Cost per usable unit equals total purchase cost divided by the quantity remaining after trimming and other unusable loss. When purchase cost is unchanged, a higher usable yield produces a lower cost per usable unit, so the shelf or quoted price alone is not enough.
Yield and trimming loss must use the same base
The USDA Food and Nutrition Service distinguishes purchased quantity from edible portion yield, using the measure “Servings per Purchase Unit, Edible Portion (EP).” The amount purchased therefore cannot be treated as the amount available after preparation.
| Measure | Calculation | What it shows |
|---|---|---|
| Purchased quantity | Amount bought | The starting quantity |
| Usable quantity | Purchased quantity minus trimmed or otherwise unusable quantity | The quantity retained |
| Usable-yield ratio | Usable quantity ÷ purchased quantity | The share retained after preparation |
| Trimming-loss ratio | Trimmed or unusable quantity ÷ purchased quantity | The share removed |
| Cost per usable unit | Total purchase cost ÷ usable quantity | The cost of each usable unit |
The same measurement basis and cost boundary should be used for every product or batch being compared. Otherwise, a higher apparent yield may simply reflect a different definition of usable output.
Why shelf price is not the final comparison
Penn State Extension recommends comparing food costs by unit cost rather than retail price. Where preparation removes part of a product, the relevant operational unit is the usable unit rather than merely the purchased unit. A lower purchase price can still result in a higher cost per usable quantity when less of the purchase remains usable.
Inventory accounting is a separate check. IAS 2 excludes abnormal amounts of wasted materials, labour and other production costs from inventory cost. That treatment does not change the physical quantity removed during trimming. An operational yield calculation should still record actual usable output, while abnormal waste may need separate treatment under the applicable accounting framework.
What the reader must still confirm
Before relying on the result, the reader should establish:
- The quantity actually billed and the cost basis shown on the purchase document.
- Which material counts as trimmed or unusable.
- Whether the measured yield represents the product and preparation method being purchased.
- Whether every comparison uses the same unit definition and documented cost boundary.
- Whether any waste is abnormal and therefore requires separate accounting treatment.
The cited materials do not provide a universal trimming percentage, standard usable yield or Hong Kong-specific cost rule. Those points must be confirmed through the reader’s own product records, purchase documents and applicable accounting treatment.