Inventory Turnover Ratio: Formula, Days and Benchmarks

The inventory turnover ratio divides the cost of goods sold by the average inventory. The result is a number of times per year: how often the entire stock has been sold and replaced within the period.

For a controller that is a financing figure rather than a logistics figure. Inventory is working capital that earns nothing while it sits on the shelf: at a cost of goods sold of € 8.4 million, every ten days of inventory represents roughly € 230,000 tied up in boxes instead of sitting in the bank account. And unlike receivables, there is no payment term behind it that someone can be reminded of — only a purchasing decision taken months ago.

What the inventory turnover ratio measures

The inventory turnover ratio, also called stock turn or inventory rotation, expresses how many times the average inventory is converted into sales during a period. A ratio of 6 means the stock is sold through six times a year, or that two months of inventory is standing in the warehouse at any moment.

The same figure appears in two forms:

• The ratio itself, in times per period. Useful for comparing product groups: twelve against three needs no further explanation.
• Days of inventory, also known as DIO or days inventory outstanding. Useful in a working capital discussion, because days can be added to the receivable and payable terms.

That second form is the inventory leg of the cash conversion cycle, the formula DSO + DIO − DPO. There the inventory term is one of three components; here it is the subject in its own right.

The inventory turnover ratio formula

Inventory turnover ratio = cost of goods sold ÷ average inventory

1. Take the cost of goods sold for the period: the purchase value of what was actually sold, excluding VAT. Not revenue — see the pitfalls below.
2. Determine the average inventory: opening balance plus closing balance divided by two. With seasonal swings, the average of twelve month-end balances is more accurate than the average of two year-end balances.
3. Divide the cost of goods sold by the average inventory. The outcome is the number of turns per period.
4. Convert to days where useful: the number of days in the period divided by the ratio.
5. Repeat the measurement monthly on the same basis. For a single month, use that month's cost of goods sold and convert with the thirty or thirty-one days of that month.

One balance sheet line and one income statement line are all you need. The difficulty lies in keeping the numerator and the denominator on the same valuation basis, and that is exactly where it tends to go wrong.

Worked example

A technical wholesaler wants the figure for a full financial year. Revenue is € 12,000,000 and the cost of goods sold € 8,400,000, a gross margin of 30%. Inventory stood at € 1,300,000 on 1 January and € 1,500,000 on 31 December.

Item	Amount
Cost of goods sold	€ 8,400,000
Inventory 1 January	€ 1,300,000
Inventory 31 December	€ 1,500,000
Average inventory	€ 1,400,000
Inventory turnover ratio	6.0 times a year
Days of inventory	61 days

8,400,000 ÷ 1,400,000 = 6.0 times a year, and 365 ÷ 6.0 = 61 days. Had the closing balance of € 1,500,000 been used on its own, the outcome would have been 5.6 times and 65 days: four days too high, purely because inventory grew during the year.

From turns to days of inventory

Days of inventory = days in the period ÷ inventory turnover ratio

Both directions work. A ratio of 6.0 equals 61 days; 61 days of inventory equals a ratio of 6.0. The days figure can also be calculated directly as (average inventory ÷ cost of goods sold) × 365, which is the same DIO that gets added to the receivable term in the cash conversion cycle.

Which form to use depends on the conversation. In a purchasing or assortment review, turns work better, because the spread between fast and slow movers is immediately visible. In a discussion about working capital or financing, days work better, because they add up with the days sales outstanding and subtract against the payable term. For a quarter, use 90 or 91 days instead of 365, and do not then multiply the result by four.

What is a healthy inventory turnover ratio?

There is no absolute benchmark. The outcome depends entirely on the business model and on how perishable the product is, so the only meaningful comparison is against your own history and against companies running the same model. The ranges below are indicative:

• Grocery and fresh produce: fifteen to twenty-five times a year. Perishability forces the pace; there is no choice.
• Non-food retail and e-commerce: four to eight times. Collections and seasons put a floor under the stock level.
• Technical wholesale: four to six times. Delivery reliability is a selling point here, so part of the stock is deliberately held as service inventory.
• Manufacturing with long lead times: two to four times, with much of the value sitting in work in progress and semi-finished goods.
• Spare parts and service items: one to two times. A low figure is not an error here but the price of a delivery guarantee.

