A supplier lot's unit cost is its total cost divided by the number of units it produced.
Key takeaways
- Keep costs from different lots separate.
- Include the component and time costs that belong to the lot.
- Check the unit count and keep your calculation with the lot record.
Why the invoice total is not your unit cost
Your shelf, your count sheet, and your inventory records need a number for each unit, though. That number is the unit cost. Getting it right matters because prices, margins, and reorder decisions all depend on it.
The tricky part is deciding what "what the lot cost you" includes. Sometimes it is only the supplier's price. Sometimes it also includes parts you add yourself, such as packaging or components you put into a finished item. Sometimes it includes the time your staff spends on the lot, a cost that does not appear on any supplier invoice.
This post walks through three worked examples, from the simplest to the one with overhead. Treat the numbers as a method you can follow by hand. The arithmetic carries over, even if your own numbers are different.
Before you start: list what belongs to the lot
Before you divide anything, write down every cost tied to the lot. Start with the supplier's price for the lot itself. Then look at the invoice for other charges that clearly belong to these units, and ask whether you added any cost yourself after the goods arrived.
Be careful about two things. First, do not mix costs from different lots. If you received two lots at different prices in the same week, keep their totals separate, or your unit costs will blend together and stop matching what you actually paid. Second, do not guess at costs you cannot find. If a charge is unclear, note it and check it with the supplier or your bookkeeper before you fold it into the unit cost.
Next, count the units the lot produced. This is the number you divide by. If some units are damaged or lost during processing, how you count them is a judgment call for you and your accountant. Pick a rule, write it down, and use it the same way every time. For help checking the count, see physical inventory reconciliation.
Example 1: one lot cost, divided by the unit count
The simplest case is a lot with one total cost and no extra steps. Microsoft's documentation gives a plain example of this kind. The cost of the lot is 200, it produces 10 units, and the unit cost is found by dividing one by the other: 200 ÷ 10 = 20. (See About calculating standard cost in Microsoft Learn.)
So each unit in that lot carries a cost of 20. You can check it quickly. Multiply 20 by 10 units and you get back to 200, the total you started with. If the check does not return the lot total, something is off, either in the total or in the unit count.
This method works best when the lot is one product, bought in one lot, with no assembly and no added work. Once you start adding parts or labor, the total you divide changes, and the next two examples show how.
Before you divide, read the invoice line by line. Make sure the total covers the goods that actually arrived in this lot, and not a balance carried over from an earlier invoice. If the invoice includes goods you returned or that were credited, take those amounts out first.
Example 2: a lot used as components in a finished item
Many shops take parts from a lot and combine them into something new, such as a kit, a bundle, or a finished product. In that case the cost of each finished item includes the cost of each part that went into it. The cost of the parts is not separate from the unit cost. It is part of the unit cost.
Microsoft's example works this through with a unit cost of 4 for each component, 3 components in each finished item, and 10 finished items. The source states it as: "The lot cost is calculated as 4 unit cost × 3 components × 10 finished items = 120." (Source: About calculating standard cost in Microsoft Learn.)
That gives a total component cost of 120 for the lot. Divide it by the 10 finished items and you get the cost of each one: 120 ÷ 10 = 12. You can check the same answer another way: 4 × 3 = 12. Two routes to the same number is a good sign that the figure is right.
Parts are not always used in equal amounts. If one component goes into some finished items and not others, the simple division shown here will not fit. Group finished items by the components they use, then calculate each group's cost separately.
Notice what this example does not include. It covers only the cost of the components. If the finished items also need labor or machine time, those costs must be added. That is the subject of the next example, and it is why a component-only unit cost can look too low for the real cost of making the item.
Example 3: adding work-center time and overhead
The third case is the one most often left out. It is easy to treat the supplier's price as the whole cost of the lot. But if your staff spends time on the goods, such as cutting, sorting, packing, or assembling, that time has a cost too. Accountants call the shared costs that support production "overhead," and the cost of time spent at a particular station or machine is often called a work-center cost.
Overhead is easy to miss because it does not appear on a supplier invoice. OpenStax, in its managerial accounting text, puts the point clearly: "An important component in determining the total production costs of a product or job is the proper allocation of overhead." (See OpenStax, Principles of Accounting, Volume 2.) If you want the true cost of one unit, you have to spread the overhead across the units it helped produce. See overhead for each unit for a related cost check.
