How do I calculate the home office deduction using the regular method?

The regular method starts with the percentage of your home used exclusively for business, usually square footage of the office divided by total home square footage. You then apply that percentage to indirect home expenses like mortgage interest or rent, utilities, insurance, and repairs, and add 100% of any direct expenses that benefit only the office, such as painting that room. Under the actual method you also depreciate the business portion of the home. It is reported on Form 8829. Compare the result to the simplified method's flat $5 per square foot (up to 300 square feet, so $1,500 max); the regular method usually wins for larger spaces or high-cost homes but requires more records.