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Inventory Turnover Days Calculator

See how many days your stock sits on the shelf before it sells.

How it is calculated

The tool needs three numbers: cost of goods sold (COGS) for a period, the average inventory value over the same period, and the length of that period in days. turnover ratio = COGS / average inventory, the number of times the stock was sold and replaced in the period. days inventory = period days / turnover ratio, the average number of days an item waits before it sells. Both money amounts must be in the same currency and valued at cost, not at selling price, because COGS is a cost figure. Use 365 for a full year, 90 for a quarter or 30 for a month, but always take COGS from that same period. Average inventory is usually the opening and closing values added and divided by two. All three inputs must be above zero.

Worked example

Annual COGS 600,000, average inventory 100,000, period 365 days. Step 1: turnover ratio = 600,000 / 100,000 = 6.00. Step 2: days inventory = 365 / 6 = 60.83 days. So the stock is sold and restocked six times a year, and an average item sits on the shelf for about 61 days. For a 90-day quarter with the same ratio, the result would be 15 days.

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Common mistakes

  • Using sales revenue instead of COGS. Revenue includes your markup, so turnover looks higher and days lower than they really are.
  • Using year-end inventory instead of the average. If stock is built up for a season, the end figure can be far from the typical level.
  • Mismatching the period: quarterly COGS with 365 days (or annual COGS with 90 days) gives a result that is wrong by a factor of about four.
  • Comparing days across industries. A fast-moving grocery business and a machinery dealer have very different normal figures, so judge your result against your own history or peers.
  • Treating a very low days figure as always good. It can also mean stock-outs, so check it against how often you run out.

About this calculator

Days Inventory Outstanding (DIO) — often called inventory turnover days — measures how long, on average, stock sits on the shelf before it’s sold, calculated as DIO = (average inventory / COGS) × days in the period (365 for a full year). Its inverse, the inventory turnover ratio, expresses the same relationship as a frequency instead of a duration: turnover = COGS / average inventory, telling you how many times inventory is fully sold and replaced over the period.

CFOs, supply chain managers and equity analysts track this pair of numbers because inventory ties up cash — every day stock sits unsold is a day that cash is locked in warehouses rather than working elsewhere in the business — and it feeds directly into the cash conversion cycle alongside days sales outstanding and days payable outstanding. A rising DIO can signal slowing sales or overstocking; a falling one can signal either operational efficiency or a stockout risk if it drops too far.

This calculator takes your cost of goods sold, average inventory value and the number of days in the period, and returns both the days inventory outstanding and the turnover ratio, so you can see how efficiently your business is moving stock.

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