How to calculate home office deductions from rent?

My girlfriend owns a 3 bedroom home and I pay half her mortgage as rent. I work from home and one of the bedrooms is used just for my home office. How can I determine the amount to write off as a business expense from my monthly rent?

If the room is used exclusively for business purposes you calculate the deduction this way. You take the total square footage of the house and get a business use percentage. You apply that percentage to the rent you pay and that amount goes on the office in home schedule. If you paid any utilities in addition to the rent you would take the same percentage of the heat and electric. Water is not included unless it is a significant part of your business. I should add that office in home is an item that attracts IRS audits so you need to be sure that this space is entirely for business, also it requires that you not have another office available in the area for your use. You need to have receipts to back up what you claim for the office and for everything else on your return because you do increase substantially your risk of audit. I agree with your other answer about looking up and reading the publication. This is a tricky deduction.