Days inventory outstanding (DIO) is the average number of days your business holds stock before selling it.
If you run a product-based business, DIO might be the most important working capital metric you are not tracking. Every day stock sits on your shelves, in your warehouse, or in transit is a day your cash is locked up in something that is not yet generating revenue.
Quick Summary
- What it measures: How long stock sits in your business before being sold
- Formula: (Average Inventory / Cost of Goods Sold) x Number of Days
- Good DIO: Depends heavily on your industry – perishable goods demand low DIO; heavy manufacturing tolerates higher
- UK average for SMEs: Roughly 3 to 72 days, depending on sector
- Why it matters: High DIO means cash is trapped in stock instead of working for your business
What Does Days Inventory Outstanding Actually Mean?
DIO answers a simple question: once you buy or make something, how long does it take to sell it?
A DIO of 30 means your average item sits in stock for 30 days before a customer buys it. A DIO of 90 means three months – and three months of cash tied up in products gathering dust.
For service businesses with no physical stock, DIO is essentially zero. But for retailers, manufacturers, wholesalers, and e-commerce businesses, DIO is often the largest component of the Cash Conversion Cycle definition. Getting it right can mean the difference between healthy cash flow and a warehouse full of goods you cannot afford to replace.
The DIO Formula
The standard formula is:
DIO = (Average Inventory / Cost of Goods Sold) x Number of Days
Average inventory is typically calculated as (opening inventory + closing inventory) / 2. Using the average smooths out fluctuations from large deliveries or seasonal stock builds.
Worked Example
Imagine you run an online kitchenware business based in Bristol. For the month of October:
- Opening inventory (1 October): £78,000
- Closing inventory (31 October): £64,000
- Average inventory: (£78,000 + £64,000) / 2 = £71,000
- Cost of goods sold for October: £52,000
- Number of days in October: 31
DIO = (£71,000 / £52,000) x 31 = 42.3 days
That means your average product sits in stock for about 42 days before it sells. Whether that is good or bad depends on your industry – for e-commerce kitchenware, it is within the normal range.
Now a quarterly view for Q4 (October to December – your peak season):
- Average inventory across Q4: £68,000
- Cost of goods sold for Q4: £185,000
- Number of days in Q4: 92
DIO = (£68,000 / £185,000) x 92 = 33.8 days
Your DIO dropped in Q4 because stock was turning over faster during the busy Christmas period. This is exactly what you would expect – and exactly why tracking DIO over time reveals seasonal patterns that a single snapshot misses.
Try it with your own numbers using our DIO Calculator.
UK Industry Benchmarks for DIO
DIO is the metric with the widest variation across sectors. A professional services firm has almost no inventory. A manufacturer might have raw materials, work-in-progress, and finished goods all sitting in stock simultaneously.
Typical UK SME benchmarks:
| Industry | Typical DIO (days) |
|---|---|
| Professional Services | 3 |
| Retail | 45 |
| Manufacturing | 72 |
| Construction | 22 |
| E-commerce | 42 |
What stands out? Manufacturing has by far the highest DIO, reflecting the reality of long production cycles, raw material stockpiling, and finished goods waiting for dispatch. Retail and e-commerce sit in the middle – these businesses need enough stock to meet demand but suffer when stock sits unsold. Construction is lower than you might expect because materials are typically purchased close to when they are needed on site rather than held in bulk storage.
The professional services figure of 3 days reflects minimal physical inventory – perhaps office supplies or equipment awaiting installation. For practical purposes, if you run a service business, DIO is not a metric you need to lose sleep over.
Why DIO Matters for Your Business
Stock is cash in a different form. Every pound locked up in inventory is a pound you cannot use for anything else. Here is why DIO deserves attention:
Cash trap
Consider a manufacturer with £400,000 of annual COGS and a DIO of 72 days. That is roughly £79,000 of cash permanently tied up in stock. Reduce DIO to 50 days and you free up about £24,000 – without changing your sales, your prices, or your headcount.
Storage and handling costs
Stock does not just tie up cash. It costs money to store, insure, move, and manage. A lower DIO reduces these carrying costs, which often go unnoticed because they are spread across multiple line items in your accounts.
