has a claim changed. This is why an entry has two sides even when only one side of the equation is involved.

The equation holds after every entry, not just at month end. A transaction that breaks it has been recorded wrong — there is no third possibility.

Revenue and expenses are not a fourth category — they are equity, moving. Revenue increases the owners' claim, expenses decrease it, and the reason they get their own accounts is that "equity went up by $2,550" is a useless sentence compared to "we sold $31,500 and it cost us $28,950". Written out in full, the equation a bookkeeper actually works with is assets = liabilities + contributed capital + revenue − expenses — the last two folded into retained earnings when the year is closed.