How to Build an Inventory Budget

An inventory budget is a plan for how much stock the company will hold at the end of each period and how much it must buy to get there, and it is the bridge between the sales forecast, the purchasing plan and the cash forecast. This guide is part of demand planning and inventory optimization, follows inventory valuation methods compared and FIFO vs. LIFO, leads to open-to-buy planning and reducing inventory without causing stockouts, and feeds the cash flow management forecast. Without it, buying decisions are made one purchase order at a time, and the total only becomes visible when cash runs short or the warehouse fills up. With it, finance can see the cash needed to support growth, planners can see the stock levels they are expected to hold, and leadership can test whether the inventory is appropriate for the sales plan. This guide shows how to build an inventory budget step by step, using the basic relationship between cost of goods sold, ending inventory and purchases, with a worked quarterly example in which every figure is checked. It also covers seasonal phasing, category-level budgets, links to the cash forecast and monthly review.

The core relationship

Every inventory budget rests on one identity: beginning inventory plus purchases minus cost of goods sold equals ending inventory. Rearranged, purchases = cost of goods sold + ending inventory - beginning inventory. The sales forecast gives cost of goods sold. A target for ending inventory gives the second term. The beginning balance is simply last period's ending balance. The purchases figure that results is what the buying team needs to commit, measured at cost. If the budget is built by month, the monthly purchase figure feeds the supplier payment schedule and the cash forecast.

Step 1: start from the sales plan

Begin with the demand forecast, converted into revenue and then into cost of goods sold using expected product margins. Use the consensus forecast from the sales and operations planning process, not a number set by one function. If the business has distinct categories with different margins or lead times, build the budget by category and add them up. At minimum, separate fast, seasonal and slow categories. See financial forecasting for how the revenue plan is built.

Step 2: set the stock target

The target for ending inventory is usually expressed as days or weeks of forward cover, as a stock-to-sales ratio, or as a turnover target. Forward cover is the most useful for planning because it looks at upcoming demand, not past sales. A target of 60 days of forward cover at the end of a quarter means holding enough stock for the first 60 days of the next quarter. Set cover by category according to lead time, demand variability and the service level you want. Fast, predictable products with short lead times can run at lower cover. Long lead time or uncertain products need more. The target should reflect safety stock and cycle stock, plus any pipeline stock or planned builds. See safety stock formula.

Step 3: calculate purchases

A worked example makes this concrete. A company expects cost of goods sold of $6.0 million in Q1, $6.5 million in Q2, $8.5 million in Q3 and $9.0 million in Q4, a total of $30.0 million (illustrative). It expects $6.2 million in Q1 of the following year. Beginning inventory is $4.2 million. The target is 60 days of forward cover, and for simplicity each quarter is treated as 90 days, so cover equals two-thirds of the following quarter's cost of goods sold.

The target ending inventory for each quarter is as follows. At the end of Q1, it is 6.5 × 2/3 = $4.33 million. At the end of Q2, 8.5 × 2/3 = $5.67 million. At the end of Q3, 9.0 × 2/3 = $6.00 million. At the end of Q4, 6.2 × 2/3 = $4.13 million.

Purchases by quarter, using purchases = cost of goods sold + ending inventory - beginning inventory:

Check the full year: purchases of 29.93 should equal cost of goods sold of 30.00 plus the change in inventory (4.13 - 4.20 = -0.07), which is 29.93. The quarterly checks also hold: for Q2, 6.50 + 5.67 - 4.33 = 7.84, and for Q3, 8.50 + 6.00 - 5.67 = 8.83. The budget shows that inventory rises through the third quarter to support peak demand, and falls in the fourth, with purchases peaking in Q3, ahead of the peak in sales. Peak inventory is $6.00 million at the end of Q3.

