Inventory turnover ratio measures how many times a business sold the equivalent of its average inventory during a defined period, using inventory cost rather than selling price. Calculate it from cost of goods sold and an average inventory balance measured on the same basis.
The number is a starting point for questions about stock, demand and service. It does not establish profitability or tell you that every item sold at the same speed. For the wider improvement process, use the inventory-turnover pillar; this page focuses on the calculation.
Inventory turnover ratio formula
inventory turnover = cost of goods sold for the period / average inventory at cost
A simple average is:
average inventory = (opening inventory + closing inventory) / 2
OpenStax’s working-capital chapter gives these formulas and converts annual turnover to an inventory conversion period using 365 divided by turnover. Match the numerator and denominator: annual cost of goods sold belongs with inventory representative of that year, not with an unrelated month’s closing balance.
Use a more representative average when opening and closing figures conceal a seasonal build, a large acquisition or a major stock reduction. Monthly or daily balances can help, provided their coverage and valuation basis are consistent. Document the averaging method so the next comparison uses the same one.
Worked example and days in inventory
Consider a hypothetical distributor with the following annual figures. All inventory values and cost of goods sold use the same currency and cost basis.
| Input or calculation | Illustrative value |
|---|---|
| Opening inventory | 80,000 |
| Closing inventory | 120,000 |
| Average inventory | (80,000 + 120,000) / 2 = 100,000 |
| Annual cost of goods sold | 500,000 |
| Annual inventory turnover | 500,000 / 100,000 = 5 times |
| Days in inventory using average inventory | 365 / 5 = 73 days |
The hypothetical business sold the cost equivalent of its average inventory five times during the year. The 73-day measure is an aggregate average-inventory relationship, not a promise that a particular receipt will sell in 73 days. Old stock and fast-moving stock can coexist behind the same total.
For a shorter period, keep the day count aligned:
days in inventory = days in the measured period / turnover for that period
For example, a hypothetical 90-day period with cost of goods sold of 150,000 and average inventory of 100,000 has turnover of 1.5 for that period and 60 days in inventory. Dividing 365 by the unannualised 1.5 would mix periods. State whether a published comparison uses actual period turnover or an annualised figure.
Some reports use ending inventory rather than average inventory for a days measure. Treat that as a different convention and label it; do not silently combine it with the average-inventory calculation above.
Why sales divided by inventory can mislead
Revenue includes the selling margin, while inventory is generally held at cost. A sales-based ratio therefore answers a different question from cost-based inventory turnover. Changing selling prices or product margins can move that ratio even when stock movement does not change.
Using the same hypothetical annual example, revenue of 800,000 divided by average inventory of 100,000 gives 8, compared with cost-based turnover of 5. Neither arithmetic result is interchangeable with the other. Label a revenue-based metric explicitly and compare it only with figures calculated the same way.
How to interpret the result
OpenStax’s inventory-management discussion describes the need to balance inventory holdings with operating and sales needs. The practical interpretation depends on what changed in your own business:
| Observation | Questions to investigate |
|---|---|
| Turnover rises and service stays stable | Did replenishment improve, or did a temporary sale accelerate demand? |
| Turnover rises and stockouts increase | Has inventory become too lean for replenishment variability? |
| Turnover falls after a planned stock build | Is this an intentional seasonal position, and is sell-through on plan? |
| Turnover falls with aged stock | Which items, locations or lifecycle decisions explain the balance? |
| Turnover changes after a valuation adjustment | How much is accounting treatment rather than physical movement? |
Compare periods with similar scope, product mix and valuation methods. A service-parts operation holding critical slow-moving components should not borrow an unsupported target from a high-volume retailer. No universal industry benchmark is used here.
Use segment-level analysis where useful, but keep allocated cost of goods sold and inventory aligned. Do not average individual SKU turnover ratios and assume the result equals the business-wide ratio; the denominators carry different weights.
Calculation checks before reporting
- Define the entity, location, product scope and start/end dates.
- Obtain cost of goods sold for that same scope and period.
- Confirm inventory is measured consistently at cost.
- Choose an average that represents the period and disclose the method.
- Check for missing balances, unusual adjustments and changes in product mix.
- Calculate turnover and days using the same period.
- Read the result beside stockouts, aged inventory, margin and service measures.
If average inventory is zero, the ratio is undefined; do not report an infinite performance improvement. A negative denominator or a negative cost figure also requires investigation before using the result operationally.
A periodic inventory system can supply the balances used in this calculation, but it is not a replenishment strategy or a substitute for demand planning. The inventory KPI guide helps place turnover alongside other operating measures. Preserve those separate measures rather than using one ratio to explain the entire business.