Working capital: the number that decides whether growth pays you or drains you

Here's a sentence that has surprised more owners than any other: a profitable company can run out of cash, and a growing one is the most likely to. The reason has a name — working capital — and once you can read it, the gap between "we made money" and "where did it go" stops being a mystery.

Working capital is the cash your day-to-day operations tie up: money you've earned but not collected, money sitting on the shelf as inventory, minus the money you owe suppliers but haven't paid yet. It's the most overlooked driver in a financial model and one of the sharpest things you can track month to month. Let's walk through all of it.

What working capital actually is

The textbook definition is current assets − current liabilities. That's the whole balance-sheet view, and it's fine for a banker checking whether you can cover the next year. But for running the business, a CFO looks at the tighter, operating version — the three accounts that move with sales:

Read it as a story. You ship a product or finish a job, so the sale lands on your P&L — but the cash hasn't arrived (that's a receivable). To make the sale you had to buy or build the thing first, and some of it is still unsold (that's inventory). The one thing working in your favor is that you haven't paid your own suppliers yet (that's a payable). Net those three and you have the cash your operation is quietly holding hostage.

Why a profitable company runs out of cash

Here's the trap. Working capital scales with revenue. Sell 30% more and — all else equal — your customers owe you 30% more, your shelves hold 30% more, and your payables rise 30% too. The receivables and inventory go up faster than the payables offset them, so the net cash tied up climbs.

For Northwind, operating working capital is $377,000 on $1.46M of revenue— about 26 cents of cash locked up for every dollar of sales. Grow revenue 30% and that figure climbs to roughly $490,000. That's ~$113,000 of cash consumed by growth — spent before a single dollar of the new profit reaches the bank. The more you grow, the more it eats. This is why a hockey-stick year can end with a scramble for a credit line, and why "we're profitable" is never the same sentence as "we have cash."

The cash conversion cycle: how long your cash is gone

Dollars are hard to compare across businesses; days aren't. So a CFO converts each working-capital account into a number of days, then nets them into one figure — the cash conversion cycle (CCC): how many days pass between paying for something and collecting the cash from selling it.

Net them: CCC = DSO + DIO − DPO = 60 + 119 − 70 = 109 days. For about three and a half months, every dollar Northwind spends on inventory is out of reach — gone into a box on a shelf, then into an unpaid invoice — before it comes back as cash. That 109-day gap, multiplied by daily spend, is the $377,000. Shorten the cycle and you free real money without selling anything more; let it drift longer and growth gets hungrier for cash every quarter.

Don't take Northwind's word for it — drop in your own revenue, cost of goods, and the three balance-sheet lines and watch your DSO, DIO, DPO, and cash cycle fall out:

In a financial model, working capital is a driver — not a plug

The most common modeling mistake is treating the balance sheet as an afterthought — holding receivables and inventory flat, or plugging them to make the model balance. That hides the exact risk you built the model to find. Done right, working capital is driven off the days assumptions and the income statement:

• A/R = DSO ÷ 365 × revenue. Faster you assume you get paid, less cash the model ties up.
• Inventory = DIO ÷ 365 × COGS. Tied to cost of goods, not sales — inventory is valued at cost.
• A/P = DPO ÷ 365 × COGS. The free financing your suppliers extend.

Now the model earns its keep. Change one blue assumption — push DSO from 60 days to 45 — and you watch the cash freed up flow straight to the bottom of the cash flow statement. Because here's the part that ties it together: it's the change in working capital, not the balance, that hits cash. Each period, Δ working capitalis a line in operating cash flow — working capital growing is cash out, shrinking is cash in. A model that drives the three accounts off days is the only one that will warn you, in advance, that next year's growth needs a $113,000 cushion.

The five numbers to watch every month

Working capital isn't a year-end exercise — it's a monthly vital sign, and like every vital sign it's the trend that matters more than the level. The free KPI tracker keeps all five on one page, month over month:

• DSO, DIO, DPO. The three day-counts, tracked month over month. A DSO creeping from 52 to 61 over two quarters is your customers slowly turning you into their bank — visible long before it shows up as a cash crunch.
• Cash conversion cycle. The one number that nets the three. Falling is good; rising means each new dollar of revenue is locking up more cash than the last.
• Working capital as a % of revenue. The modeling driver, read as a KPI. If it drifts up, your growth is getting more cash-hungry — and your forecast needs to know.
• Current & quick ratio.The liquidity backstop — can current assets cover what's due? The quick ratio strips out inventory, the slowest asset to turn into cash.
• A/R and A/P aging. The detail behind the averages: which customers are past terms, and which payables you could stretch without burning a supplier.

How to actually shrink it

Every day you cut off the cash cycle is cash back in your account, free. The levers, in roughly the order they pay off:

• Collect faster (DSO). Invoice the day you deliver, not at month-end. Tighten terms on new customers, take deposits on big jobs, and work the aging by name — most overdue cash is in a handful of accounts.
• Carry less stock (DIO).Usually the biggest lever and the slowest dollar. Find the dead SKUs that haven't moved in a year, tighten reorder points, and stop pre-buying to chase a volume discount that locks up more cash than it saves.
• Use your terms (DPO). Pay on the last fair day, not the first — supplier credit is free financing. The one exception: take an early-pay discount when the implied rate beats your cost of capital, and skip it otherwise.
• Mind the mix.Customers and products that pay cash-up-front or turn quickly are worth more than their margin alone suggests, because they don't tie up the cash that funds everything else.