Step 4: test the result

Convert the budget into measures that leadership understands. Average inventory over the year using the five quarter-end balances (4.20, 4.33, 5.67, 6.00, 4.13) is the simple average of the opening and quarter-end points: (4.20 + 4.33 + 5.67 + 6.00 + 4.13) / 5 = 24.33 / 5 = $4.87 million. Turnover is 30.0 / 4.87 = 6.2 times, and days of inventory are 365 / 6.2 = about 59 days. Check: 4.87 × 6.2 = 30.2, which agrees with 30.0 within rounding. Compare with last year and with the target. If the result is worse than the company's goal, the questions are whether cover can be reduced in specific categories, whether the peak can be shifted or whether purchases can be better phased. If it is better than last year, check that the service-level assumptions are realistic and that there is room for variation in demand.

Step 5: phase by month

Quarterly budgets hide the seasonal peak within the quarter. Move to monthly once the quarterly shape is agreed. Use the monthly sales forecast and forward cover targets to compute monthly ending inventory and purchases. Add known timing effects: supplier shutdowns, shipping lead times, port delays and promotional builds. Where purchases must be committed far ahead of the sales, as with long lead time imports, build the budget on commitment dates and show the delivery and payment dates separately. This allows finance to see when cash goes out, not just when goods arrive. See how inventory decisions affect cash flow.

Step 6: link to the cash forecast

Purchases at cost become supplier payments after the payment terms. If the company pays 45 days after receipt, a purchase received in March is paid in mid-April. Map purchases to payments, then combine with collections and other cash items. The resulting cash curve will show the funding needed to carry the inventory. If the peak is higher than the credit line, either the inventory plan must change or the financing must increase. See working capital for the wider picture.

Step 7: budget by category and location

A single company-wide number hides imbalances. Break the budget down by category, and where relevant by warehouse or channel. Allocate the total to categories using their share of sales and their individual cover targets. Review the result for items that would be disproportionately large, and challenge them. A category budget also gives buyers a limit to work within, which is the foundation of open-to-buy planning. See open-to-buy planning.

Step 8: include new products and reserves

New products are often missing from budgets or included at optimistic volumes. Add a line for launches with a realistic first-order quantity, and make the assumptions explicit. Include a line for planned markdowns, write-downs and the reserve for excess and obsolete stock, because these affect the carrying value and the cash recovered. See new product forecasting and inventory write-downs.

Monitoring and revising

An inventory budget is a plan, and it should be compared with actual results every month. Track actual ending inventory against budget by category, actual purchases against budget, and cover against target. Investigate variances that exceed a set threshold, such as 10 percent, and note whether the cause is demand (sales higher or lower than forecast), supply (late deliveries) or decisions (buying earlier or later than planned). Reforecast the remainder of the year using actuals, so that the budget remains a working tool. Report to leadership a short summary of the total, the biggest variances and the actions taken.

Common mistakes

Setting inventory as a percentage of sales without accounting for seasonality leads to unrealistic targets. Using last year's balance sheet as a guide ignores changes in lead times and mix. Building the budget in isolation from the cash forecast leaves finance surprised. Leaving out new product launches understates the need. Ignoring minimum order quantities and pack sizes yields purchase numbers that cannot be placed. Failing to update the budget after the sales forecast changes leaves it out of date. Treating the budget as a target to be hit, and not as a plan to be revised, encourages buying to match the number and not the demand.

FAQs

What is an inventory budget?

A plan for the inventory balance at the end of each period and the purchases needed to reach it, based on the sales forecast and stock targets.

How do you calculate budgeted purchases?

Purchases equal cost of goods sold plus target ending inventory minus beginning inventory.

How do I set the ending inventory target?

Use days of forward cover, a stock-to-sales ratio or a turnover goal, set by category according to lead time, variability and the service level you need.

How is an inventory budget linked to cash?

Budgeted purchases become supplier payments after the payment terms, and the resulting cash curve shows the funding needed to carry the stock.

How often should the inventory budget be updated?

Review it monthly against actuals, and reforecast when the sales forecast or lead times change.