Somewhere in your operation, money is disappearing.
Not from theft. Not from bad vendors. From a number you've never actually calculated โ your true inventory cost.
Most operations managers know what they paid per unit. Some track freight. Almost none track everything. And that gap, the costs sitting between "what I paid" and "what it actually costs to hold this stock," is where working capital quietly drains.
Here's what the full picture actually looks like.
The costs your current system is probably ignoring
When you calculate inventory cost, you're likely using this math:
Unit price + shipping = landed cost
That's the starting point. It's not the full number.
The real cost of holding inventory includes:
Storage and warehousing. Whether you own the space or lease it, every SKU occupies square footage that costs money per month. Most teams never divide that cost down to the unit level.
Insurance. Inventory on hand is an asset. That asset is insured. That insurance cost is almost never factored into per-unit economics.
Obsolescence and spoilage. Products expire, go out of season, or get discontinued. That loss is a real cost spread across every unit you hold โ but it rarely appears in your per-SKU calculation.
Capital cost. Every peso or dollar tied up in inventory is a peso or dollar not invested elsewhere. The opportunity cost of your working capital is one of the most overlooked inventory expenses in small and mid-size operations.
Receiving and inspection labor. Someone unboxed it, counted it, logged it. That labor cost belongs to the inventory.
Add these up across your top 20 SKUs. The number is almost always 20โ40% higher than your purchase price alone.
Why this matters more than you think
Getting the number wrong cascades through every decision downstream.
Your reorder point is based on a cost assumption. If that assumption is wrong, you're either holding too much stock (cash tied up, carrying costs rising) or too little (stockouts, rush orders, margin erosion).
Your pricing is built on margin calculations. If your true cost per unit is 30% higher than you thought, your margin is 30% thinner than it looks.
Your supplier negotiations are based on unit cost comparisons. The cheapest unit price often comes with the highest total cost once freight, lead time, and quality failure rates are included.
One wrong number. Five wrong decisions.
The five metrics that tell the real story
You don't need an ERP system to get clarity on your inventory. You need five numbers:
1. Inventory Turnover Ratio How many times does your stock cycle through in a year? Low ratio means slow-moving inventory โ cash sitting still. High ratio means you're running lean, but watch your stockout risk.
Formula: Cost of Goods Sold รท Average Inventory Value
2. Days Inventory Outstanding (DIO) How many days of supply do you have on hand right now? Benchmark varies by industry, but anything over 60โ90 days in most product categories deserves a second look.
Formula: (Average Inventory รท COGS) ร 365
3. Carrying Cost Percentage What percentage of your inventory value does it cost you to hold that inventory for one year? Industry average sits between 20โ30%. If you're above that, your storage or capital costs need attention.
4. Stockout Rate How often are you unable to fulfill an order because the item wasn't in stock? Each stockout has a direct revenue cost and a harder-to-measure customer trust cost.
5. Order Fill Rate What percentage of orders ship complete and on time? This is the single best measure of whether your inventory system is working.
Most operations managers can tell you their unit cost. Almost none can answer these five questions without digging through three spreadsheets.
What to do with the numbers
Once you have all five metrics, the picture becomes clear fast.
High DIO + low turnover = you're overstocked. Time to review reorder quantities and minimum order commitments with suppliers.
Low fill rate + frequent stockouts = you're undercounting demand or your reorder point is set too low. Review your safety stock formula.
High carrying cost percentage = your storage, insurance, or capital costs are too high relative to your inventory value. Time to look at SKU rationalization โ which products are worth holding versus which should be made-to-order.
The math is not complicated. The problem is most teams never sit down and run it.
Get the Inventory Health Calculator โ free
The Inventory Health Calculator is a ready-to-use Excel and Google Sheets tool that calculates all five metrics from your existing data.
Enter your COGS, average inventory value, and a few operational inputs. The tool returns your turnover ratio, DIO, carrying cost estimate, and a simple health flag โ overstocked, optimal, or at risk โ for each SKU you track.
No formulas to build. No consultants required. Ten minutes to run.
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