Last reviewed: July 2026.
Inventory turnover days estimate how long inventory is held before sale or use. The measure is also called inventory holding period or stock turnover period.
It is most useful when calculated consistently and interpreted with product mix, seasonality, supply risk, gross margin and net realisable value.
Inventory turnover days formula
Average inventory ÷ Cost of sales × 365
Average inventory is commonly opening inventory plus closing inventory divided by two.
Inventory turnover ratio
Cost of sales ÷ Average inventory
The turnover ratio shows how many times average inventory is sold or consumed. Days are approximately 365 divided by turnover.
Worked example
Opening inventory is 180,000 CU, closing inventory is 220,000 CU and annual cost of sales is 1,460,000 CU.
- Average inventory = (180,000 + 220,000) ÷ 2 = 200,000 CU
- Turnover ratio = 1,460,000 ÷ 200,000 = 7.3 times
- Inventory days = 200,000 ÷ 1,460,000 × 365 = 50 days
Using closing inventory only
When opening inventory is unavailable, closing inventory may be used, but the result is less representative for seasonal or fast-changing businesses.
State the denominator clearly.
Why cost of sales is used
Inventory is measured at cost, so cost of sales is normally a better matching flow than revenue. Using sales can distort comparisons when margins differ.
Review the cost of goods sold guide.
Interpretation of high days
Longer holding periods may indicate slow-moving stock, overbuying, weak demand, obsolete items or deliberate safety inventory.
They can also arise from expansion, supply-chain disruption or inventory purchased before price increases.
Interpretation of low days
Lower days may indicate efficient purchasing and fast sales. Extremely low levels can signal stock-outs, lost sales, weak resilience or underinvestment.
Trend analysis
Compare several periods using the same formula. Investigate changes in categories, locations and business conditions rather than relying only on the total ratio.
Industry comparison
Perishable goods, fashion, industrial spares and property development have very different normal holding periods. Benchmark against comparable businesses and products.
Seasonality
Opening and closing balances can miss peaks within the year. Monthly or quarterly average inventory provides a more reliable denominator for seasonal businesses.
Inventory mix
A total ratio can hide fast-moving products and obsolete categories. Calculate turnover by product family, location, age and value.
IAS 2 valuation connection
IAS 2 measures inventory at the lower of cost and net realisable value. Slow turnover can indicate damage, obsolescence or selling prices below cost.
Use the stocktaking and IAS 2 controls guide.
Cash conversion cycle
Inventory days are one component of the cash conversion cycle:
Inventory days + Receivables days − Payables days
See the cash conversion cycle guide.
Improving inventory turnover
- improve demand forecasting;
- reduce low-value product proliferation;
- set reorder points and safety stock by risk;
- strengthen supplier lead-time data;
- discount or dispose of obsolete goods;
- improve production scheduling;
- review minimum order quantities.
Raw materials, work in progress and finished goods
Manufacturers should calculate separate holding periods for raw materials, work in progress and finished goods. Each stage has different drivers and improvement actions.
Standard cost and variances
When inventory uses standard cost, large purchase-price or production variances can distort cost of sales and average inventory. Reconcile standards to actual cost and review whether standards are current.
Days for new or fast-growing businesses
Rapid growth can increase inventory before sales occur, temporarily increasing days. A new product launch may also make opening and closing averages unrepresentative.
Inflation and price changes
Rising input prices can increase closing inventory values even when physical quantities are stable. Analyse units and values together.
Do not optimise the ratio alone
Reducing inventory may improve the ratio while increasing lost sales, emergency freight, production downtime or supplier dependency.
Balance working capital with service level and resilience.
Gross margin interaction
High-margin products can justify longer holding periods, while low-margin stock requires tighter control. Analyse gross profit generated per unit of inventory investment.
Inventory turnover and profitability
Faster turnover can improve return on capital when margins and service are maintained. However, turnover growth from discounting may reduce profit.
Use the broader financial ratio analysis guide.
Data quality and controls
- reconcile inventory records to the general ledger;
- use reliable physical counts;
- exclude non-inventory balances;
- review negative and zero quantities;
- separate write-downs and abnormal losses;
- calculate average balances consistently;
- document formula definitions.
Forecast turnover days
Budgets should forecast inventory days by category and connect assumptions to sales, purchasing, lead times and production plans. Compare actual days with forecast and explain volume, price and mix differences.
Inventory ageing
An ageing report complements turnover ratios. Long-aged items may require specific NRV analysis even when total turnover appears acceptable.
Common mistakes
- using revenue instead of cost of sales without explanation;
- using closing inventory for a seasonal business;
- comparing unrelated industries;
- ignoring inventory write-downs;
- treating lower days as always better;
- mixing raw materials, work in progress and finished goods;
- failing to investigate obsolete stock.
Key takeaway
Inventory turnover days measure how long cost is tied up in stock. Use average inventory, consistent definitions and operational context before deciding whether turnover is healthy.
Official references: IAS 2 Inventories and IAS 2 supporting material.