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Guide · Cash Flow

Days Inventory Outstanding: Cost-Based Timing

Calculate inventory days using average inventory at cost and cost of goods sold, with limits for seasonal stock and slow-moving items.

By PriceAndProfit Editorial Updated 3 min read

On this page
  1. Work from the cost records
  2. Connect a change in days to units and purchasing
  3. Handle unusual periods openly

Days inventory outstanding, or DIO, expresses the inventory balance in days of cost of goods sold. It estimates how much stock the business carries relative to its cost flow. Because inventory is recorded at cost in this calculation, the denominator should also be a cost measure. Using sales revenue instead can make inventory days look artificially low when the selling price includes a margin.

Inventory days = average inventory at cost ÷ cost of goods sold × days in the period

Work from the cost records

Assume a hypothetical 90-day period starts with $36,000 of inventory and ends with $44,000. The simple average is $40,000. Cost of goods sold during the period is $120,000. Inventory days are $40,000 divided by $120,000, multiplied by 90, or 30 days. The daily goods-sold cost is $1,333.33 when displayed to cents.

MeasureAmount
Opening inventory at cost $36,000
Closing inventory at cost $44,000
Average inventory at cost $40,000
Period cost of goods sold $120,000
Inventory days 30

If revenue is $200,000, using revenue in the denominator would produce 18 days. That 18-day result does not measure the same inventory cycle. The difference arises from the valuation base, not from faster stock movement. Keep revenue-based ratios labeled separately if the business uses them for another purpose.

Connect a change in days to units and purchasing

At the same daily cost flow, reducing average inventory from 30 to 24 days corresponds to $8,000 less inventory: six days multiplied by $120,000 divided by 90. This is a steady-state balance scenario. It does not mean a purchase order can necessarily be reduced by exactly $8,000 immediately, because order minimums, lead times, seasonality, and service requirements may constrain the change.

Break the total into product groups. Fast-moving stock can offset a large amount of slow stock in the blended ratio. Review units on hand, expected demand, and age by item before assuming a lower overall ratio represents better service or lower risk. Stockouts can reduce inventory while also losing contribution from unfilled orders.

Handle unusual periods openly

A seasonal buildup before a selling period can raise DIO even when it follows the plan. A two-point average may miss a large midperiod stock peak; use more frequent balances when available. Write-downs can lower the inventory value without producing cash or moving physical units, so explain those effects separately.

When cost of goods sold is zero, inventory days are undefined, not zero. The business may have stock but no completed sales in the period. The formula is an indicator, not a recommended stock level or a substitute for replenishment planning. Maintain a dated purchasing and cash schedule alongside it.

Method references: OpenStax: What Is Working Capital?. Accessed October 22, 2026. All dollar figures are hypothetical worked examples.

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