How much stock does next month’s sales plan require?

To calculate a planned purchase quantity, add the units you expect to sell to the usable stock you want left at the end, then subtract the usable stock already available at the start. Check delivery dates and existing orders before turning that calculation into a purchase.

The result differs from a reorder trigger, which tells a buyer when a supplied replenishment policy calls for action. This calculation builds the quantity needed for a defined planning period.

Count what the month needs

Take an invented retailer planning May. It starts with fifty identical usable units, expects to sell three hundred and chooses to finish with eighty. There are no open orders, returns, damaged units or goods already promised outside the sales forecast. Nothing expires during the month, and storage can accommodate the plan.

The purchase calculation is 300 plus 80 minus 50: 330 units. After buying them, the retailer has 380 units available across opening stock and deliveries. Selling three hundred leaves the chosen eighty.

Buying only the forecast three hundred would leave fifty. Buying 380 would forget the fifty already held and leave 130. Neither is the stated plan.

The eighty-unit closing target is a supplied policy, not an optimal buffer established by this example. It still needs a reason connected to the following month's demand and deliveries. OpenStax's operating-budget explanation uses forecast sales, desired ending inventory and beginning inventory to calculate production or merchandise purchases.

The purchase is not the expense on every unit sold

Every unit, including opening stock, costs $12. That figure contains the complete inventory cost in this model; no separate freight or buying charge is omitted. All units sell for $20. The 330-unit purchase costs $3,960.

The modeled cost of the three hundred units sold is $3,600. Reconcile it through stock values: $600 opening inventory plus $3,960 purchased, less $960 closing inventory, equals $3,600 cost of goods sold.

The $360 difference between purchases and cost of goods sold is the increase in stock held: thirty additional units at $12. That increase is still inventory, not another expense charged merely because the supplier has issued a bill.

Assume all three hundred sales are earned and collected during May. Other operating expenses of $1,700 are incurred and paid in May, covering all remaining operating costs, including owner-management pay. With no depreciation, financing or income tax in this simplified model, the stated operating profit is $6,000 less $3,600 less $1,700: $700.

Put the supplier on the calendar

The supplier delivers all 330 units ready to sell on May 1, before any modeled sale. Its supplied terms require half the bill then and half on June 1. There are no opening supplier payables, and the retailer actually has $4,000 opening cash available for this plan. Neither branch below assumes borrowing.

May's supplier payment is $1,980. With $6,000 customer receipts and $1,700 other payments, cash increases by $2,320 and closes at $6,320. The remaining $1,980 supplier liability is still due on June 1. Paying it would leave $4,340 before any other June activity.

The $2,320 cash increase is larger than the $700 operating profit because the business has not yet paid the full stock bill. Start with profit, subtract the $360 increase in inventory and add the $1,980 increase in supplier payables: $2,320. The unpaid bill supplies timing, not extra earnings.

If the supplier instead required the full $3,960 on May 1, May would close at $4,340 and have no supplier payable from this order. Its operating profit would remain $700. The starting $4,000 covers that initial purchase, and no other payment occurs before the first customer receipt in this full-payment branch.

When the sales forecast is wrong

Return to the half-now, half-June terms and the same completed 330-unit purchase. Suppose only 240 units sell in May, still at $20, and nothing else changes. The retailer ends with 140 usable units, sixty above its chosen target. It has not obtained those units for free: their $720 cost is part of the order already made.

Revenue is $4,800; the units sold cost $2,880. Subtract the same $1,700 other expenses and operating profit is $220. May cash closes at $5,120 after the $1,980 supplier payment. Another $1,980 remains due on June 1. The extra stock cannot be treated as cash for that bill.

Chapter 3 of Infinite Upside connects stock with service, waste and the timing of demand. For a real order, establish the units already usable, incoming quantities and dates, the reason for the closing target, and the evidence behind expected sales. Then attach the supplier's payment schedule. Each change belongs in the calculation it actually affects.