QuickBooks Desktop, depending on the version you're using, may offer either one or two unit of measure (U/M) options – Single U/M Per Item or Multiple U/M Per Item. QuickBooks Pro doesn't offer U/M. The U/M feature first must be enabled from the Items & Inventory Company Preferences.

UOM is enabled under Items & Inventory Company Preferences.
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Next, depending upon the specific version of QuickBooks you're using, select the specific U/M you need for your items and inventory.
For example, if you have QuickBooks Premier "generic" installed, you only have the single U/M per item option available. Should you need multiple U/M per item you will need to convert your "generic" flavor of Premier to either "Contractor" or "Manufacturing & Wholesale."

Not all QB Desktop versions offer U/M options, and some only offer the Single U/M per item option.
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Since both the process and ramifications of changing units of measure will depend upon which option is in use, we first should review the fundamentals of the two options.
Single U/M per Item Option
With this option, you can assign different units of measure to different items, but each item can have only one unit of measure assigned to it. Select this Single U/M per item option only if you buy, stock and sell each item by the same unit of measure.

Setting-up Single U/M is simple, but much more difficult to 'undo'.
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For example, choose this option if you buy an item by the foot, keep track of it in inventory by the foot, and sell it by the foot.
Multiple U/M per Item Option
Using this option, you can create any number of units of measure – called a unit of measure set – for each item. This option is helpful when you buy in one unit of measure, but sell in a different unit of measure.
A unit of measure set consists of a base unit (usually the smallest unit used to track a certain type of item) and any number of related units (defined as containing a certain number of base units).
For example, you could create a unit of measure set called "Length by the inch" with a base unit of inch and related units of foot (containing 12 inches) and yard (containing 36 inches).

If you thought Multiple U/M was complex, just how much of a mess do you think trying to change it will be?
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Another benefit of choosing Multiple mode is the ability to change or convert the unit of measure when you enter an item on a transaction form, such as an invoice or purchase order.
For example, suppose you have an item called Cable that you've assigned a unit of measure set that contains the inch, foot and yard units of measure. If you enter Cable on an invoice with a quantity of 30 feet, you then could change the unit of measure to yards (causing 30 feet to become 30 yards) or convert the unit of measure to yards (causing 30 feet to become 10 yards).
Think Before You Change Units of Measure
The way you change a unit of measure assignment for an item depends on which of the two Unit of Measure options you're currently using. And while there are clearly defined steps by which you actually can make changes, the question is should you make such changes.
Under most circumstances, the answer is no. So, this article is not going to outline the steps you could take to make changes, rather we're going to clearly point out the ramification of making such changes.
Ramifications of changing U/M assignments when using the Single U/M Per Item Option
You can change the U/M of any service, non-inventory part or group item without affecting the accuracy of any data stored in QuickBooks. But changing the U/M of an inventory part or inventory assembly significantly can alter the quantity on hand and cost information for that item, in some cases resulting in inaccuracies.
For example, suppose one of your inventory part items is nylon cord and you currently have 600 feet of nylon cord in inventory. If you change the item's unit of measure from foot to yard, your quantity on hand still will be 600, but should actually be changed 200 to be accurate.
QuickBooks will not make this math change for you when you reassign the U/M.
If you proceed with making a U/M change of this type, the old U/M will appear in all previously saved transactions using the original U/M. But in order to show that the prior U/M shown in the transaction is obsolete, QuickBooks actually displays the U/M within brackets [feet].
Let’s take this one step further. If the current cost of the nylon cord is 0.25 per foot, you would need to manually change the cost to 0.75 per yard after changing the unit of measure to yard. Once again, QuickBooks will not automatically make this change for you when you reassign the U/M.
As a result of this oddity, Intuit actually recommends you create a new item with the correct U/M and record the adjusted quantity and cost in the new item based upon the new U/M. Use an inventory adjustment to reduce the on-hand quantity of the item being made inactive, and post the quantity and cost of the new item being created.
There is another oddity you should be aware of as well. If you change a time-based U/M associated with a service item used in time sheets or time tracking functions, any U/M besides hour will cause rate calculations to be inaccurate, because in all QuickBooks time sheets "hour" is the only recognized time unit.
Ramifications of changing U/M assignments when using the Multiple U/M Per Item Option
If you thought things sounded bad when we were talking about changing just "single" U/M per item assignments, we were only beginning to see the complications that are even more significant when "multiple" U/M per item assignments are in force.
When changing an item's unit of measure set, you should only change to another set with the same base unit, under certain circumstances. You can change the base unit of measure of a service, non-inventory part, or group item without affecting the accuracy of any information stored in QuickBooks.
But changing the base unit of any inventory part or inventory assembly can significantly alter the quantity on hand and cost information for that item, resulting in inaccuracies in most cases.
You should only rename the base unit or give it a new abbreviation to correct a spelling error or to make clearer what the current base unit is.
Intuit recommends that you never actually change the base unit to an entirely different U/M that would require a conversion of quantities and rates, such as changing from inch to foot. Such a change would require you redefine all related units in the set, manually adjust inventory quantities, and change quantities and rates in every saved transaction that includes items with the current base unit.
If you need to use a new base unit, Intuit recommends you create an entirely new U/M set with the different base unit, and then assign the new set only to the appropriate items for use from that point forward.
Ramifications of Turning-off Units of Measure (U/M)
There is one additional way that some users choose to change Units of Measure, and that's to simply abandon the use of any U/M after they have previously set them up and used them.
Because U/M is one of the most misunderstood features of QuickBooks, many times they're used inappropriately. This more commonly occurs when using Multiple U/M per item than with Single U/M per item.

Turning-off (Disabling) Units of Measure isn't any better an idea then changing your U/M set-up.
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As a result, some QuickBooks users simply decide to disable the U/M feature under the Items & Inventory Company Preferences. Before doing that, it's import to understand what really happens when U/M is disabled, and the possible ramifications of turning-off the feature:
- When U/M is disabled QuickBooks simply hides U/M throughout QuickBooks, the U/M column on QuickBooks forms disappears.
- All item quantities, prices and costs that had been based upon U/M now will be shown in terms of their base (or original) units.
- All previously configured related units of measure for an item in a transaction now will display the quantity, price and cost of that item in their base (or original) units.
- Disabling U/M significantly can alter the quantity on hand, cost and price information for your items, in many cases resulting in inaccuracies.
- Disabling U/M should really only be done when the U/M feature actually has not been used even though it has been enabled.
Consider correcting the problems with U/M of measure using the steps discussed herein, specifically creating new items with the correct U/M or U/M sets and making inventory adjustments to adjust quantities to zero for the prior item with incorrect U/M before making such item inactive, and an inventory adjustment to post the correct quantity for the correct U/M, along with the correct cost for the U/M (or U/M set) for the new (replacement) item.