What inventory days means

We’ve spent the last few posts on profitability — how much of every rupee of sales a company keeps. Inventory days (also called Days Inventory Outstanding, or DIO) is the first of a different family of ratios: efficiency, or how well a company manages the cash tied up in running the business day to day.

Inventory days answers a simple operational question: on average, how many days does stock sit around — as raw material, work in progress, or finished goods — before it’s sold? A snacks company holding 60 days of inventory is carrying two months of unsold stock at any given time; one holding 20 days turns its stock over much faster.

The formula

Inventory Days = Inventory / COGS × 365

We divide by COGS rather than revenue, because inventory is carried at its cost to the company, not at what it’ll eventually sell for.

We use the closing (year-end) inventory here, as is common for the days and liquidity ratios; the return ratios (ROE, ROCE, ROA) average the opening and closing balances instead. Either works, as long as you compare like with like.

Worked example: Desi Bites Foods, FY25

  ₹ Lakh
Inventory (FY25 closing) 211
COGS (FY25) 1,607
Inventory Days 48 days

Worked example: Britannia Industries, FY25

From Britannia Industries’ audited consolidated FY25 results (year ended 31 March 2025, filed 8 May 2025). For illustration only, not a signal to act on.

  ₹ Crore
Inventory (FY25 closing) 1,236.51
COGS (FY25) 10,604.05
Inventory Days 42.6 days

Britannia turns its inventory faster than Desi Bites — 42.6 days versus 48. That’s consistent with the advantages of scale and distribution reach in packaged foods — a bigger network can move stock out of factories and depots faster — though the ratio alone can’t tell you the cause.

One caveat, flagged in the gross margin post: Britannia’s COGS here is materials-only, since the filing has no single COGS line. A smaller denominator makes inventory days look longer, so on a full-cost basis Britannia’s figure would be a little shorter still.

🧒 Explain it like I'm 10 (optional — skip if this is already clear)

Imagine a fruit stall. If a crate of mangoes sits unsold for 10 days, that’s 10 days the stall owner’s money is stuck in mangoes instead of in cash. A stall that sells its mangoes in 3 days gets its money back — and can buy the next crate — much faster than one that takes 10. Inventory days measures exactly that: how long money is “stuck” in stock before it turns back into cash.

Common mistakes

  • Comparing inventory days across very different industries. A jewellery retailer and a bakery have completely different natural inventory cycles — one sells through slowly by design, the other has to move stock daily. Compare within the same industry, or against the same company’s own history.
  • Assuming falling inventory days is always good. It can mean genuinely better efficiency — or it can mean the company is running low on stock and risking stockouts, which shows up later as lost sales.
  • Assuming rising inventory days is always bad. A company stocking up ahead of a big seasonal push (festive season for an Indian FMCG company, for instance) will show temporarily higher inventory days without anything being wrong.
  • Reading one year-end snapshot without checking seasonality. Inventory levels can swing a lot within a year — a single balance-sheet date doesn’t always represent the average.

Takeaway: inventory days measures how long a company’s cash stays tied up in unsold stock — lower is generally more efficient, but the number only means something when read against the company’s own industry and history, not in isolation.