Inventory Turnover Ratio: Formula, DIO and Examples
Inventory turnover ratio explained: the formula, how to calculate days inventory outstanding, a worked example with numbers, and practical ways to improve it.
The inventory turnover ratio shows how many times a business sells and replaces its stock in a period. The formula is cost of goods sold divided by average inventory. Divide 365 by the result to get days inventory outstanding (DIO), the average number of days stock sits before it sells. Higher turnover and lower DIO usually mean less cash tied up.
For any business that sells physical products, this is one of the most useful numbers to track. This guide covers the formula, how to calculate DIO, a step-by-step worked example, how to read the results for your industry, and practical ways to improve turnover without running out of stock.
The inventory turnover ratio formula
Inventory turnover ratio = cost of goods sold / average inventory
Where:
Average inventory = (beginning inventory + ending inventory) / 2
Cost of goods sold (COGS) is the direct cost of the products you sold during the period: purchase cost, inbound freight and, for manufacturers, direct materials and labour. If you are not sure how to calculate it, see our guide to cost of goods sold. Inventory should be valued at cost, the same basis as COGS, so you are comparing like with like.
Using an average rather than just the year-end figure smooths out the effect of a large delivery or a clearance sale just before the balance sheet date. If stock levels swing a lot through the year, average the 12 month-end balances instead of only the opening and closing figures.
Why use COGS rather than sales?
Some sources calculate turnover as sales divided by inventory. Because sales include your markup and inventory does not, that version overstates turnover. If your gross margin is 40%, a sales-based ratio will be roughly two thirds higher than the COGS-based one. Use COGS for your own tracking, and check which version any benchmark uses before comparing. Our markup vs margin guide explains why the difference matters.
Days inventory outstanding (DIO)
Turnover is easier to act on when converted into days:
Days inventory outstanding = (average inventory / cost of goods sold) x 365
or, equivalently:
DIO = 365 / inventory turnover ratio
If you calculate for a quarter, use the quarter's COGS and 91 or 92 days instead of 365. DIO is also called days sales of inventory (DSI) or days in inventory. It is one of the three building blocks of the cash conversion cycle, along with days sales outstanding and days payables outstanding.
Why bother with days? Because days connect directly to supplier terms and cash. If your suppliers give you 30 days to pay and stock sits for 61 days, you are funding at least 31 days of inventory from your own cash or borrowing, before you even wait for customers to pay. Expressing turnover in days makes that gap obvious, and it gives you a concrete target to work towards, such as cutting a week from average stock holding over the next six months.
Worked example: calculating the inventory turnover ratio
Ridgeway Cycles is an independent bike shop. From its accounts for last year:
- Revenue: $760,000
- Cost of goods sold: $480,000
- Inventory at start of year (at cost): $90,000
- Inventory at end of year (at cost): $70,000
Step 1: average inventory = ($90,000 + $70,000) / 2 = $80,000
Step 2: inventory turnover ratio = $480,000 / $80,000 = 6.0
Step 3: days inventory outstanding = 365 / 6.0 = about 61 days
Ridgeway sells through its average stock about six times a year, and a typical item sits in the shop for around two months. For comparison, the sales-based version would give $760,000 / $80,000 = 9.5, which looks far better but does not reflect how quickly the stock really moves.
Breaking it down by category
The overall figure hides big differences. Ridgeway's owner split the numbers by category:
| Category | Annual COGS | Average inventory | Turnover | DIO (days) |
|---|---|---|---|---|
| Complete bikes | $300,000 | $60,000 | 5.0 | 73 |
| Parts and accessories | $150,000 | $15,000 | 10.0 | 37 |
| Clothing | $30,000 | $5,000 | 6.0 | 61 |
| Total | $480,000 | $80,000 | 6.0 | 61 |
Complete bikes tie up three quarters of the inventory and turn slowest. That is where the cash is.
How much cash faster turnover releases
Suppose Ridgeway reduces DIO on complete bikes from 73 to 55 days by ordering smaller batches more often, with no change in sales. Average bike inventory needed becomes $300,000 x 55 / 365 = about $45,200, down from $60,000. That releases roughly $14,800 of cash, money that can pay suppliers early for a discount, fund marketing or simply act as a buffer. It also cuts storage space, insurance and the risk of being left with last season's models.
What is a good inventory turnover ratio?