Microsoft's example shows how this works for a single work center. Work center 200 contributes 4 to each item's cost: (10 setup minutes + 1 run minute × 10 items) × a rate of 2 for each minute ÷ 10 items.
Work through that formula step by step. One run minute for each of 10 items is 10 minutes. Add the 10 setup minutes and the total time is 20 minutes. The setup time is shared across the whole lot, which is why it matters how many items come out of each setup.
The separate total work-center calculation uses a direct unit cost of 7: 7 × 10 items = 70, then 70 ÷ 10 = 7. (Source: About calculating standard cost in Microsoft Learn.) The contribution of 4 from work center 200 is not the total direct unit cost of 7.
Spreading overhead always involves a choice. You have to decide what to spread it by, such as minutes of machine time, hours of staff time, or the number of units. Different choices can give different unit costs, so pick the basis that matches how the work is really done, and write it down.
For your own shop, count the setup time and run time spent on the lot. Add those times, multiply by the applicable rate, and divide the cost by the number of units.
Keep your time records honest. If staff record time as rough estimates, the work-center figure will be rough too. Note how the times were gathered, so the next person who reads the figure knows how much weight to put on it.
Checking your numbers
Try the check on Example 3: 7 × 10 = 70.
Watch for three common errors. The first is dividing by the wrong count, such as the number of boxes instead of the number of units. The second is leaving out a cost that belongs to the lot, such as setup time you did not record. The third is mixing units, such as minutes in one place and hours in another. Each of these can make a unit cost look reasonable and still be wrong by a noticeable amount.
Rounding is another point to decide in advance. If you round a unit cost, multiplying the rounded figure back will not always return the exact total. A small difference is normal, but a large one means you should look again.
It helps to keep a short worksheet for each lot. List the supplier cost, the other costs you added, the unit count, the steps you took, and the final figure. When a question comes up months later, the worksheet tells you how the number was made, and you do not have to rebuild it from memory.
Illustrative example: check a simple lot
An illustrative lot costs 200 and contains 10 units, with no added costs. Divide 200 by 10 to get a unit cost of 20. Check the result: 20 × 10 = 200.
Using the unit cost after you have it
A unit cost is useful only if it reaches the places that depend on it. Record it where your stock system keeps item costs, and note which lot it came from. If the next lot of the same item arrives at a different price or needs different work, the unit cost will change. Do not carry the old number forward without checking.
Stock staff often see the cost on the shelf label or the bin card before anyone else does. Make sure they know which figure applies and what it includes. If the unit cost covers only the supplier price, and the item also needs packing time, someone should say so. Otherwise the number will be read as a complete cost, and it is not.
Owners and managers can use unit cost to judge whether prices cover what the goods cost to get and prepare. The unit cost is one input among several. Compare it to the price you charge, the sales volume you expect, and other costs the business carries, such as rent, wages, and shipping.
Side by side: which method fits your lot
Each of the three examples answers a different question, so start with the one that matches what happened to the lot.
- The lot was resold as received. Divide the lot cost by the unit count, as in Example 1.
- Parts from the lot went into a finished item. Total the component costs across the lot, then divide by the finished items, as in Example 2.
- Staff time was spent on the lot. Add the time cost to the lot cost, then divide by the units, as in Example 3.
Many lots need more than one method. A kit built from bought parts, for instance, may need both the component cost and the time cost before the unit figure makes sense. Work through each piece separately, then add the results, and keep the steps on your worksheet.
Start with one invoice today
Choose a recent supplier invoice and match it to the units received. Use the method above that fits the lot, then save the calculation beside the invoice.
Common questions
Should I include shipping in the lot cost? It depends on whether the charge belongs to this lot. Read the wording on the invoice and ask your bookkeeper how your business treats it. Whatever you decide, apply the same choice to every lot so the figures compare fairly.
What if I do not know how long the work took? Estimate it, mark the figure as an estimate on your worksheet, and replace it with real numbers once you have them. An estimate labeled as one is far more useful than a number that looks exact but is not.
Can the unit cost change after I record it? Yes, a later lot can have a different unit cost. If a later invoice for the same item comes in at a different price, or your staff time changes, the unit cost changes too. Record the new figure with its own date and lot, and keep the old one on file so you can see how the cost moved.