Q. What do you mean by Inventory days?

(Average inventory / Cost of sales)     x      365 days

Average inventory can be arrived by taking this year's and last year's inventory values and dividing by 2 - (Opening inventories + closing inventories) / 2.  This ratio tells how long the inventory stays in the company before it is sold.  The lower the ratio the more efficient company is trading, however this may result in low levels of inventories to meet demand. A lengthening inventory period may indicate a slowdown in trade and an excessive build up of inventories, resulting in additional costs.