Days inventory outstanding (DIO)

Definition

Days inventory outstanding (DIO), or stock days, is the average number of days stock sits in the business before it is sold. It is derived from stock turnover and measures how long cash stays frozen as inventory.

In plain terms

A DIO of 60 means, on average, stock sits for two months between arriving and selling. The longer it sits, the longer your cash is trapped and the higher your working-capital need. Fast-moving businesses aim to keep DIO low.

Why it matters

DIO is the stock leg of the cash conversion cycle. Reducing it — clearing slow stock, ordering little and often — releases cash directly. See how to reduce stock holding.

In practice

For a UK limited company holding physical stock, DIO shows up long before it shows up as a cash problem on paper. A retailer or wholesaler might look profitable on the income statement while cash is tight, because the margin earned on a sale is locked up in shelves and storerooms until that stock actually moves. Directors often notice the squeeze first as slower supplier payments or a fuller overdraft, not as a change in reported profit.

A practical walk-through: a business orders stock ahead of a busy period, pays its supplier on the usual terms, then finds the season runs later than expected. The stock sits. Meanwhile rent, wages and supplier invoices keep falling due on their normal schedule. The gap between cash going out for stock and cash coming in from sales is exactly what DIO is measuring, and it widens whenever buying gets ahead of selling.

How lenders read it

Lenders reviewing a limited company's accounts tend to look at DIO alongside stock turnover and the wider operating cycle rather than in isolation, because a rising DIO can mean very different things. It might reflect deliberate bulk-buying to secure better terms, a seasonal build-up ahead of expected demand, or it might signal stock that is ageing, slow-moving or increasingly hard to sell. The direction of travel over several periods, and the reason behind it, usually matters more to a lender than a single snapshot.

Businesses that can explain a stock build with a clear, credible sales plan are in a different position to those where stock is quietly accumulating without one. Keeping a simple narrative alongside the numbers — why stock levels moved, and what happens to it next — tends to make this part of a lending conversation more straightforward.