Cash Flow Basics

The simplest vital sign

Cash flow is money in minus money out, over a period: net cash flow = inflows − outflows. Suppose a month brings $5,000 of income against $4,200 of outflows: net +$800. Positive cash flow is what funds savings, investing, and every plan you have; negative cash flow quietly consumes reserves until something must change. It is the financial vital sign for a household — and, as the statements lesson showed, the third report every company must publish measures exactly the same thing.

Key point: Net worth is what you HAVE; cash flow is which direction you are MOVING, and how fast.

Why paper wealth cannot pay bills

Recall the liquidity lesson's apartment owner: $300,000 of wealth, months from cash. Bills are paid from flows and liquid reserves, never from net worth directly. This is how a profitable-looking household with an expensive house ends up borrowing at card rates, and how businesses with full order books fail — the classic phrase is that companies die not from lack of profit, but from running out of cash. A strong position needs both stocks AND flows: net worth for the long game, cash flow for staying in the game.

Burn rate and runway — the negative case, quantified

When cash flow is negative, two numbers matter. The burn rate is how much cash the period consumes; the runway is how long reserves last at that rate: runway = liquid reserves ÷ burn rate. Suppose reserves are $12,000 and the monthly burn is $800: runway = 12,000 ÷ 800 = 15 months. Startup investors live by these numbers, but they apply identically to a household between jobs — runway converts anxiety into a deadline, and a deadline can be planned for.

Same income, opposite directions

Two households each earn $6,000 a month. Household A spends $5,100: net +$900, about $10,800 a year available to invest or buffer. Household B spends $6,300 using card credit for the gap: net −$300, burning reserves (or accumulating 20%-rate debt) every month. After five years, A has funded a portfolio; B has funded a bank's. The income statement looked identical; the cash flow decided everything.