Inventory Carrying Cost Calculator

The maths behind it

Carrying cost is the annual cost of holding stock, expressed either in money or as a percentage of average inventory value. The components are capital cost, storage, service costs such as insurance and handling, and risk costs: shrinkage, damage, obsolescence and markdown.

The conventional total is 20% to 30% a year, and for anything with fashion or expiry risk it is considerably higher. Businesses that estimate it at 10% are almost always leaving obsolescence and markdown out.

A real example

Average inventory of $3,132. Capital at 9% is $282. Storage at 4% is $125. Insurance, handling and administration at 4% is $125. Risk, shrinkage, damage and markdown, at 8% is $251.

Total is $783 a year, which is 25% of average inventory value. Per unit that is $4.50 a year, or 37.5 cents a month.

Now the decision that number informs: holding an extra 100 units of safety stock costs $450 a year. If that extra stock prevents four stockouts a year on a product contributing $40 a unit and losing 8 sales each time, it saves $1,280. The buffer pays for itself nearly three times over, which is not obvious until both sides are priced.

The usual mistakes

Storage cost is often treated as zero because the warehouse is already paid for, and that is only true while there is spare space. The moment stock levels force a larger unit or overflow storage, the marginal cost jumps from nothing to a great deal all at once.

Obsolescence is also chronically understated because it shows up as markdowns and write-offs in a different line of the accounts. A business marking down 6% of stock a year has a 6% obsolescence cost whether or not anyone attributes it to carrying inventory.

Using the result

Calculate the percentage once for the business and apply it per product, adjusting upward for anything with a short shelf life or a fashion cycle. A single rate across a mixed catalogue understates the cost of the risky stock and overstates the safe stock.

Then use it in every stock decision. Order quantity, safety stock, whether to accept a volume discount and whether to keep a slow-moving SKU all turn on this number, and a business that has never calculated it is making those decisions on instinct.

Why the volume discount often loses

A supplier offering 8% off for a year's stock instead of a quarter's is offering an immediate saving against a carrying cost that runs for the whole year. With carrying cost at 25%, holding an extra nine months of stock costs roughly 18.75% of its value.

On the example product, a quarter's stock is 520 units and a year's is 2,080. The 8% discount saves $2,995. The extra average inventory of about 780 units at $18 costs 25% of $14,040, $3,510 a year. The discount loses by $515 before counting the obsolescence risk on a year of stock.

That calculation flips the moment carrying cost is low or the discount is deep, which is why it has to be run rather than assumed. What it reliably shows is that discounts under about 10% rarely justify more than doubling the order quantity, and suppliers offer them precisely because most buyers never check.