Working Capital Forecasting: AR, AP, and Inventory

Working capital explains part of the gap between profit and cash. A company can show a profit every month of a forecast and still run out of money if receivables, payables, or inventory move differently than the model assumes. Forecasting those accounts makes the cash effect visible.

Three accounts do most of the work: accounts receivable, accounts payable, and inventory. Each is forecast as a number of days against the flow it relates to.

The three drivers

Accounts receivable is revenue you have recognized and not collected. Forecast it with days sales outstanding, or days sales outstanding (DSO): the average number of days between invoicing and payment.

Accounts receivable = DSO / 365 * annual revenue  

If invoices are due in 30 days but customers actually pay in 52, use 52. Payment terms describe the contract; collections history describes the cash flow. Use the history when it is available, and show an operational improvement explicitly rather than hiding it in the DSO assumption.

Accounts payable is the mirror image: expenses you have incurred and not paid. Forecast it with days payable outstanding (DPO) against cost of goods sold (COGS), or against total cash operating expenses if you want it broader.

Accounts payable = DPO / 365 * annual COGS  

Stretching payables preserves cash in the period when payment is delayed. The benefit does not repeat unless DPO keeps increasing, and longer delays can damage vendor relationships or change purchasing terms. A forecast with DPO rising every year is making an operating assumption that should be explained.

Inventory is product purchased or produced but not yet sold. Forecast it with days inventory outstanding (DIO) against COGS.

Inventory = DIO / 365 * annual COGS  

Software companies generally have no inventory to forecast. For a company selling physical products, inventory may be the largest working capital account because cash leaves before the related revenue is collected.

The cash conversion cycle

Put the three together and you get the number that summarizes the whole picture:

Cash conversion cycle = DSO + DIO - DPO  

The cash conversion cycle estimates how many days the business finances its operations before recovering cash from customers. A positive cycle means the company generally pays suppliers before collecting from customers, so growth requires more cash. With a negative cycle, customer cash arrives first and can help finance growth. Some marketplaces and subscription businesses that collect annually in advance have this advantage, although deferred revenue and other operating costs still matter.

If the cycle stays positive while revenue grows, working capital will usually consume cash because the balance tied up in operations is increasing. The income statement does not show that use of cash, which is how a profitable forecast can still produce a funding need.

How it flows through the model

Working capital lives on the balance sheet, but its effect shows up on the cash flow statement as the change in each account, not the balance:

• Receivables go up, cash goes down. You sold more and collected proportionally less.
• Payables go up, cash goes up. You are holding onto money longer.
• Inventory goes up, cash goes down.

The cash flow statement therefore uses the period-over-period change in each account, with the sign reversed for assets. A sign error should break the balance-sheet check. If it does not, the model has a second problem: its cash flow statement is not properly connected to the balance sheet.

Where this is already built

The Standard Financial Model forecasts all three accounts from days-based assumptions and rolls the changes through the balance sheet and cash flow statement. The Runway Tool is narrower: it does not carry a full balance sheet, but it does model when expenses are paid, which captures the timing relevant to runway.

For a business in distress, use a 13 week cash flow forecast instead. It forecasts receipts and disbursements directly, which is more useful for near-term liquidity decisions than deriving cash from accrual accounts.

What people get wrong

One mistake is forecasting working capital as a percentage of revenue without checking the operating relationship underneath it. The shortcut can work while payment terms and customer mix remain stable. When either changes, the assumption no longer describes how the business collects and pays.

Another is putting the working capital balance, rather than its change, on the cash flow statement. The balance sheet carries the ending balance; the cash flow statement carries the movement during the period. Confusing the two can make the first forecast period wrong by an order of magnitude while leaving the output superficially plausible.