The IRS summarizes the rule as follows:
Generally, you cannot deduct personal, living, or family expenses.
However, if you have an expense for something that is used partly for
business and partly for personal purposes, divide the total cost
between the business and personal parts. You can deduct the business
For example, if you borrow money and use 70% of it for business and
the other 30% for a family vacation, you can deduct 70% of the
interest as a business expense. The remaining 30% is personal interest
and is not deductible. Refer to chapter 4 of Publication 535, Business
Expenses, for information on deducting interest and the allocation
Business Use of Your Home
If you use part of your home for business, you may be able to deduct
expenses for the business use of your home. These expenses may include
mortgage interest, insurance, utilities, repairs, and depreciation.
Refer to Home Office Deduction and Publication 587, Business Use of
Your Home, for more information.
Business Use of Your Car
If you use your car in your business, you can deduct car expenses. If
you use your car for both business and personal purposes, you must
divide your expenses based on actual mileage. Refer to Publication
463, Travel, Entertainment, Gift, and Car Expenses. For a list of
current and prior year mileage rates see the Standard Mileage Rates.
In practice, a mixed use expense will often be disallowed unless you can document in some way the percentage of use that was business related (e.g. with some kind of log).
For inexpensive items, it is often cheaper and easier from a bookkeeping perspective to have dedicated business and dedicated personal items, even if it means that there is some duplication of purchases.
Also, if property is owned by the LLC, but used for personal purposes by the owner of the LLC, this will very likely destroy the limited liability protection associated with the LLC. Comingling of business and personal property is a leading reason to pierce the company veil.