It's time to grab a measuring tape! Measure the actual length and width (in feet) of the space that you "regularly and exclusively" use for business. Area (in square feet) = length times width.
Now, you need the total area of your entire home. You can measure each room, as above, and add them together. More conveniently, the realtor's listing from when you bought your home or the "property card" maintained by your tax assessor's office (online in most counties) will both have the area measurement, without you doing the work.
Divide the area used for business by the total area of the home, and that is your percentage.