There is no single good number, because business models differ so much. As general patterns:
| Type of business | Typical turnover pattern | Why |
|---|---|---|
| Grocery, food service, florists | High | Perishable goods must sell quickly |
| General retail, e-commerce | Moderate | Mix of fast and slow lines, seasonal peaks |
| Furniture, jewellery, high-end goods | Low | Expensive items, wide ranges, slower purchase decisions |
| Industrial parts, spares | Low | Many lines held for availability rather than volume |
Use your own trend as the main yardstick. If turnover is falling year on year while sales are flat, stock is building up. If it is rising and customers are not complaining about availability, your buying is getting more efficient. Industry associations, trade bodies or your accountant may be able to give you sector benchmarks that are calculated on the same basis as yours.
Reading high and low results
Low turnover (high DIO) often points to:
- Over-ordering or buying large quantities to get a volume discount.
- Slow-moving, damaged or obsolete stock that should be cleared.
- Weak sales or pricing that is out of line with the market.
- Inaccurate stock records, where the system shows stock that does not exist.
Very high turnover (low DIO) can mean excellent stock control, but check for:
- Frequent stockouts and lost sales.
- Rush orders and high delivery costs from ordering too often.
- Customers waiting longer than competitors make them wait.
Inventory also affects liquidity ratios. A business with slow-moving stock will show a large gap between its current ratio and quick ratio; see current ratio vs quick ratio for how that works.
How to improve your inventory turnover ratio
- Analyse by product. Rank items by sales and by turnover. A small share of products usually accounts for most of the slow stock.
- Clear dead stock. Discount, bundle or return slow items to suppliers where possible. Cash is worth more than items gathering dust.
- Order smaller batches more often. Compare the cost of extra deliveries with the cash and storage savings.
- Improve forecasting. Use past sales by month, including seasonal peaks, to set reorder points. A simple inventory management spreadsheet is enough for many small businesses.
- Negotiate supplier terms. Shorter lead times and smaller minimum orders let you hold less safety stock.
- Review pricing. Items priced well above market move slowly; items priced too low may sell fast but erode margin. Check gross margin alongside turnover, as covered in gross profit vs net profit.
- Count stock regularly. Cycle counts keep records accurate, so the ratio reflects reality.
Common calculation mistakes
- Mixing sales and cost. Dividing revenue by inventory at cost inflates turnover, as the example above shows.
- Using only year-end stock. Many businesses run stock down before year end, so a single closing figure makes turnover look better than it is during the rest of the year. Use an average, ideally of monthly balances.
- Mismatched periods. Quarterly COGS divided by average inventory gives a quarterly turnover. Multiply by four to compare with annual figures, or convert to days using the number of days in the quarter.
- Ignoring stock that cannot be sold. Damaged, returned or obsolete items still sitting in the inventory figure drag the ratio down and overstate assets. Write them down in line with your accounting policy.
- Comparing different valuation methods. FIFO, weighted average and other methods give different inventory values when prices are changing, so comparisons between businesses may not be like for like.
Quarterly calculation example
Ridgeway's spring quarter (91 days) shows COGS of $150,000, opening inventory of $95,000 and closing inventory of $75,000. Average inventory is $85,000. Quarterly turnover is $150,000 / $85,000 = 1.76, which annualizes to about 7.1. DIO for the quarter is $85,000 / $150,000 x 91 = about 52 days. The busy season turns stock faster than the annual average of 61 days, which is exactly what you would expect and why seasonal businesses should track quarterly or monthly figures.
Manufacturers and online sellers
Manufacturers hold three kinds of inventory: raw materials, work in progress and finished goods. The overall ratio is still COGS divided by average total inventory, but it helps to track each stage separately. Raw materials turnover shows how well purchasing matches production, while finished goods turnover shows how well production matches demand. A build-up of work in progress often points to bottlenecks on the production floor rather than weak sales.
Online sellers who use third-party fulfilment centres should count stock held there as their own inventory, along with goods in transit that they already own. Storage fees at fulfilment centres can rise for items held a long time, so slow turnover has a direct, visible cost. Sellers who use drop shipping hold little or no inventory, so the ratio matters much less to them than gross margin and return rates.
Inventory turnover, invoicing and cash flow
Faster turnover only helps cash flow if customers pay promptly once the stock is sold. For businesses selling on account, such as wholesalers or trade suppliers, the cash conversion cycle combines DIO with how long customers take to pay. Selling stock in 40 days is less useful if invoices then wait 60 days for payment.
