What percentage of my mortgage payment can I categorize as business since my house is my office?

@everythingtreestn 

The answer is surprisingly complicated.  If you're talking about deducting a portion of your mortgage payment, you're implying that you will use the Regular Method, as the IRS calls it.  First, using the Regular Method, you can only deduct mortgage interest, not principal (paying down the principal portion of a mortgage is not an expense; it's a reduction in your liability and an increase in your home equity).  Second, you can only deduct the percentage of the mortgage interest in proportion to the amount of square footage used for the home office.  If your home office is 10% of your home's square footage, and your mortgage payment is $1,000, comprised of $600 interest and $400 principal, the deduction would be $60/month ($600 X 10%).  However, you cannot deduct the mortgage interest if you claim the standard deduction.  There are other expenses you can deduct if you use the Regular Method. 

You also have the option of using the Simplified Method, which is deducting a simple $5/sq. ft. for your home office space (300 sq. ft. maximum).  If your home office is 150 sq. ft., that would be $1,050/yr.  You can take this deduction even if you claim the standard deduction.

Your tax accountant is your best source of info for your specific situation.