Obsolescence and waste risk
The longer stock sits, the greater the risk it becomes outdated, damaged, or unsellable. This is acute for businesses selling technology, fashion, or perishable goods, but it affects every product-based business to some degree. Unsold stock that gets written off is cash you will never recover.
Demand signal
DIO trends tell you something about demand. A rising DIO might mean sales are slowing and stock is piling up. A falling DIO might mean demand is strong – or that you are at risk of running out. Both signals are valuable for planning.
How to Interpret Your DIO
As with all working capital metrics, context is everything.
Compare DIO to your industry. A DIO of 50 is excellent for a manufacturer but concerning for an e-commerce business. Use the benchmarks above as a starting point, then track your own trend.
Watch for seasonal patterns. Most product businesses have predictable DIO swings. A retailer might build stock in September (high DIO) and sell through it by December (low DIO). The pattern is normal – the question is whether you are building the right amount.
Break DIO down by product category. Your overall DIO might be 40 days, but that could mask a mix of fast-moving bestsellers at 15 days and slow-moving lines at 120 days. Identifying which products drag your DIO up helps you make better purchasing and discontinuation decisions.
Look at DIO alongside DSO and DPO. DIO is most useful as part of the Cash Conversion Cycle guide. A manufacturer with a DIO of 72, DSO of 52, and DPO of 38 has a cash conversion cycle of 86 days. That is nearly three months of cash locked up in the business cycle. Understanding all three metrics together shows you where the biggest improvement opportunities lie.
How to Optimise Your DIO
Lowering DIO is not about running out of stock. It is about holding the right amount of the right products at the right time.
Demand forecasting
Better forecasting means less guesswork and fewer excess orders. Even simple approaches – reviewing last year’s sales by month, tracking which lines are trending up or down – can meaningfully reduce overstock.
Supplier lead time management
If your supplier can deliver in five days instead of fifteen, you can hold less buffer stock. Shorter, more reliable lead times directly reduce the inventory you need to carry.
Regular stock reviews
Schedule monthly reviews of slow-moving stock. Products that have not sold in 90 days need a plan – whether that is a promotion, a price reduction, or a decision to discontinue and clear.
Just-in-time purchasing
Where practical, ordering stock closer to when you need it reduces holding periods. This works best when suppliers are reliable and demand is predictable. It works less well for imported goods with long shipping times or products with unpredictable demand spikes.
Product range discipline
More SKUs generally means higher DIO. Every new product line adds inventory that needs to earn its place. Regularly reviewing whether each product justifies its shelf space keeps your range focused and your DIO in check.
For manufacturers dealing with high DIO specifically, our Manufacturing Working Capital guide covers sector-specific strategies for reducing the time cash spends locked in production cycles.
DIO and the Cash Conversion Cycle
DIO sits at the heart of the cash conversion cycle for any product-based business:
Cash Conversion Cycle = DSO + DIO – DPO
For a retailer with a DSO of 8, DIO of 45, and DPO of 38, the CCC is 15 days. The low DSO (customers pay at the till) and reasonable DPO (monthly supplier terms) keep the cycle short despite holding 45 days of stock.
For a manufacturer with a DSO of 52, DIO of 72, and DPO of 38, the CCC is 86 days. Here, DIO is the dominant factor. Reducing DIO by even 10 days would free up meaningful cash without needing to chase customers harder or delay supplier payments.
The Cash Conversion Cycle guide explains how these three metrics work together and where to focus your improvement efforts for the biggest impact.
Frequently Asked Questions
What if my business has no inventory?
If you sell services rather than products, your DIO is effectively zero and you can largely ignore this metric. Your cash conversion cycle will be driven by DSO and DPO instead.
How often should I calculate DIO?
Monthly is ideal for businesses with significant inventory. It helps you spot seasonal trends and catch slow-moving stock early. Quarterly is sufficient for businesses where inventory levels are relatively stable.
Can DIO be too low?
Yes. A very low DIO might mean you are frequently running out of stock, turning away customers, or placing expensive emergency orders. The goal is not zero inventory – it is the minimum inventory needed to reliably meet demand.
This article is for informational purposes only and does not constitute financial advice. Inventory management needs vary by business and sector. If you are concerned about stock levels or working capital, consider speaking with a qualified accountant or financial adviser.