Raise invoices as soon as goods ship, show clear payment terms and itemize products so customers can match invoices to deliveries quickly. The free invoice generator lets you list line items with quantities and unit prices, apply one tax rate and a discount, and download a PDF to send. You can also start from a clean invoice template if you want a particular look. Keeping working capital healthy depends on both sides: stock that sells and invoices that get paid.
Inventory valuation methods and write-downs can affect taxable profit, so check with a qualified accountant before changing how you value or write off stock.
Conclusion
The inventory turnover ratio is simple to calculate: cost of goods sold divided by average inventory, with DIO giving the same answer in days. Calculate it overall and by category, watch the trend, and focus on the slow lines where cash is tied up. Pair better stock control with prompt invoicing and collection, and the improvement shows up in your bank balance. Create your next invoice with the free invoice generator and explore more guides in business finance.
Frequently asked questions
How do you calculate inventory turnover ratio?
Divide cost of goods sold for a period by average inventory for the same period. Average inventory is usually the beginning inventory plus ending inventory, divided by two. For example, if cost of goods sold for the year is $480,000 and average inventory is $80,000, inventory turnover is 6.0, meaning the business sold and replaced its average stock six times during the year. Use inventory valued at cost so both figures are on the same basis.
What is days inventory outstanding (DIO)?
Days inventory outstanding is the average number of days it takes to sell inventory. It is calculated as average inventory divided by cost of goods sold, multiplied by the number of days in the period, usually 365. You can also divide 365 by the inventory turnover ratio. A turnover of 6.0 equals about 61 days. Lower DIO means stock sells faster and less cash is tied up on shelves or in the warehouse.
What is a good inventory turnover ratio?
It depends heavily on the industry. Businesses selling perishable or fast-moving goods, such as groceries or restaurant supplies, typically turn stock over many times a year. Businesses selling expensive, slow-moving items, such as furniture, jewellery or industrial parts, turn over far less often. Rather than chasing a universal number, compare your ratio with your own past results and with similar businesses, and check whether stockouts or excess stock are causing problems.
Should I use sales or cost of goods sold for inventory turnover?
Cost of goods sold is the better choice because inventory is recorded at cost. Using sales revenue divides a figure that includes your markup by a figure that does not, which inflates the ratio and makes it look better than it is. Some industry reports and lenders still use sales, so check which version a benchmark is based on before comparing. Whatever you choose, use the same method consistently from period to period.
Is a high inventory turnover ratio always good?
Usually it indicates efficient stock management and less cash tied up, but a very high ratio can be a warning sign. It may mean you are holding too little stock and losing sales through stockouts, ordering in small, expensive batches, or failing to meet customer demand quickly. The aim is a turnover high enough to keep cash moving but not so high that customers find empty shelves or wait too long for orders.
What causes a low inventory turnover ratio?
Common causes include over-ordering, slow-moving or obsolete products, weak sales, poor demand forecasting, buying in bulk for discounts that are not needed, and stock recorded in the system that no longer physically exists or cannot be sold. A low ratio ties up cash and increases storage, insurance and obsolescence costs. Reviewing turnover by product or category usually reveals that a small number of items are responsible for most of the problem.
How does inventory turnover affect cash flow?
Money spent on stock is not available for anything else until the stock is sold and the customer pays. The slower inventory turns, the longer cash is tied up. Days inventory outstanding is one of the three components of the cash conversion cycle, along with days sales outstanding and days payables outstanding. Reducing DIO by even a week or two can release a meaningful amount of cash for a stock-heavy business.
How often should I calculate inventory turnover?
At minimum, calculate it annually from your accounts. Most product businesses benefit from a monthly or quarterly calculation, using the cost of goods sold for that period and annualizing it or converting it to days. For seasonal businesses, monthly figures are more useful because a single year-end inventory count may not reflect typical stock levels. Calculating by product category as well as overall shows where to act.
Do service businesses need to track inventory turnover?
Most pure service businesses hold little or no inventory, so the ratio is not relevant. However, businesses that sell materials as part of their service, such as trades, salons, repair shops or print studios, may carry meaningful stock. For them, tracking turnover on parts and supplies helps avoid tying up cash in materials that sit on shelves. Billing those materials promptly on customer invoices matters too.
Invoice Creator Editorial Team
Guides on invoicing, cash flow and small-business finance from Invoice Creator, operated by Next Software Development Company. Content is reviewed for accuracy against the fields our own invoice generator produces.
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