The spread underneath matters more than the level. A healthy 6 may consist of fast movers turning twenty times plus a quarter of the stock value that has been standing still for two years. Higher is also not automatically better: if the ratio rises because stock is too tight, the cost shows up in lost sales, rush orders and air freight. Read the outcome alongside the gross margin and the service level.

One further nuance: the quick ratio deliberately excludes inventory altogether. That is not a contradiction but a judgement, because stock that turns six times a year is a very different asset from stock that turns once. The lower the turnover, the less inventory deserves to be treated as a liquid asset.

Pitfalls

• Revenue in the numerator. Inventory sits on the balance sheet at purchase value, so the numerator has to as well. In the example above, revenue of € 12,000,000 produces a ratio of 8.6 and 43 days instead of 6.0 and 61 days. Those eighteen days are exactly the 30% gross margin — no achievement, just an arithmetic error, and by far the most common one.
• Using the closing balance only. In a seasonal business, a measurement on the closing balance mostly measures the balance sheet date. A closing balance taken after the autumn purchasing round and one taken in February give very different outcomes on identical sales.
• One figure for the whole assortment. An average hides the slow movers that are the actual problem. Split at least by product group or ABC class before drawing any conclusion.
• Reading a write-down as an improvement. Writing off obsolete stock lowers the denominator and lifts the ratio instantly, without a single extra unit having been sold. Always read the outcome alongside the write-downs in the income statement.
• Treating work in progress inconsistently. In project and manufacturing businesses, much of the pre-financing sits under semi-finished goods or work in progress. If those balances go into the denominator, make sure the matching cost goes into the numerator.

Frequently asked questions about the inventory turnover ratio

What is a good inventory turnover ratio?

It depends on the sector and on the shelf life of the product. A grocer runs at fifteen to twenty-five times a year, a technical wholesaler at four to six and a spare parts supplier at one to two. So never compare against a general average; compare against your own twelve-month series and against the level your supplier lead times allow.

What is the difference between inventory turnover and days of inventory?

Nothing in substance: they are two ways of writing the same figure. Turnover is expressed in times per period, days of inventory in days, and you convert between them by dividing the days in the period by the ratio. Use turns to compare product groups and days to place the figure next to the receivable and payable terms.

How do you calculate the inventory turnover ratio in Excel?

Set up three columns per month: cost of goods sold, opening inventory and closing inventory, all three excluding VAT. The fourth column averages the two inventory balances, the fifth divides the cost by that average and the sixth converts the result into days. Extend the series across twelve months and put a chart underneath it; the trend line tells you more than any single month.

Do you use purchase value or sales value?

Purchase value, on both sides. Inventory is carried on the balance sheet at purchase or production cost, so the denominator is already at purchase value; the numerator then has to be cost of goods sold rather than revenue. Only a retailer who values stock at selling price, under the gross profit method, works consistently at sales value on both sides.

From ratio to monthly steering

Inventory changes every day, while in many organisations this figure only gets calculated for the annual accounts. That is far too late to base a purchasing decision on. In Smartbooks you capture the ratio as a metric: a custom KPI driven by a formula across the general ledger accounts, with the opening balance pulled from the prior period through the PreviousPeriod function, so the average updates itself every month. The same metric can be split by cost center or business dimension and placed on a dashboard as a KPI tile with a trend line, referenced against budget or against last year. That puts the ratio in the same reporting set as the result, and across multiple entities also at consolidated level.

Start with twelve months of history on a fixed basis and break it down by the five largest product groups; almost all of the gain sits there. Then place the adjacent measures beside it: cash flow calculation for the cash movement itself and the cash conversion cycle for the full working capital lead time. The rest of the set is in the knowledge center.